Mammal#Locomotion

{{Main|Mammal classification}}

{{See also|List of placental mammals|List of monotremes and marsupials|List of mammal genera|List of mammal species}}

{{Pie chart

|caption=Over 70% of mammal species are in the orders Rodentia, Chiroptera, and Eulipotyphla.

|label1=Rodentia |value1=40.5 |color1=#63aafe

|label2=Chiroptera |value2=22.2 |color2=#dd2d32

|label3=Eulipotyphla |value3=8.8 |color3=#fff58c

|label4=Primates |value4=7.8 |color4=#4ee257

|label5=Artiodactyla |value5=5.4 |color5=#fea746

|label6=Carnivora |value6=4.7 |color6=#6711ff

|label7=Diprotodontia |value7=2.3 |color7=#865357

|label8=Didelphimorphia |value8=1.9 |color8=#00ccff

|label9=Lagomorpha |value9=1.7 |color9=#a2bd90

|label10=Dasyuromorphia |value10=1.3 |color10=#ccffcc

|label11=Afrosoricida |value11=0.8 |color11=#ffff99

|label12=Cingulata |value12=0.3 |color12=#ff99cc

|label13=Macroscelidea |value13=0.3 |color13=#33cccc

|label14=Peramelemorphia |value14=0.3 |color14=#cc99ff

|label15=Perissodactyla |value15=0.3 |color15=#3366ff

|label16=Pilosa |value16=0.3 |color16=#99cc00

|label17=Scandentia |value17=0.3 |color17=#ffcc99

|label18=Paucituberculata |value18=0.1

|label19=Pholidota |value19=0.1

|label20=Hyracoidea |value20=0.09 |color20=#99ccff

|label21=Monotremata |value21=0.08 |color21=#ff6600

|label22=Sirenia |value22=0.06

|label23=Proboscidea |value23=0.05 |color23=#003366

|label24=Dermoptera |value24=0.03 |color24=#ccffff

|label25=Microbiotheria |value25=0.03

|label26=Notoryctemorphia |value26=0.03

|label27=Tubulidentata |value27=0.02

}}

Mammal classification has been through several revisions since Carl Linnaeus initially defined the class, and at present{{when|date=August 2024}}, no classification system is universally accepted. McKenna & Bell (1997) and Wilson & Reeder (2005) provide useful recent compendiums.{{cite book | vauthors = Vaughan TA, Ryan JM, Czaplewski NJ |year=2013 |chapter=Classification of Mammals |title=Mammalogy |edition=6th |publisher=Jones and Bartlett Learning |isbn=978-1-284-03209-3}} Simpson (1945){{cite journal | vauthors = Simpson GG |author-link=George Gaylord Simpson |title=Principles of classification, and a classification of mammals |journal=American Museum of Natural History |volume=85 |year=1945}} provides systematics of mammal origins and relationships that had been taught universally until the end of the 20th century.

However, since 1945, a large amount of new and more detailed information has gradually been found: The paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematisation itself, partly through the new concept of cladistics. Though fieldwork and lab work progressively outdated Simpson's classification, it remains the closest thing to an official classification of mammals, despite its known issues.{{cite journal | vauthors = Szalay FS |year=1999 |title=Classification of mammals above the species level: Review |journal=Journal of Vertebrate Paleontology |volume=19 |number=1 |pages=191–195 |jstor=4523980 |doi=10.1080/02724634.1999.10011133 |issn=0272-4634}}

Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders in numbers of species are Rodentia: mice, rats, porcupines, beavers, capybaras, and other gnawing mammals; Chiroptera: bats; and Eulipotyphla: shrews, moles, and solenodons. The next three biggest orders, depending on the biological classification scheme used, are the primates: apes, monkeys, and lemurs; Cetartiodactyla: whales and even-toed ungulates; and Carnivora which includes cats, dogs, weasels, bears, seals, and allies.{{MSW3 |heading=Preface and introductory material |page=xxvi |name-list-style=vanc }} According to Mammal Species of the World, 5,416 species were identified in 2006. These were grouped into 1,229 genera, 153 families and 29 orders. In 2008, the International Union for Conservation of Nature (IUCN) completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species.{{cite web |title=Mammals |work=The IUCN Red List of Threatened Species |date=April 2010 |publisher=International Union for Conservation of Nature (IUCN) |url=https://www.iucnredlist.org/initiatives/mammals |access-date=23 August 2016 |archive-date=3 September 2016 |archive-url=https://web.archive.org/web/20160903200637/http://www.iucnredlist.org/initiatives/mammals |url-status=live }} According to research published in the Journal of Mammalogy in 2018, the number of recognised mammal species is 6,495, including 96 recently extinct.{{cite journal | vauthors = Burgin CJ, Colella JP, Kahn PL, Upham NS |date=1 February 2018 |title=How many species of mammals are there? |journal=Journal of Mammalogy |volume=99 |issue=1 |pages=1–14 |doi=10.1093/jmammal/gyx147 |doi-access=free}}

The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap"). In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes (echidnas and platypuses) and therians (marsupials and placentals) and all descendants of that ancestor.{{cite journal |vauthors=Rowe T |year=1988 |title=Definition, diagnosis, and origin of Mammalia |journal=Journal of Vertebrate Paleontology |volume=8 |issue=3 |pages=241–264 |url=https://www.geo.utexas.edu/faculty/rowe/Publications/pdf/010%20Rowe%201988.pdf |doi=10.1080/02724634.1988.10011708 |bibcode=1988JVPal...8..241R |access-date=25 January 2024 |archive-date=18 January 2024 |archive-url=https://web.archive.org/web/20240118163712/http://www.geo.utexas.edu/faculty/rowe/Publications/pdf/010%20Rowe%201988.pdf |url-status=live }} Since this ancestor lived in the Jurassic period, Rowe's definition excludes all animals from the earlier Triassic, despite the fact that Triassic fossils in the Haramiyida have been referred to the Mammalia since the mid-19th century.{{cite book |title=The Student's Elements of Geology | vauthors = Lyell C |author-link=Charles Lyell |year=1871 |publisher=John Murray |location=London |page=347 |url={{Google books|plainurl=yes|id=634gAQAAIAAJ|page=347}} |isbn=978-1-345-18248-4}} If Mammalia is considered as the crown group, its origin can be roughly dated as the first known appearance of animals more closely related to some extant mammals than to others. Ambondro is more closely related to monotremes than to therian mammals while Amphilestes and Amphitherium are more closely related to the therians; as fossils of all three genera are dated about {{ma|167|million years ago}} in the Middle Jurassic, this is a reasonable estimate for the appearance of the crown group.{{cite journal |vauthors=Cifelli RL, Davis BM |title=Paleontology. Marsupial origins |journal=Science |volume=302 |issue=5652 |pages=1899–1900 |date=December 2003 |pmid=14671280 |doi=10.1126/science.1092272|s2cid=83973542 }}

T. S. Kemp has provided a more traditional definition: "Synapsids that possess a dentary–squamosal jaw articulation and occlusion between upper and lower molars with a transverse component to the movement" or, equivalently in Kemp's view, the clade originating with the last common ancestor of Sinoconodon and living mammals.{{cite book |url=https://doc.rero.ch/record/200125/files/PAL_E3904.pdf |location=United Kingdom |title=The Origin and Evolution of Mammals |vauthors=Kemp TS |year=2005 |publisher=Oxford University Press |isbn=978-0-19-850760-4 |page=3 |oclc=232311794 |access-date=25 January 2024 |archive-date=26 September 2023 |archive-url=https://web.archive.org/web/20230926060152/https://doc.rero.ch/record/200125/files/PAL_E3904.pdf |url-status=live }} The earliest-known synapsid satisfying Kemp's definitions is Tikitherium, dated {{ma|225|Ma}}, so the appearance of mammals in this broader sense can be given this Late Triassic date.{{cite journal |vauthors=Datta PM |year=2005 |title=Earliest mammal with transversely expanded upper molar from the Late Triassic (Carnian) Tiki Formation, South Rewa Gondwana Basin, India |journal=Journal of Vertebrate Paleontology |volume=25 |issue=1 |pages=200–207 |doi=10.1671/0272-4634(2005)025[0200:EMWTEU]2.0.CO;2|s2cid=131236175 }}{{cite journal | vauthors = Luo ZX, Martin T |year=2007 |title=Analysis of Molar Structure and Phylogeny of Docodont Genera |journal=Bulletin of Carnegie Museum of Natural History |volume=39 |pages=27–47 |doi=10.2992/0145-9058(2007)39[27:AOMSAP]2.0.CO;2 |s2cid=29846648 |url=https://xa.yimg.com/kq/groups/13543816/1018125207/name/Luo+y+Martin+2007-+molar+structure+and+phylogeny+of+docodonts.pdf |access-date=8 April 2013 |archive-url=https://web.archive.org/web/20160303225517/http://xa.yimg.com/kq/groups/13543816/1018125207/name/Luo+y+Martin+2007-+molar+structure+and+phylogeny+of+docodonts.pdf |archive-date=3 March 2016 |url-status=dead }} However, this animal may have actually evolved during the Neogene.{{Cite journal |last1=Averianov |first1=Alexander O. |last2=Voyta |first2=Leonid L. |date=March 2024 |title=Putative Triassic stem mammal Tikitherium copei is a Neogene shrew |url=https://link.springer.com/10.1007/s10914-024-09703-w |journal=Journal of Mammalian Evolution |language=en |volume=31 |issue=1 |doi=10.1007/s10914-024-09703-w |issn=1064-7554|url-access=subscription }}

File:OrthoMaM v10b 2019 116genera circular tree.svg.{{cite journal | vauthors = Scornavacca C, Belkhir K, Lopez J, Dernat R, Delsuc F, Douzery EJ, Ranwez V | title = OrthoMaM v10: Scaling-up orthologous coding sequence and exon alignments with more than one hundred mammalian genomes | journal = Molecular Biology and Evolution | volume = 36 | issue = 4 | pages = 861–862 | date = April 2019 | pmid = 30698751 | pmc = 6445298 | doi = 10.1093/molbev/msz015 }} The major clades are coloured: marsupials (magenta), xenarthrans (orange), afrotherians (red), laurasiatherians (green), and euarchontoglirans (blue).]]

As of the early 21st century, molecular studies based on DNA analysis have suggested new relationships among mammal families. Most of these findings have been independently validated by retrotransposon presence/absence data.{{cite journal | vauthors = Kriegs JO, Churakov G, Kiefmann M, Jordan U, Brosius J, Schmitz J | title = Retroposed elements as archives for the evolutionary history of placental mammals | journal = PLOS Biology | volume = 4 | issue = 4 | pages = e91 | date = April 2006 | pmid = 16515367 | pmc = 1395351 | doi = 10.1371/journal.pbio.0040091 | doi-access = free }} Classification systems based on molecular studies reveal three major groups or lineages of placentals—Afrotheria, Xenarthra and Boreoeutheria—which diverged in the Cretaceous. The relationships between these three lineages is contentious, and all three possible hypotheses have been proposed with respect to which group is basal. These hypotheses are Atlantogenata (basal Boreoeutheria), Epitheria (basal Xenarthra) and Exafroplacentalia (basal Afrotheria).{{cite journal | vauthors = Nishihara H, Maruyama S, Okada N | title = Retroposon analysis and recent geological data suggest near-simultaneous divergence of the three superorders of mammals | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 106 | issue = 13 | pages = 5235–5240 | date = March 2009 | pmid = 19286970 | pmc = 2655268 | doi = 10.1073/pnas.0809297106 | bibcode = 2009PNAS..106.5235N | doi-access = free }} Boreoeutheria in turn contains two major lineages—Euarchontoglires and Laurasiatheria.

Estimates for the divergence times between these three placental groups range from 105 to 120 million years ago, depending on the type of DNA used (such as nuclear or mitochondrial){{cite journal | vauthors = Springer MS, Murphy WJ, Eizirik E, O'Brien SJ | title = Placental mammal diversification and the Cretaceous–Tertiary boundary | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 3 | pages = 1056–1061 | date = February 2003 | pmid = 12552136 | pmc = 298725 | doi = 10.1073/pnas.0334222100 | bibcode = 2003PNAS..100.1056S | doi-access = free }} and varying interpretations of paleogeographic data.

{{clear}}

Mammal phylogeny according to Álvarez-Carretero et al., 2022:{{cite journal |vauthors=Álvarez-Carretero S, Tamuri AU, Battini M, Nascimento FF, Carlisle E, Asher RJ, Yang Z, Donoghue PC, dos Reis M |display-authors=8 |title=A species-level timeline of mammal evolution integrating phylogenomic data |journal=Nature |volume= 602|issue=7896 |pages=263–267 |date=2022 |pmid= 34937052|pmc= |doi=10.1038/s41586-021-04341-1|bibcode=2022Natur.602..263A |hdl=1983/de841853-d57b-40d9-876f-9bfcf7253f12 |s2cid=245438816 |url=https://qmro.qmul.ac.uk/xmlui/handle/123456789/75979 |hdl-access=free }}

{{Clade | style=font-size:90%;line-height:70%

|label1=Mammalia

|1={{Clade

|label1=Yinotheria

|1=Monotremata File:Genera mammalium Ornithorhynchus anatinus.jpg

|label2=Theria

|2={{Clade

|label1=Marsupialia

|1={{Clade

|1=Paucituberculata 50 px

|2={{Clade

|1=Didelphimorphia 50 px

|label2=Australidelphia

|2={{Clade

|1=Microbiotheria

|label2=Eomarsupialia

|2={{Clade

|label1=Agreodontia

|1={{Clade

|1=Notoryctemorphia 50 px

|2={{Clade

|1=Peramelemorphia 50 px

|2=Dasyuromorphia 50 px

}}

}}

|2=Diprotodontia File:A monograph of the Macropodidæ, or family of kangaroos (9398404841) white background.jpg

}}

}}

}}

}}

|label2=Placentalia

|2={{Clade

|label1=Atlantogenata

|1={{Clade

|label1=Xenarthra

|1={{Clade

|1=Cingulata File:Nine-banded-Armadillo white background.jpg

|2=Pilosa File:Natural history of the animal kingdom for the use of young people (Plate XV) (Myrmecophaga tridactyla).jpg

}}

|label2=Afrotheria

|2={{Clade

|label1=Paenungulata

|1={{Clade

|1=Hyracoidea 50 px

|label2=Tethytheria

|2={{Clade

|1=Sirenia File:Manatee white background.jpg

|2=Proboscidea File:Indian elephant white background.jpg

}}

}}

|label2=Afroinsectiphilia

|2={{Clade

|1=Tubulidentata 50 px

|label2=Afroinsectivora

|2={{Clade

|1=Macroscelidea 50 px

|2=Afrosoricida 50 px

}}

}}

}}

}}

|label2=Boreoeutheria

|2={{Clade

|label1=Laurasiatheria

|1={{Clade

|1=Eulipotyphla File:Mole white background.jpg

|label2=Scrotifera

|2={{Clade

|1=Chiroptera File:Vampire bat white background.jpg

|label2=Ferungulata

|2={{Clade

|1=Pholidota File:FMIB 46859 Pangolin a grosse queue white background.jpeg

|2={{Clade

|1=Carnivora File:Cynailurus guttata - 1818-1842 - Print - Iconographia Zoologica - Special Collections University of Amsterdam - (white background).jpg

|label2=Euungulata

|2={{Clade

|1=Perissodactyla File:Equus quagga (white background).jpg

|2=Artiodactyla File:Walia ibex illustration white background.png

}}

}}

}}

}}

}}

|label2=Euarchontoglires

|2={{Clade

|1={{Clade

|1=Scandentia 50 px

|label2=Glires

|2={{Clade

|1=Lagomorpha File:Bruno Liljefors - Hare studies 1885 white background.jpg

|2=Rodentia File:Ruskea rotta.png

}}

}}

|2={{Clade

|label1=Primatomorpha

|1={{Clade

|1=Dermoptera 50 px

|2=Primates File:Die Säugthiere in Abbildungen nach der Natur, mit Beschreibungen (Plate 8) (white background).jpg

}}

}}

}}

}}

}}

}}

}}

}}

{{Main|Evolution of mammals}}

Synapsida, a clade that contains mammals and their extinct relatives, originated during the Pennsylvanian subperiod (~323 million to ~300 million years ago), when they split from the reptile lineage. Crown group mammals evolved from earlier mammaliaforms during the Early Jurassic. The cladogram takes Mammalia to be the crown group.{{cite journal | vauthors = Meng J, Wang Y, Li C | title = Transitional mammalian middle ear from a new Cretaceous Jehol eutriconodont | journal = Nature | volume = 472 | issue = 7342 | pages = 181–185 | date = April 2011 | pmid = 21490668 | doi = 10.1038/nature09921 | bibcode = 2011Natur.472..181M | s2cid = 4428972 }}

{{clade

|label1=Mammaliaformes

|1={{clade

|1=Morganucodontidae 50px

|2={{clade

|1=Docodonta 50px

|2={{clade

|1=Haldanodon

|label2=Mammalia

|2={{clade

|1=Australosphenida (incl. Monotremata) 50px

|2={{clade

|1=Fruitafossor

|2={{clade

|1={{clade

|1=Haramiyavia

|2=Multituberculata 50px

}}

|2=Tinodon

|3=Eutriconodonta (incl. Gobiconodonta) 50px

|4=Trechnotheria (incl. Theria) 50px

}}

}}

}}

}}

}}

}}

}}

File:Skull synapsida 1.png behind the orbitals, in a fairly low position on the skull (lower right in this image). This opening might have assisted in containing the jaw muscles of these organisms which could have increased their biting strength.]]

The first fully terrestrial vertebrates were amniotes. Like their amphibious early tetrapod predecessors, they had lungs and limbs. Amniotic eggs, however, have internal membranes that allow the developing embryo to breathe but keep water in. Hence, amniotes can lay eggs on dry land, while amphibians generally need to lay their eggs in water.

The first amniotes apparently arose in the Pennsylvanian subperiod of the Carboniferous. They descended from earlier reptiliomorph amphibious tetrapods,{{cite journal| vauthors = Ahlberg PE, Milner AR |date=April 1994| title=The Origin and Early Diversification of Tetrapods | journal= Nature | volume=368 | pages=507–514| doi=10.1038/368507a0| issue=6471|bibcode = 1994Natur.368..507A |s2cid=4369342}} which lived on land that was already inhabited by insects and other invertebrates as well as ferns, mosses and other plants. Within a few million years, two important amniote lineages became distinct: the synapsids, which would later include the common ancestor of the mammals; and the sauropsids, which now include turtles, lizards, snakes, crocodilians and dinosaurs (including birds).{{cite web | url = https://palaeos.com/Vertebrates/Units/190Reptilomorpha/190.400.html#Amniota | archive-url = https://web.archive.org/web/20101220194106/http://palaeos.com/Vertebrates/Units/190Reptilomorpha/190.400.html#Amniota | archive-date = 20 December 2010 | title = Amniota – Palaeos}} Synapsids have a single hole (temporal fenestra) low on each side of the skull. Primitive synapsids included the largest and fiercest animals of the early Permian such as Dimetrodon.{{cite web | url=https://palaeos.com/Vertebrates/Units/390Synapsida/390.000.html | archive-url=https://web.archive.org/web/20101220193822/http://palaeos.com/Vertebrates/Units/390Synapsida/390.000.html | archive-date=20 December 2010 | title=Synapsida overview – Palaeos}} Nonmammalian synapsids were traditionally—and incorrectly—called "mammal-like reptiles" or pelycosaurs; we now know they were neither reptiles nor part of reptile lineage.

Therapsids, a group of synapsids, evolved in the Middle Permian, about 265 million years ago, and became the dominant land vertebrates.{{cite journal | vauthors = Kemp TS | title = The origin and early radiation of the therapsid mammal-like reptiles: a palaeobiological hypothesis | journal = Journal of Evolutionary Biology | volume = 19 | issue = 4 | pages = 1231–1247 | date = July 2006 | pmid = 16780524 | doi = 10.1111/j.1420-9101.2005.01076.x | s2cid = 3184629 | url = https://users.ox.ac.uk/~tskemp/pdfs/jeb2006.pdf | access-date = 14 January 2012 | archive-date = 8 March 2021 | archive-url = https://web.archive.org/web/20210308061139/http://users.ox.ac.uk/~tskemp/pdfs/jeb2006.pdf | url-status = dead }} They differ from basal eupelycosaurs in several features of the skull and jaws, including: larger skulls and incisors which are equal in size in therapsids, but not for eupelycosaurs. The therapsid lineage leading to mammals went through a series of stages, beginning with animals that were very similar to their early synapsid ancestors and ending with probainognathian cynodonts, some of which could easily be mistaken for mammals. Those stages were characterised by:{{cite book | vauthors = Kermack DM, Kermack KA | title=The evolution of mammalian characters|location=Washington, DC | publisher=Croom Helm | year=1984 | isbn=978-0-7099-1534-8 |oclc=10710687}}

• The gradual development of a bony secondary palate.
• Abrupt acquisition of endothermy among Mammaliamorpha, thus prior to the origin of mammals by 30–50 millions of years {{cite journal |last1=Araújo |title=Inner ear biomechanics reveals a Late Triassic origin for mammalian endothermy |journal=Nature |volume=607 |date= 28 July 2022 |issue=7920 |pages=726–731 |doi=10.1038/s41586-022-04963-z |pmid=35859179 |bibcode=2022Natur.607..726A |s2cid=236245230 |display-authors=etal}}.
• Progression towards an erect limb posture, which would increase the animals' stamina by avoiding Carrier's constraint. But this process was slow and erratic: for example, all herbivorous nonmammaliaform therapsids retained sprawling limbs (some late forms may have had semierect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semisprawling hindlimbs. In fact, modern monotremes still have semisprawling limbs.
• The dentary gradually became the main bone of the lower jaw which, by the Triassic, progressed towards the fully mammalian jaw (the lower consisting only of the dentary) and middle ear (which is constructed by the bones that were previously used to construct the jaws of reptiles).

The Permian–Triassic extinction event about 252 million years ago, which was a prolonged event due to the accumulation of several extinction pulses, ended the dominance of carnivorous therapsids.{{cite journal| vauthors = Tanner LH, Lucas SG, Chapman MG |title=Assessing the record and causes of Late Triassic extinctions |journal=Earth-Science Reviews |volume=65 |issue=1–2 |pages=103–139 |year=2004 |doi=10.1016/S0012-8252(03)00082-5 |url=https://nmnaturalhistory.org/pdf_files/TJB.pdf |bibcode=2004ESRv...65..103T |url-status=dead |archive-url=https://web.archive.org/web/20071025225841/http://nmnaturalhistory.org/pdf_files/TJB.pdf |archive-date=25 October 2007 }} In the early Triassic, most medium to large land carnivore niches were taken over by archosaurs{{cite journal | vauthors = Brusatte SL, Benton MJ, Ruta M, Lloyd GT | title = Superiority, competition, and opportunism in the evolutionary radiation of dinosaurs | journal = Science | volume = 321 | issue = 5895 | pages = 1485–1488 | date = September 2008 | pmid = 18787166 | doi = 10.1126/science.1161833 | bibcode = 2008Sci...321.1485B | hdl = 20.500.11820/00556baf-6575-44d9-af39-bdd0b072ad2b | s2cid = 13393888 | url = https://www.pure.ed.ac.uk/ws/files/8232088/PDF_Brusatteetal2008SuperiorityCompetition.pdf | access-date = 12 October 2019 | archive-date = 19 July 2018 | archive-url = https://web.archive.org/web/20180719005836/https://www.pure.ed.ac.uk/ws/files/8232088/PDF_Brusatteetal2008SuperiorityCompetition.pdf | url-status = live }} which, over an extended period (35 million years), came to include the crocodylomorphs,{{cite book | vauthors = Gauthier JA |year=1986 |chapter=Saurischian monophyly and the origin of birds |title=The Origin of Birds and the Evolution of Flight. Memoirs of the California Academy of Sciences | veditors = Padian K |volume=8 |publisher=California Academy of Sciences |location=San Francisco |pages=1–55}} the pterosaurs and the dinosaurs;{{cite journal| vauthors = Sereno PC |year=1991|title=Basal archosaurs: phylogenetic relationships and functional implications|journal=Memoirs of the Society of Vertebrate Paleontology|volume=2|pages=1–53|doi=10.2307/3889336|jstor=3889336}} however, large cynodonts like Trucidocynodon and traversodontids still occupied large sized carnivorous and herbivorous niches respectively. By the Jurassic, the dinosaurs had come to dominate the large terrestrial herbivore niches as well.{{cite journal| vauthors = MacLeod N, Rawson PF, Forey PL, Banner FT, Boudagher-Fadel MK, Bown PR, Burnett JA, Chambers P, Culver S, Evans SE, Jeffery C | display-authors = 6 |title=The Cretaceous–Tertiary biotic transition|year=1997|journal=Journal of the Geological Society|volume=154|issue=2|pages=265–292|doi=10.1144/gsjgs.154.2.0265|bibcode=1997JGSoc.154..265M | s2cid = 129654916 }}

The first mammals (in Kemp's sense) appeared in the Late Triassic epoch (about 225 million years ago), 40 million years after the first therapsids. They expanded out of their nocturnal insectivore niche from the mid-Jurassic onwards;{{cite book|url={{Google books|plainurl=yes|id=APWwBAAAQBAJ|page=73}}| vauthors = Hunt DM, Hankins MW, Collin SP, Marshall NJ |title=Evolution of Visual and Non-visual Pigments|year= 2014|location=London|publisher=Springer|page=73|isbn=978-1-4614-4354-4|oclc=892735337}} the Jurassic Castorocauda, for example, was a close relative of true mammals that had adaptations for swimming, digging and catching fish.{{cite web | url=https://news.nationalgeographic.com/news/2006/02/0223_060223_beaver.html| archive-url=https://web.archive.org/web/20060303071809/http://news.nationalgeographic.com/news/2006/02/0223_060223_beaver.html| url-status=dead| archive-date=3 March 2006| vauthors = Bakalar N |year=2006| title=Jurassic "Beaver" Found; Rewrites History of Mammals|access-date=28 May 2016|work=National Geographic News}} Most, if not all, are thought to have remained nocturnal (the nocturnal bottleneck), accounting for much of the typical mammalian traits.{{cite journal | vauthors = Hall MI, Kamilar JM, Kirk EC | title = Eye shape and the nocturnal bottleneck of mammals | journal = Proceedings of the Royal Society B: Biological Sciences| volume = 279 | issue = 1749 | pages = 4962–4968 | date = December 2012 | pmid = 23097513 | pmc = 3497252 | doi = 10.1098/rspb.2012.2258 }} The majority of the mammal species that existed in the Mesozoic Era were multituberculates, eutriconodonts and spalacotheriids.{{cite journal | vauthors = Luo ZX | title = Transformation and diversification in early mammal evolution | journal = Nature | volume = 450 | issue = 7172 | pages = 1011–1019 | date = December 2007 | pmid = 18075580 | doi = 10.1038/nature06277 | bibcode = 2007Natur.450.1011L | s2cid = 4317817 }} The earliest-known fossil of the Metatheria ("changed beasts") is Sinodelphys, found in 125-million-year-old Early Cretaceous shale in China's northeastern Liaoning Province. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues.{{cite web | url=https://news.nationalgeographic.com/news/2003/12/1215_031215_oldestmarsupial.html | archive-url=https://web.archive.org/web/20031217024049/http://news.nationalgeographic.com/news/2003/12/1215_031215_oldestmarsupial.html | url-status=dead | archive-date=17 December 2003 | vauthors = Pickrell J |year=2003| title=Oldest Marsupial Fossil Found in China| publisher=National Geographic News|access-date=28 May 2016}}

File:Juramaia NT.jpg, the oldest-known eutherian (160 mya)]]

The oldest-known fossil among the Eutheria ("true beasts") is the small shrewlike Juramaia sinensis, or "Jurassic mother from China", dated to 160 million years ago in the late Jurassic.{{cite journal | vauthors = Luo ZX, Yuan CX, Meng QJ, Ji Q | title = A Jurassic eutherian mammal and divergence of marsupials and placentals | journal = Nature | volume = 476 | issue = 7361 | pages = 442–5 | date = August 2011 | pmid = 21866158 | doi = 10.1038/nature10291 | bibcode = 2011Natur.476..442L | s2cid = 205225806 }} A later eutherian relative, Eomaia, dated to 125 million years ago in the early Cretaceous, possessed some features in common with the marsupials but not with the placentals, evidence that these features were present in the last common ancestor of the two groups but were later lost in the placental lineage.{{cite journal | vauthors = Ji Q, Luo ZX, Yuan CX, Wible JR, Zhang JP, Georgi JA | title = The earliest known eutherian mammal | journal = Nature | volume = 416 | issue = 6883 | pages = 816–822 | date = April 2002 | pmid = 11976675 | doi = 10.1038/416816a | bibcode = 2002Natur.416..816J | s2cid = 4330626 }} In particular, the epipubic bones extend forwards from the pelvis. These are not found in any modern placental, but they are found in marsupials, monotremes, other nontherian mammals and Ukhaatherium, an early Cretaceous animal in the eutherian order Asioryctitheria. This also applies to the multituberculates.{{cite journal | vauthors = Novacek MJ, Rougier GW, Wible JR, McKenna MC, Dashzeveg D, Horovitz I | title = Epipubic bones in eutherian mammals from the late Cretaceous of Mongolia | journal = Nature | volume = 389 | issue = 6650 | pages = 483–486 | date = October 1997 | pmid = 9333234 | doi = 10.1038/39020 | bibcode = 1997Natur.389..483N | s2cid = 205026882 }} They are apparently an ancestral feature, which subsequently disappeared in the placental lineage. These epipubic bones seem to function by stiffening the muscles during locomotion, reducing the amount of space being presented, which placentals require to contain their fetus during gestation periods. A narrow pelvic outlet indicates that the young were very small at birth and therefore pregnancy was short, as in modern marsupials. This suggests that the placenta was a later development.{{cite book|chapter-url={{Google books|plainurl=yes|id=G1exWxU3QHIC|page=68}} | vauthors = Power ML, Schulkin J |year=2012|title=Evolution of the Human Placenta|publisher=Johns Hopkins University Press|location=Baltimore| isbn=978-1-4214-0643-5|page=68|chapter=Evolution of Live Birth in Mammals}}

One of the earliest-known monotremes was Teinolophos, which lived about 120 million years ago in Australia.{{cite journal | vauthors = Rowe T, Rich TH, Vickers-Rich P, Springer M, Woodburne MO | title = The oldest platypus and its bearing on divergence timing of the platypus and echidna clades | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 105 | issue = 4 | pages = 1238–1242 | date = January 2008 | pmid = 18216270 | pmc = 2234122 | doi = 10.1073/pnas.0706385105 | bibcode = 2008PNAS..105.1238R | doi-access = free }} Monotremes have some features which may be inherited from the original amniotes such as the same orifice to urinate, defecate and reproduce (cloaca)—as reptiles and birds also do—{{cite book|chapter-url={{Google books|plainurl=yes|id=FASJWgDhxIsC|page=55}}| vauthors = Grant T |year=1995|title=The Platypus: A Unique Mammal|chapter=Reproduction|publisher=University of New South Wales|location= Sydney|page=55|isbn=978-0-86840-143-0|oclc=33842474}} and they lay eggs which are leathery and uncalcified.{{cite journal | vauthors = Goldman AS | title = Evolution of immune functions of the mammary gland and protection of the infant | journal = Breastfeeding Medicine | volume = 7 | issue = 3 | pages = 132–142 | date = June 2012 | pmid = 22577734 | doi = 10.1089/bfm.2012.0025 }}

Hadrocodium, whose fossils date from approximately 195 million years ago, in the early Jurassic, provides the first clear evidence of a jaw joint formed solely by the squamosal and dentary bones; there is no space in the jaw for the articular, a bone involved in the jaws of all early synapsids.{{cite book|url={{Google books|plainurl=yes|id=lyGqD_GWQ7oC&|page=82}}| vauthors = Rose KD |year=2006|title=The Beginning of the Age of Mammals|location=Baltimore|publisher=Johns Hopkins University Press|pages=82–83|isbn=978-0-8018-8472-6|oclc=646769601}}

File:Thrinaxodon Lionhinus.jpg at the National Museum of Natural History]]

The earliest clear evidence of hair or fur is in fossils of Castorocauda and Megaconus, from 164 million years ago in the mid-Jurassic. In the 1950s, it was suggested that the foramina (passages) in the maxillae and premaxillae (bones in the front of the upper jaw) of cynodonts were channels which supplied blood vessels and nerves to vibrissae (whiskers) and so were evidence of hair or fur;{{cite journal | vauthors = Brink AS | title=A study on the skeleton of Diademodon | journal=Palaeontologia Africana | volume=3 | pages=3–39 |year=1955 }}{{cite book | vauthors = Kemp TS | title=Mammal-like reptiles and the origin of mammals | publisher=Academic Press | year=1982 | location=London | page=363 | isbn=978-0-12-404120-2|oclc=8613180}} it was soon pointed out, however, that foramina do not necessarily show that an animal had vibrissae, as the modern lizard Tupinambis has foramina that are almost identical to those found in the nonmammalian cynodont Thrinaxodon.{{cite book| vauthors = Bennett AF, Ruben JA |year=1986|chapter=The metabolic and thermoregulatory status of therapsids|pages=207–218| veditors = Hotton III N, MacLean JJ, Roth J, Roth EC |title=The ecology and biology of mammal-like reptiles|publisher=Smithsonian Institution Press|location=Washington, DC|isbn=978-0-87474-524-5}}{{cite journal | vauthors = Estes R | title=Cranial anatomy of the cynodont reptile Thrinaxodon liorhinus | journal=Bulletin of the Museum of Comparative Zoology | pages=165–180 |year=1961|issue=1253 }} Popular sources, nevertheless, continue to attribute whiskers to Thrinaxodon.{{cite web |url=https://news.nationalgeographic.com/news/2009/02/photogalleries/darwin-birthday-evolution/#/thrinaxodon-missing-link_7787_600x450.jpg |archive-url=https://web.archive.org/web/20090214205210/http://news.nationalgeographic.com/news/2009/02/photogalleries/darwin-birthday-evolution/#/thrinaxodon-missing-link_7787_600x450.jpg |url-status=dead |archive-date=14 February 2009 |title=Thrinaxodon: The Emerging Mammal |date=11 February 2009 |publisher=National Geographic Daily News |access-date=26 August 2012}} Studies on Permian coprolites suggest that non-mammalian synapsids of the epoch already had fur, setting the evolution of hairs possibly as far back as dicynodonts.{{cite journal |title= Microbiota and food residues including possible evidence of pre-mammalian hair in Upper Permian coprolites from Russia | vauthors = Bajdek P, Qvarnström M, Owocki K, Sulej T, Sennikov AG, Golubev VK, Niedźwiedzki G |year=2015 |journal=Lethaia |volume=49 |issue=4 |pages=455–477 |doi=10.1111/let.12156 }}

When endothermy first appeared in the evolution of mammals is uncertain, though it is generally agreed to have first evolved in non-mammalian therapsids.{{cite journal | vauthors = Botha-Brink J, Angielczyk KD |title=Do extraordinarily high growth rates in Permo–Triassic dicynodonts (Therapsida, Anomodontia) explain their success before and after the end-Permian extinction? |year=2010 |doi= 10.1111/j.1096-3642.2009.00601.x |volume=160 |issue=2 |pages=341–365 |journal=Zoological Journal of the Linnean Society |doi-access=free}} Modern monotremes have lower body temperatures and more variable metabolic rates than marsupials and placentals,{{cite book | vauthors = Paul GS | title=Predatory Dinosaurs of the World | publisher=Simon and Schuster | year=1988 | location=New York | page=[https://archive.org/details/predatorydinosau00paul/page/464 464] | isbn=978-0-671-61946-6 | oclc=18350868 | url=https://archive.org/details/predatorydinosau00paul/page/464 }} but there is evidence that some of their ancestors, perhaps including ancestors of the therians, may have had body temperatures like those of modern therians.{{Cite journal|journal=Australian Journal of Zoology|title=Monotreme Cell-Cycles and the Evolution of Homeothermy| vauthors = Watson JM, Graves JA |volume=36|issue=5|pages=573–584|year=1988| doi = 10.1071/ZO9880573}} Likewise, some modern therians like afrotheres and xenarthrans have secondarily developed lower body temperatures.{{cite journal| vauthors = McNab BK |year=1980|title=Energetics and the limits to the temperate distribution in armadillos|journal=Journal of Mammalogy|volume=61|issue=4|pages=606–627|doi=10.2307/1380307|jstor=1380307}}

The evolution of erect limbs in mammals is incomplete—living and fossil monotremes have sprawling limbs. The parasagittal (nonsprawling) limb posture appeared sometime in the late Jurassic or early Cretaceous; it is found in the eutherian Eomaia and the metatherian Sinodelphys, both dated to 125 million years ago.{{cite journal | vauthors=Kielan-Jaworowska Z, Hurum JH | title=Limb posture in early mammals: Sprawling or parasagittal | journal=Acta Palaeontologica Polonica | volume=51 | issue=3 | pages=10237–10239 | year=2006 | url=https://app.pan.pl/archive/published/app51/app51-393.pdf | access-date=25 January 2024 | archive-date=25 January 2024 | archive-url=https://web.archive.org/web/20240125191351/https://app.pan.pl/archive/published/app51/app51-393.pdf | url-status=live }} Epipubic bones, a feature that strongly influenced the reproduction of most mammal clades, are first found in Tritylodontidae, suggesting that it is a synapomorphy between them and Mammaliaformes. They are omnipresent in non-placental Mammaliaformes, though Megazostrodon and Erythrotherium appear to have lacked them.{{cite book| vauthors = Lillegraven JA, Kielan-Jaworowska Z, Clemens WA |title=Mesozoic Mammals: The First Two-Thirds of Mammalian History |publisher=University of California Press |year=1979 |page=321 |isbn=978-0-520-03951-3 |oclc=5910695}}

It has been suggested that the original function of lactation (milk production) was to keep eggs moist. Much of the argument is based on monotremes, the egg-laying mammals.{{cite journal | vauthors = Oftedal OT | title = The mammary gland and its origin during synapsid evolution | journal = Journal of Mammary Gland Biology and Neoplasia | volume = 7 | issue = 3 | pages = 225–252 | date = July 2002 | pmid = 12751889 | doi = 10.1023/A:1022896515287 | s2cid = 25806501 }}

{{cite journal | vauthors = Oftedal OT | title = The origin of lactation as a water source for parchment-shelled eggs | journal = Journal of Mammary Gland Biology and Neoplasia | volume = 7 | issue = 3 | pages = 253–266 | date = July 2002 | pmid = 12751890 | doi = 10.1023/A:1022848632125 | s2cid = 8319185 }} In human females, mammary glands become fully developed during puberty, regardless of pregnancy.{{cite web|url = https://www.texaschildrens.org/health/breast-development |title = Breast Development | work = Texas Children's Hospital |access-date =13 January 2021|archive-url=https://web.archive.org/web/20210113081725/https://www.texaschildrens.org/health/breast-development|archive-date=13 January 2021}}

File:Hyaenodon horridus, Niobrara County, Wyoming, USA, Late Oligocene - Royal Ontario Museum - DSC00114.JPG horridus at the Royal Ontario Museum. The genus Hyaenodon was among the most successful mammals of the late Eocene-early Miocene epochs spanning for most of the Paleogene and some of the Neogene periods, undergoing many endemic radiations in North America, Europe, and Asia.{{cite journal|last1=Pfaff|first1=Cathrin|last2=Nagel|first2=Doris|last3=Gunnell|first3=Gregg|last4=Weber|first4=Gerhard W.|last5=Kriwet|first5=Jürgen|last6=Morlo|first6=Michael|last7=Bastl|first7=Katharina|year=2017|title=Palaeobiology of Hyaenodon exiguus (Hyaenodonta, Mammalia) based on morphometric analysis of the bony labyrinth|journal=Journal of Anatomy|volume=230|issue=2|pages=282–289|doi=10.1111/joa.12545|pmid=27666133 |pmc=5244453 }}]]

Therians took over the medium- to large-sized ecological niches in the Cenozoic, after the Cretaceous–Paleogene extinction event approximately 66 million years ago emptied ecological space once filled by non-avian dinosaurs and other groups of reptiles, as well as various other mammal groups,{{cite journal | vauthors = Sahney S, Benton MJ, Ferry PA | title = Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land | journal = Biology Letters | volume = 6 | issue = 4 | pages = 544–547 | date = August 2010 | pmid = 20106856 | pmc = 2936204 | doi = 10.1098/rsbl.2009.1024 }} and underwent an exponential increase in body size (megafauna).{{cite journal | vauthors = Smith FA, Boyer AG, Brown JH, Costa DP, Dayan T, Ernest SK, Evans AR, Fortelius M, Gittleman JL, Hamilton MJ, Harding LE, Lintulaakso K, Lyons SK, McCain C, Okie JG, Saarinen JJ, Sibly RM, Stephens PR, Theodor J, Uhen MD | display-authors = 6 | title = The evolution of maximum body size of terrestrial mammals | journal = Science | volume = 330 | issue = 6008 | pages = 1216–1219 | date = November 2010 | pmid = 21109666 | doi = 10.1126/science.1194830 | citeseerx = 10.1.1.383.8581 | bibcode = 2010Sci...330.1216S | s2cid = 17272200 }} The increase in mammalian diversity was not, however, solely because of expansion into large-bodied niches.{{Cite journal |last1=Benevento |first1=Gemma Louise |last2=Benson |first2=Roger B. J. |last3=Close |first3=Roger A. |last4=Butler |first4=Richard J. |date=16 June 2023 |title=Early Cenozoic increases in mammal diversity cannot be explained solely by expansion into larger body sizes |journal=Palaeontology |language=en |volume=66 |issue=3 |page=12653 |doi=10.1111/pala.12653 |issn=0031-0239 |doi-access=free |bibcode=2023Palgy..6612653B }} Mammals diversified very quickly, displaying an exponential rise in diversity. For example, the earliest-known bat dates from about 50 million years ago, only 16 million years after the extinction of the non-avian dinosaurs.{{cite journal | vauthors = Simmons NB, Seymour KL, Habersetzer J, Gunnell GF | title = Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation | journal = Nature | volume = 451 | issue = 7180 | pages = 818–821 | date = February 2008 | pmid = 18270539 | doi = 10.1038/nature06549 | url = https://www.nature.com/articles/nature06549.epdf?referrer_access_token=vvnCZKFsJpI9PyHTMCW0AtRgN0jAjWel9jnR3ZoTv0OIjnS1VOVvLEmlM3pVKicSGILcK5-gC0KUTwfjLtGsWX-Jl3sk3aQbBWjeluiuMfZh8gMqZJ4qV9dfir4OcYHZFaqbm8GbWK-9JqFbcMEjN3G_-d7t8hJWLm5RDaih5vZhy47BSgJOQDuz8aRMBtHCvZEWV7affpdxsSefk_6x80U5fE1N1SjLKUHVdainKEM%3D&tracking_referrer=arstechnica.com | bibcode = 2008Natur.451..818S | hdl = 2027.42/62816 | s2cid = 4356708 | hdl-access = free | access-date = 25 January 2024 | archive-date = 25 January 2024 | archive-url = https://web.archive.org/web/20240125191352/https://www.nature.com/articles/nature06549.epdf?referrer_access_token=vvnCZKFsJpI9PyHTMCW0AtRgN0jAjWel9jnR3ZoTv0OIjnS1VOVvLEmlM3pVKicSGILcK5-gC0KUTwfjLtGsWX-Jl3sk3aQbBWjeluiuMfZh8gMqZJ4qV9dfir4OcYHZFaqbm8GbWK-9JqFbcMEjN3G_-d7t8hJWLm5RDaih5vZhy47BSgJOQDuz8aRMBtHCvZEWV7affpdxsSefk_6x80U5fE1N1SjLKUHVdainKEM%3D&tracking_referrer=arstechnica.com | url-status = live }}

Molecular phylogenetic studies initially suggested that most placental orders diverged about 100 to 85 million years ago and that modern families appeared in the period from the late Eocene through the Miocene.{{cite journal | vauthors = Bininda-Emonds OR, Cardillo M, Jones KE, MacPhee RD, Beck RM, Grenyer R, Price SA, Vos RA, Gittleman JL, Purvis A | display-authors = 6 | title = The delayed rise of present-day mammals | journal = Nature | volume = 446 | issue = 7135 | pages = 507–512 | date = March 2007 | pmid = 17392779 | doi = 10.1038/nature05634 | url = https://www.utheria.org/uploads/nature05634.pdf | bibcode = 2007Natur.446..507B | s2cid = 4314965 | access-date = 25 January 2024 | archive-date = 25 January 2024 | archive-url = https://web.archive.org/web/20240125191351/https://www.utheria.org/uploads/nature05634.pdf | url-status = live }} However, no placental fossils have been found from before the end of the Cretaceous. The earliest undisputed fossils of placentals come from the early Paleocene, after the extinction of the non-avian dinosaurs.{{cite journal | vauthors = Wible JR, Rougier GW, Novacek MJ, Asher RJ | title = Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary | journal = Nature | volume = 447 | issue = 7147 | pages = 1003–1006 | date = June 2007 | pmid = 17581585 | doi = 10.1038/nature05854 | bibcode = 2007Natur.447.1003W | s2cid = 4334424 }} (Scientists identified an early Paleocene animal named Protungulatum donnae as one of the first placental mammals,{{cite journal | vauthors = O'Leary MA, Bloch JI, Flynn JJ, Gaudin TJ, Giallombardo A, Giannini NP, Goldberg SL, Kraatz BP, Luo ZX, Meng J, Ni X, Novacek MJ, Perini FA, Randall ZS, Rougier GW, Sargis EJ, Silcox MT, Simmons NB, Spaulding M, Velazco PM, Weksler M, Wible JR, Cirranello AL | display-authors = 6 | title = The placental mammal ancestor and the post-K-Pg radiation of placentals | journal = Science | volume = 339 | issue = 6120 | pages = 662–667 | date = February 2013 | pmid = 23393258 | doi = 10.1126/science.1229237 | url = https://www.science.org/doi/abs/10.1126/science.1229237 | bibcode = 2013Sci...339..662O | hdl = 11336/7302 | s2cid = 206544776 | hdl-access = free | access-date = 30 June 2022 | archive-date = 10 November 2021 | archive-url = https://web.archive.org/web/20211110174432/https://www.science.org/doi/abs/10.1126/science.1229237 | url-status = live }} but it has since been reclassified as a non-placental eutherian.){{cite journal | vauthors = Halliday TJ, Upchurch P, Goswami A | title = Resolving the relationships of Paleocene placental mammals | journal = Biological Reviews of the Cambridge Philosophical Society | volume = 92 | issue = 1 | pages = 521–550 | date = February 2017 | pmid = 28075073 | pmc = 6849585 | doi = 10.1111/brv.12242 }} Recalibrations of genetic and morphological diversity rates have suggested a Late Cretaceous origin for placentals, and a Paleocene origin for most modern clades.{{cite journal | vauthors = Halliday TJ, Upchurch P, Goswami A | title = Eutherians experienced elevated evolutionary rates in the immediate aftermath of the Cretaceous-Palaeogene mass extinction | journal = Proceedings. Biological Sciences| volume = 283 | issue = 1833 | pages = 20153026 | date = June 2016 | pmid = 27358361 | pmc = 4936024 | doi = 10.1098/rspb.2015.3026 }}

The earliest-known ancestor of primates is Archicebus achilles{{cite journal | vauthors = Ni X, Gebo DL, Dagosto M, Meng J, Tafforeau P, Flynn JJ, Beard KC | title = The oldest known primate skeleton and early haplorhine evolution | journal = Nature | volume = 498 | issue = 7452 | pages = 60–64 | date = June 2013 | pmid = 23739424 | doi = 10.1038/nature12200 | bibcode = 2013Natur.498...60N | s2cid = 4321956 }} from around 55 million years ago. This tiny primate weighed 20–30 grams (0.7–1.1 ounce) and could fit within a human palm.

{{anchor|Anatomy and morphology}}

Living mammal species can be identified by the presence of sweat glands, including those that are specialised to produce milk to nourish their young.{{cite book | vauthors = Romer SA, Parsons TS |year=1977 |title=The Vertebrate Body |location=Philadelphia |publisher=Holt-Saunders International |pages=129–145 |isbn=978-0-03-910284-5 |oclc=60007175}} In classifying fossils, however, other features must be used, since soft tissue glands and many other features are not visible in fossils.{{cite book |url= {{Google books|plainurl=yes|id=kS-h84pMJw4C|page=593}} | vauthors = Purves WK, Sadava DE, Orians GH, Helle HC |year=2001 |title= Life: The Science of Biology|location=New York|edition=6th|publisher=Sinauer Associates, Inc. |page=593 |isbn=978-0-7167-3873-2 |oclc=874883911 }}

Many traits shared by all living mammals appeared among the earliest members of the group:

• Jaw joint – The dentary (the lower jaw bone, which carries the teeth) and the squamosal (a small cranial bone) meet to form the joint. In most gnathostomes, including early therapsids, the joint consists of the articular (a small bone at the back of the lower jaw) and quadrate (a small bone at the back of the upper jaw).
• Middle ear – In crown-group mammals, sound is carried from the eardrum by a chain of three bones, the malleus, the incus and the stapes. Ancestrally, the malleus and the incus are derived from the articular and the quadrate bones that constituted the jaw joint of early therapsids.{{cite journal | vauthors = Anthwal N, Joshi L, Tucker AS | title = Evolution of the mammalian middle ear and jaw: adaptations and novel structures | journal = Journal of Anatomy | volume = 222 | issue = 1 | pages = 147–160 | date = January 2013 | pmid = 22686855 | pmc = 3552421 | doi = 10.1111/j.1469-7580.2012.01526.x }}
• Tooth replacement – Teeth can be replaced once (diphyodonty) or (as in toothed whales and murid rodents) not at all (monophyodonty).{{cite journal | vauthors = van Nievelt AF, Smith KK |year=2005 |title=To replace or not to replace: the significance of reduced functional tooth replacement in marsupial and placental mammals |journal=Paleobiology |volume=31 |issue=2 |pages=324–346 |doi=10.1666/0094-8373(2005)031[0324:trontr]2.0.co;2|s2cid=37750062 }} Elephants, manatees, and kangaroos continually grow new teeth throughout their life (polyphyodonty).{{cite journal | vauthors = Libertini G, Ferrara N | title = Aging of perennial cells and organ parts according to the programmed aging paradigm | journal = Age | volume = 38 | issue = 2 | pages = 35 | date = April 2016 | pmid = 26957493 | pmc = 5005898 | doi = 10.1007/s11357-016-9895-0 }}
• Prismatic enamel – The enamel coating on the surface of a tooth consists of prisms, solid, rod-like structures extending from the dentin to the tooth's surface.{{cite journal | vauthors = Mao F, Wang Y, Meng J | title = A Systematic Study on Tooth Enamel Microstructures of Lambdopsalis bulla (Multituberculate, Mammalia) – Implications for Multituberculate Biology and Phylogeny | journal = PLOS ONE | volume = 10 | issue = 5 | pages = e0128243 | year = 2015 | pmid = 26020958 | pmc = 4447277 | doi = 10.1371/journal.pone.0128243 | bibcode = 2015PLoSO..1028243M | doi-access = free }}
• Occipital condyles – Two knobs at the base of the skull fit into the topmost neck vertebra; most other tetrapods, in contrast, have only one such knob.{{cite journal |jstor=2453526 | vauthors = Osborn HF |year=1900 |title=Origin of the Mammalia, III. Occipital Condyles of Reptilian Tripartite Type|journal=The American Naturalist|volume=34|number=408| pages=943–947 |doi=10.1086/277821|doi-access=free | bibcode = 1900ANat...34..943O }}

For the most part, these characteristics were not present in the Triassic ancestors of the mammals.{{cite journal| vauthors = Crompton AW, Jenkins Jr FA |year=1973|title=Mammals from Reptiles: A Review of Mammalian Origins|journal=Annual Review of Earth and Planetary Sciences|volume=1|pages=131–155|doi=10.1146/annurev.ea.01.050173.001023|bibcode=1973AREPS...1..131C}} Nearly all mammaliaforms possess an epipubic bone, the exception being modern placentals.

File:Aurochsfeatures.jpg, the extinct wild ancestor of cattle]]

On average, male mammals are larger than females, with males being at least 10% larger than females in over 45% of investigated species. Most mammalian orders also exhibit male-biased sexual dimorphism, although some orders do not show any bias or are significantly female-biased (Lagomorpha). Sexual size dimorphism increases with body size across mammals (Rensch's rule), suggesting that there are parallel selection pressures on both male and female size. Male-biased dimorphism relates to sexual selection on males through male–male competition for females, as there is a positive correlation between the degree of sexual selection, as indicated by mating systems, and the degree of male-biased size dimorphism. The degree of sexual selection is also positively correlated with male and female size across mammals. Further, parallel selection pressure on female mass is identified in that age at weaning is significantly higher in more polygynous species, even when correcting for body mass. Also, the reproductive rate is lower for larger females, indicating that fecundity selection selects for smaller females in mammals. Although these patterns hold across mammals as a whole, there is considerable variation across orders.{{cite book |vauthors=Lindenfors P, Gittleman JL, Jones KE |title=Sex, Size and Gender Roles: Evolutionary Studies of Sexual Size Dimorphism |chapter=Sexual size dimorphism in mammals |date=2007 |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-920878-4 |pages=16–26 |url=https://swepub.kb.se/bib/swepub:oai:DiVA.org:su-16290?tab2=abs&language=en |access-date=25 January 2024 |archive-date=25 January 2024 |archive-url=https://web.archive.org/web/20240125191352/https://swepub.kb.se/bib/swepub:oai:DiVA.org:su-16290?tab2=abs&language=en |url-status=live }}

{{Main|Biological system}}

The majority of mammals have seven cervical vertebrae (bones in the neck). The exceptions are the manatee and the two-toed sloth, which have six, and the three-toed sloth which has nine.{{cite book|url={{Google books|plainurl=yes|id=FIIgDk9i_GkC|page=154}} | vauthors = Dierauf LA, Gulland FM |title=CRC Handbook of Marine Mammal Medicine: Health, Disease, and Rehabilitation|location=Boca Raton|edition=2nd|publisher=CRC Press |year= 2001|page=154 |isbn=978-1-4200-4163-7|oclc=166505919}} All mammalian brains possess a neocortex, a brain region unique to mammals.{{cite journal | vauthors = Lui JH, Hansen DV, Kriegstein AR | title = Development and evolution of the human neocortex | journal = Cell | volume = 146 | issue = 1 | pages = 18–36 | date = July 2011 | pmid = 21729779 | pmc = 3610574 | doi = 10.1016/j.cell.2011.06.030 }} Placental brains have a corpus callosum, unlike monotremes and marsupials.{{cite journal | vauthors = Keeler CE | title = Absence of the Corpus Callosum as a Mendelizing Character in the House Mouse | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 19 | issue = 6 | pages = 609–611 | date = June 1933 | pmid = 16587795 | pmc = 1086100 | doi = 10.1073/pnas.19.6.609 | bibcode = 1933PNAS...19..609K | jstor = 86284 | doi-access = free }}

{{multiple image

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The mammalian heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers.{{cite book| vauthors = Standring S, Borley NR |year=2008|title=Gray's anatomy: the anatomical basis of clinical practice|edition=40th|location=London|publisher=Churchill Livingstone|pages=960–962|isbn=978-0-8089-2371-8|oclc=213447727}} The heart has four valves, which separate its chambers and ensures blood flows in the correct direction through the heart (preventing backflow). After gas exchange in the pulmonary capillaries (blood vessels in the lungs), oxygen-rich blood returns to the left atrium via one of the four pulmonary veins. Blood flows nearly continuously back into the atrium, which acts as the receiving chamber, and from here through an opening into the left ventricle. Most blood flows passively into the heart while both the atria and ventricles are relaxed, but toward the end of the ventricular relaxation period, the left atrium will contract, pumping blood into the ventricle. The heart also requires nutrients and oxygen found in blood like other muscles, and is supplied via coronary arteries.{{cite book|vauthors=Betts JF, Desaix P, Johnson E, Johnson JE, Korol O, Kruse D, Poe B, Wise JA, Womble M, Young KA|display-authors=6|title=Anatomy & physiology|year=2013|isbn=978-1-938168-13-0|oclc=898069394|url=https://cnx.org/content/m46676/latest/?collection=col11496/latest|location=Houston|publisher=Rice University Press|pages=787–846|access-date=25 January 2024|archive-date=23 February 2022|archive-url=https://web.archive.org/web/20220223063018/https://openstax.org/books/anatomy-and-physiology/pages/19-1-heart-anatomy|url-status=live}}

File:Lung expansion simulation with Raccoon.gif lungs being inflated manually]]

{{Main|Respiratory system#Mammals}}

The lungs of mammals are spongy and honeycombed. Breathing is mainly achieved with the diaphragm, which divides the thorax from the abdominal cavity, forming a dome convex to the thorax. Contraction of the diaphragm flattens the dome, increasing the volume of the lung cavity. Air enters through the oral and nasal cavities, and travels through the larynx, trachea and bronchi, and expands the alveoli. Relaxing the diaphragm has the opposite effect, decreasing the volume of the lung cavity, causing air to be pushed out of the lungs. During exercise, the abdominal wall contracts, increasing pressure on the diaphragm, which forces air out quicker and more forcefully. The rib cage is able to expand and contract the chest cavity through the action of other respiratory muscles. Consequently, air is sucked into or expelled out of the lungs, always moving down its pressure gradient.{{cite book| vauthors = Levitzky MG |title=Pulmonary physiology|date=2013|publisher=McGraw-Hill Medical|location=New York|isbn=978-0-07-179313-1|chapter=Mechanics of Breathing |edition=8th|oclc=940633137}} This type of lung is known as a bellows lung due to its resemblance to blacksmith bellows.{{cite book|chapter-url={{Google books|plainurl=yes|id=DoC48j2-LnkC|page=12}} |location=New Delhi| vauthors = Umesh KB |year=2011|title=Handbook of Mechanical Ventilation|chapter=Pulmonary Anatomy and Physiology|publisher=Jaypee Brothers Medical Publishing |page=12|isbn=978-93-80704-74-6|oclc=945076700}}

{{break}}

File:The skin of mammals.jpg, (2) epidermis, (3) sebaceous gland, (4) Arrector pili muscle, (5) dermis, (6) hair follicle, (7) sweat gland. Not labelled, the bottom layer: hypodermis, showing round adipocytes]]

The integumentary system (skin) is made up of three layers: the outermost epidermis, the dermis and the hypodermis. The epidermis is typically 10 to 30 cells thick; its main function is to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is 15 to 40 times thicker than the epidermis. The dermis is made up of many components, such as bony structures and blood vessels. The hypodermis is made up of adipose tissue, which stores lipids and provides cushioning and insulation. The thickness of this layer varies widely from species to species;{{rp|97}} marine mammals require a thick hypodermis (blubber) for insulation, and right whales have the thickest blubber at {{convert|20|in|cm}}.{{cite book| vauthors = Tinker SW |title=Whales of the World|year=1988|publisher=Brill Archive|isbn=978-0-935848-47-2|page=51|url={{Google books|plainurl=yes|id=ASIVAAAAIAAJ|page=51}}}} Although other animals have features such as whiskers, feathers, setae, or cilia that superficially resemble it, no animals other than mammals have hair. It is a definitive characteristic of the class, though some mammals have very little.{{cite book|url={{Google books|plainurl=yes|id=udCnKce9hfoC|page=97}}| vauthors = Feldhamer GA, Drickamer LC, Vessey SH, Merritt JF, Krajewski C |year=2007|title=Mammalogy: Adaptation, Diversity, Ecology|edition=3rd|location=Baltimore|publisher=Johns Hopkins University Press|isbn=978-0-8018-8695-9|oclc=124031907}}{{rp|61}}

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Herbivores have developed a diverse range of physical structures to facilitate the consumption of plant material. To break up intact plant tissues, mammals have developed teeth structures that reflect their feeding preferences. For instance, frugivores (animals that feed primarily on fruit) and herbivores that feed on soft foliage have low-crowned teeth specialised for grinding foliage and seeds. Grazing animals that tend to eat hard, silica-rich grasses, have high-crowned teeth, which are capable of grinding tough plant tissues and do not wear down as quickly as low-crowned teeth.{{cite book| vauthors = Romer AS |year=1959|title=The vertebrate story| url = https://archive.org/details/vertebratestory00rome | url-access = registration |publisher=University of Chicago Press|location=Chicago|isbn=978-0-226-72490-4|edition=4th}} Most carnivorous mammals have carnassial teeth (of varying length depending on diet), long canines and similar tooth replacement patterns.{{cite journal| vauthors = de Muizon C, Lange-Badré B |year=1997 |title=Carnivorous dental adaptations in tribosphenic mammals and phylogenetic reconstruction |journal=Lethaia |volume=30 |issue=4 |pages=353–366 |doi=10.1111/j.1502-3931.1997.tb00481.x |bibcode=1997Letha..30..353D }}

The stomach of even-toed ungulates (Artiodactyla) is divided into four sections: the rumen, the reticulum, the omasum and the abomasum (only ruminants have a rumen). After the plant material is consumed, it is mixed with saliva in the rumen and reticulum and separates into solid and liquid material. The solids lump together to form a bolus (or cud), and is regurgitated. When the bolus enters the mouth, the fluid is squeezed out with the tongue and swallowed again. Ingested food passes to the rumen and reticulum where cellulolytic microbes (bacteria, protozoa and fungi) produce cellulase, which is needed to break down the cellulose in plants.{{cite journal | vauthors = Langer P | title = Comparative anatomy of the stomach in mammalian herbivores | journal = Quarterly Journal of Experimental Physiology | volume = 69 | issue = 3 | pages = 615–625 | date = July 1984 | pmid = 6473699 | doi = 10.1113/expphysiol.1984.sp002848 | s2cid = 30816018 }} Perissodactyls, in contrast to the ruminants, store digested food that has left the stomach in an enlarged cecum, where it is fermented by bacteria.{{cite book|chapter-url={{Google books|plainurl=yes|id=Mqv4Lo1vpk4C|page=322}}| vauthors = Vaughan TA, Ryan JM, Czaplewski NJ |title=Mammalogy | name-list-style = vanc |edition=5th |publisher=Jones and Bartlett |year=2011 |page=322 |isbn=978-0-7637-6299-5 |chapter=Perissodactyla |oclc=437300511}} Carnivora have a simple stomach adapted to digest primarily meat, as compared to the elaborate digestive systems of herbivorous animals, which are necessary to break down tough, complex plant fibres. The cecum is either absent or short and simple, and the large intestine is not sacculated or much wider than the small intestine.{{cite book |url={{Google books|plainurl=yes|id=B3crAAAAYAAJ|page=496}}| vauthors = Flower WH, Lydekker R |author2-link=Richard Lydekker |year=1946 |title=An Introduction to the Study of Mammals Living and Extinct |publisher=Adam and Charles Black |location=London |page=496|isbn=978-1-110-76857-8}}

File:Glycerination of Bovine kidney.jpg

File:Image from page 702 of "Outlines of zoology" (1895) (20732795545).jpg of a male and female rabbit]]

The mammalian excretory system involves many components. Like most other land animals, mammals are ureotelic, and convert ammonia into urea, which is done by the liver as part of the urea cycle.{{cite book | vauthors = Sreekumar S |title=Basic Physiology|publisher=PHI Learning Pvt. Ltd.|year=2010|pages=180–181|isbn=978-81-203-4107-4|url ={{Google books|plainurl=yes|id=IxYR9wTeXQgC|page=180}}}} Bilirubin, a waste product derived from blood cells, is passed through bile and urine with the help of enzymes excreted by the liver.{{cite book| vauthors = Cheifetz AS |title=Oxford American Handbook of Gastroenterology and Hepatology|year=2010|publisher=Oxford University Press, US|location=Oxford|isbn=978-0-19-983012-1|page=165}} The passing of bilirubin via bile through the intestinal tract gives mammalian feces a distinctive brown coloration.{{cite book| vauthors = Kuntz E |title=Hepatology: Textbook and Atlas|year=2008|publisher=Springer|location=Germany|isbn=978-3-540-76838-8|page=38}} Distinctive features of the mammalian kidney include the presence of the renal pelvis and renal pyramids, and of a clearly distinguishable cortex and medulla, which is due to the presence of elongated loops of Henle. Only the mammalian kidney has a bean shape, although there are some exceptions, such as the multilobed reniculate kidneys of pinnipeds, cetaceans and bears.{{cite journal | vauthors = Ortiz RM | title = Osmoregulation in marine mammals | journal = The Journal of Experimental Biology | volume = 204 | issue = Pt 11 | pages = 1831–1844 | date = June 2001 | doi = 10.1242/jeb.204.11.1831 | pmid = 11441026 | url = https://jeb.biologists.org/cgi/content/short/204/11/1831 | doi-access = free | bibcode = 2001JExpB.204.1831O | access-date = 25 January 2024 | archive-date = 25 January 2024 | archive-url = https://web.archive.org/web/20240125191351/https://jeb.biologists.org/cgi/content/short/204/11/1831 | url-status = live }} Most adult placentals have no remaining trace of the cloaca. In the embryo, the embryonic cloaca divides into a posterior region that becomes part of the anus, and an anterior region that has different fates depending on the sex of the individual: in females, it develops into the vestibule or urogenital sinus that receives the urethra and vagina, while in males it forms the entirety of the penile urethra.{{cite book|last=Linzey|first=Donald W.|title=Vertebrate Biology: Systematics, Taxonomy, Natural History, and Conservation|publisher=Johns Hopkins University Press|year=2020|page=306|isbn=978-1-42143-733-0|url=https://books.google.com/books?id=Rur4DwAAQBAJ&pg=PA306|access-date=22 January 2024|archive-date=22 January 2024|archive-url=https://web.archive.org/web/20240122190826/https://books.google.com/books?id=Rur4DwAAQBAJ&pg=PA306|url-status=live}} However, the afrosoricids and some shrews retain a cloaca as adults.{{Cite journal |url=http://journals.cambridge.org/action//displayFulltext?type=6&fid=290402&jid=BRE&volumeId=80&issueId=01&aid=275550&fulltextType=RV&fileId=S1464793104006566 |title=Biological Reviews – Cambridge Journals |journal=Biological Reviews |date=February 2005 |volume=80 |issue=1 |pages=93–128 |doi=10.1017/S1464793104006566 |access-date=21 January 2017 |archive-date=22 November 2015 |archive-url=https://web.archive.org/web/20151122104221/http://journals.cambridge.org/action//displayFulltext?type=6&fid=290402&jid=BRE&volumeId=80&issueId=01&aid=275550&fulltextType=RV&fileId=S1464793104006566 |url-status=live |last1=Symonds |first1=Matthew R. E. |pmid=15727040 |url-access=subscription }} In marsupials, the genital tract is separate from the anus, but a trace of the original cloaca does remain externally.{{cite book| vauthors = Roman AS, Parsons TS |year=1977|title=The Vertebrate Body|publisher=Holt-Saunders International|location=Philadelphia|pages=396–399|isbn=978-0-03-910284-5}} Monotremes, which translates from Greek into "single hole", have a true cloaca.{{cite book|url={{Google books|plainurl=yes|https://books.google.com/books?id=wojUDAAAQBAJ|page=281}} |vauthors = Dawkins R, Wong Y |year=2016|title=The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution|edition=2nd|publisher=Mariner Books|location=Boston|page=281|isbn=978-0-544-85993-7}} Urine flows from the ureters into the cloaca in monotremes and into the bladder in placentals.

File:Animal echolocation.svg

As in all other tetrapods, mammals have a larynx that can quickly open and close to produce sounds, and a supralaryngeal vocal tract which filters this sound. The lungs and surrounding musculature provide the air stream and pressure required to phonate. The larynx controls the pitch and volume of sound, but the strength the lungs exert to exhale also contributes to volume. More primitive mammals, such as the echidna, can only hiss, as sound is achieved solely through exhaling through a partially closed larynx. Other mammals phonate using vocal folds. The movement or tenseness of the vocal folds can result in many sounds such as purring and screaming. Mammals can change the position of the larynx, allowing them to breathe through the nose while swallowing through the mouth, and to form both oral and nasal sounds; nasal sounds, such as a dog whine, are generally soft sounds, and oral sounds, such as a dog bark, are generally loud.

File:Beluga_vocalizations.ogg echolocation sounds]]

Some mammals have a large larynx and thus a low-pitched voice, namely the hammer-headed bat (Hypsignathus monstrosus) where the larynx can take up the entirety of the thoracic cavity while pushing the lungs, heart, and trachea into the abdomen.{{cite journal| vauthors = Langevin P, Barclay RM |year=1990 |title= Hypsignathus monstrosus|journal=Mammalian Species|issue=357 |pages=1–4 |doi=10.2307/3504110|jstor=3504110 |doi-access=free }} Large vocal pads can also lower the pitch, as in the low-pitched roars of big cats.{{cite journal | vauthors = Weissengruber GE, Forstenpointner G, Peters G, Kübber-Heiss A, Fitch WT | title = Hyoid apparatus and pharynx in the lion (Panthera leo), jaguar (Panthera onca), tiger (Panthera tigris), cheetah (Acinonyxjubatus) and domestic cat (Felis silvestris f. catus) | journal = Journal of Anatomy | volume = 201 | issue = 3 | pages = 195–209 | date = September 2002 | pmid = 12363272 | pmc = 1570911 | doi = 10.1046/j.1469-7580.2002.00088.x }} The production of infrasound is possible in some mammals such as the African elephant (Loxodonta spp.) and baleen whales.{{cite journal | vauthors = Stoeger AS, Heilmann G, Zeppelzauer M, Ganswindt A, Hensman S, Charlton BD | title = Visualizing sound emission of elephant vocalizations: evidence for two rumble production types | journal = PLOS ONE | volume = 7 | issue = 11 | pages = e48907 | year = 2012 | pmid = 23155427 | pmc = 3498347 | doi = 10.1371/journal.pone.0048907 | bibcode = 2012PLoSO...748907S | doi-access = free }}{{cite journal| vauthors = Clark CW |year=2004 |title= Baleen whale infrasonic sounds: Natural variability and function|journal=Journal of the Acoustical Society of America |volume=115 |issue=5|doi=10.1121/1.4783845|page=2554|bibcode=2004ASAJ..115.2554C}} Small mammals with small larynxes have the ability to produce ultrasound, which can be detected by modifications to the middle ear and cochlea. Ultrasound is inaudible to birds and reptiles, which might have been important during the Mesozoic, when birds and reptiles were the dominant predators. This private channel is used by some rodents in, for example, mother-to-pup communication, and by bats when echolocating. Toothed whales also use echolocation, but, as opposed to the vocal membrane that extends upward from the vocal folds, they have a melon to manipulate sounds. Some mammals, namely the primates, have air sacs attached to the larynx, which may function to lower the resonances or increase the volume of sound.{{cite book |chapter-url=https://homepage.univie.ac.at/tecumseh.fitch/wp-content/uploads/2010/08/Fitch2006MammalVocalProd.pdf |vauthors=Fitch WT |chapter=Production of Vocalizations in Mammals |year=2006 |title=Encyclopedia of Language and Linguistics |veditors=Brown K |publisher=Elsevier |location=Oxford |pages=115–121 |access-date=25 January 2024 |archive-date=1 June 2024 |archive-url=https://web.archive.org/web/20240601150858/https://homepage.univie.ac.at/tecumseh.fitch/wp-content/uploads/2010/08/Fitch2006MammalVocalProd.pdf/ |url-status=live }}

The vocal production system is controlled by the cranial nerve nuclei in the brain, and supplied by the recurrent laryngeal nerve and the superior laryngeal nerve, branches of the vagus nerve. The vocal tract is supplied by the hypoglossal nerve and facial nerves. Electrical stimulation of the periaqueductal grey (PEG) region of the mammalian midbrain elicit vocalisations. The ability to learn new vocalisations is only exemplified in humans, seals, cetaceans, elephants and possibly bats; in humans, this is the result of a direct connection between the motor cortex, which controls movement, and the motor neurons in the spinal cord.

{{Main|Fur}}

File:Stekelvarken Aiguilles Porc-épic.jpgs use their spines for defence.]]

The primary function of the fur of mammals is thermoregulation. Others include protection, sensory purposes, waterproofing, and camouflage.{{cite journal | vauthors = Dawson TJ, Webster KN, Maloney SK | title = The fur of mammals in exposed environments; do crypsis and thermal needs necessarily conflict? The polar bear and marsupial koala compared | journal = Journal of Comparative Physiology B | volume = 184 | issue = 2 | pages = 273–284 | date = February 2014 | pmid = 24366474 | doi = 10.1007/s00360-013-0794-8 | s2cid = 9481486 }} Different types of fur serve different purposes:{{rp|99}}

• Definitive – which may be shed after reaching a certain length
• Vibrissae – sensory hairs, most commonly whiskers
• Pelage – guard hairs, under-fur, and awn hair
• Spines – stiff guard hair used for defence (such as in porcupines)
• Bristles – long hairs usually used in visual signals. (such as a lion's mane)
• Velli – often called "down fur" which insulates newborn mammals
• Wool – long, soft and often curly

Hair length is not a factor in thermoregulation: for example, some tropical mammals such as sloths have the same length of fur length as some arctic mammals but with less insulation; and, conversely, other tropical mammals with short hair have the same insulating value as arctic mammals. The denseness of fur can increase an animal's insulation value, and arctic mammals especially have dense fur; for example, the musk ox has guard hairs measuring {{cvt|30|cm}} as well as a dense underfur, which forms an airtight coat, allowing them to survive in temperatures of {{cvt|-40|C}}.{{rp|162–163}} Some desert mammals, such as camels, use dense fur to prevent solar heat from reaching their skin, allowing the animal to stay cool; a camel's fur may reach {{cvt|70|C}} in the summer, but the skin stays at {{cvt|40|C}}.{{rp|188}} Aquatic mammals, conversely, trap air in their fur to conserve heat by keeping the skin dry.{{rp|162–163}}

File:Great male Leopard in South Afrika-JD.JPG's disruptively coloured coat provides camouflage for this ambush predator.]]

Mammalian coats are coloured for a variety of reasons, the major selective pressures including camouflage, sexual selection, communication, and thermoregulation. Coloration in both the hair and skin of mammals is mainly determined by the type and amount of melanin; eumelanins for brown and black colours and pheomelanin for a range of yellowish to reddish colours, giving mammals an earth tone.{{cite journal | vauthors = Slominski A, Tobin DJ, Shibahara S, Wortsman J | title = Melanin pigmentation in mammalian skin and its hormonal regulation | journal = Physiological Reviews | volume = 84 | issue = 4 | pages = 1155–1228 | date = October 2004 | pmid = 15383650 | doi = 10.1152/physrev.00044.2003 | s2cid = 21168932 }}{{cite journal | url=https://www.hiltonpond.org/ArticleAnimalColorsMain.html | title=South Carolina Wildlife | publisher=Hilton Pond Center | journal=Animal Colors | year=1996 | access-date=26 November 2011 | vauthors=Hilton Jr B | pages=10–15 | volume=43 | issue=4 | archive-date=25 January 2024 | archive-url=https://web.archive.org/web/20240125191353/https://www.hiltonpond.org/ArticleAnimalColorsMain.html | url-status=live }} Some mammals have more vibrant colours; certain monkeys such mandrills and vervet monkeys, and opossums such as the Mexican mouse opossums and Derby's woolly opossums, have blue skin due to light diffraction in collagen fibres. Many sloths appear green because their fur hosts green algae; this may be a symbiotic relation that affords camouflage to the sloths.{{cite journal | vauthors = Suutari M, Majaneva M, Fewer DP, Voirin B, Aiello A, Friedl T, Chiarello AG, Blomster J | display-authors = 6 | title = Molecular evidence for a diverse green algal community growing in the hair of sloths and a specific association with Trichophilus welckeri (Chlorophyta, Ulvophyceae) | journal = BMC Evolutionary Biology | volume = 10 | issue = 86 | pages = 86 | date = March 2010 | pmid = 20353556 | pmc = 2858742 | doi = 10.1186/1471-2148-10-86 | doi-access = free | bibcode = 2010BMCEE..10...86S }}

Camouflage is a powerful influence in a large number of mammals, as it helps to conceal individuals from predators or prey.{{cite journal | vauthors = Caro T |year=2005 |title= The Adaptive Significance of Coloration in Mammals |journal=BioScience |volume= 55 | issue = 2 |pages= 125–136 |doi=10.1641/0006-3568(2005)055[0125:tasoci]2.0.co;2 |doi-access=free }} In arctic and subarctic mammals such as the arctic fox (Alopex lagopus), collared lemming (Dicrostonyx groenlandicus), stoat (Mustela erminea), and snowshoe hare (Lepus americanus), seasonal color change between brown in summer and white in winter is driven largely by camouflage.{{cite journal | vauthors = Mills LS, Zimova M, Oyler J, Running S, Abatzoglou JT, Lukacs PM | title = Camouflage mismatch in seasonal coat color due to decreased snow duration | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 110 | issue = 18 | pages = 7360–7365 | date = April 2013 | pmid = 23589881 | pmc = 3645584 | doi = 10.1073/pnas.1222724110 | bibcode = 2013PNAS..110.7360M |bibcode-access=free | doi-access = free }} Some arboreal mammals, notably primates and marsupials, have shades of violet, green, or blue skin on parts of their bodies, indicating some distinct advantage in their largely arboreal habitat due to convergent evolution.{{cite journal | vauthors = Prum RO, Torres RH | title = Structural colouration of mammalian skin: convergent evolution of coherently scattering dermal collagen arrays | journal = The Journal of Experimental Biology | volume = 207 | issue = Pt 12 | pages = 2157–2172 | date = May 2004 | pmid = 15143148 | doi = 10.1242/jeb.00989 | bibcode = 2004JExpB.207.2157P | url = https://jeb.biologists.org/content/207/12/2157.full.pdf | hdl = 1808/1599 | s2cid = 8268610 | access-date = 25 January 2024 | archive-date = 5 June 2024 | archive-url = https://web.archive.org/web/20240605171545/https://jeb.biologists.org/content/207/12/2157.full.pdf | url-status = live }}

Aposematism, warning off possible predators, is the most likely explanation of the black-and-white pelage of many mammals which are able to defend themselves, such as in the foul-smelling skunk and the powerful and aggressive honey badger.{{cite journal | vauthors = Caro T | title = Contrasting coloration in terrestrial mammals | doi-access = free | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 364 | issue = 1516 | pages = 537–548 | date = February 2009 | pmid = 18990666 | pmc = 2674080 | doi = 10.1098/rstb.2008.0221 }} Coat color is sometimes sexually dimorphic, as in many primate species.{{cite journal | vauthors = Plavcan JM | title = Sexual dimorphism in primate evolution | journal = American Journal of Physical Anthropology | volume = Suppl 33 | issue = 33 | pages = 25–53 | year = 2001 | pmid = 11786990 | doi = 10.1002/ajpa.10011 | s2cid = 31722173 |s2cid-access=free | doi-access = free }} Differences in female and male coat color may indicate nutrition and hormone levels, important in mate selection.{{cite journal | vauthors = Bradley BJ, Gerald MS, Widdig A, Mundy NI |year=2012 |title=Coat Color Variation and Pigmentation Gene Expression in Rhesus Macaques (Macaca Mulatta) |journal=Journal of Mammalian Evolution |volume=20 |issue=3 |pages=263–270 |doi=10.1007/s10914-012-9212-3 |s2cid=13916535 |url=https://www.eva.mpg.de/pks/staff/widdig/pdf/Bradley_et_al_2012.pdf |url-status=dead |archive-url=https://web.archive.org/web/20150924004623/http://www.eva.mpg.de/pks/staff/widdig/pdf/Bradley_et_al_2012.pdf |archive-date=24 September 2015 }} Coat color may influence the ability to retain heat, depending on how much light is reflected. Mammals with a darker coloured coat can absorb more heat from solar radiation, and stay warmer, and some smaller mammals, such as voles, have darker fur in the winter. The white, pigmentless fur of arctic mammals, such as the polar bear, may reflect more solar radiation directly onto the skin.{{rp|166–167}} The dazzling black-and-white striping of zebras appear to provide some protection from biting flies.{{cite journal | vauthors = Caro T, Izzo A, Reiner RC, Walker H, Stankowich T | title = The function of zebra stripes |bibcode-access=free | journal = Nature Communications | volume = 5 | pages = 3535 | date = April 2014 | pmid = 24691390 | doi = 10.1038/ncomms4535 | author1-link = Tim Caro | bibcode = 2014NatCo...5.3535C | s2cid = 9849814 |s2cid-access=free | doi-access = free }}

{{Main|Mammalian reproduction}}

File:Goat family.jpg kids stay with their mother until they are weaned.]]

Mammals reproduce by internal fertilisation{{Cite book|last=Naguib|first=Marc|url=https://books.google.com/books?id=KgTeDwAAQBAJ&pg=PA65|title=Advances in the Study of Behavior|date=19 April 2020|publisher=Academic Press|isbn=978-0-12-820726-0}} and are solely gonochoric (an animal is born with either male or female genitalia, as opposed to hermaphrodites where there is no such schism).{{Cite book| vauthors = Kobayashi K, Kitano T, Iwao Y, Kondo M |url=https://books.google.com/books?id=g4teDwAAQBAJ&q=mammal+gonochorism&pg=PA290|title=Reproductive and Developmental Strategies: The Continuity of Life |date=2018|publisher=Springer|isbn=978-4-431-56609-0|pages=290|language=en}} Male mammals ejaculate semen during copulation through a penis, which may be contained in a prepuce when not erect. Male placentals also urinate through a penis, and some placentals also have a penis bone (baculum).{{cite book| vauthors = Lombardi J |title=Comparative Vertebrate Reproduction|url=https://books.google.com/books?id=cqQX9RMPAegC|date= 1998|publisher=Springer Science & Business Media|isbn=978-0-7923-8336-9}}{{cite book |author=Libbie Henrietta Hyman |url=https://books.google.com/books?id=VKlWjdOkiMwC&pg=PA583 |title=Hyman's Comparative Vertebrate Anatomy |date=15 September 1992 |publisher=University of Chicago Press |isbn=978-0-226-87013-7 |pages=583–}} Marsupials typically have forked penises,{{cite book| vauthors = Tyndale-Biscoe H, Renfree M |title=Reproductive Physiology of Marsupials|url=https://books.google.com/books?id=HpjovN0vXW4C|date=1987|publisher=Cambridge University Press|isbn=978-0-521-33792-2}} while the echidna penis generally has four heads with only two functioning.{{Cite journal | doi=10.1086/522847| pmid=18171162| title=One-Sided Ejaculation of Echidna Sperm Bundles| journal=The American Naturalist| volume=170| issue=6| pages=E162–E164| year=2007| vauthors = Johnston SD, Smith B, Pyne M, Stenzel D, Holt WV | bibcode=2007ANat..170E.162J| s2cid=40632746| url=https://espace.library.uq.edu.au/view/UQ:130591/UQ130591_OA.pdf}} Depending on the species, an erection may be fuelled by blood flow into vascular, spongy tissue or by muscular action. The testicles of most mammals descend into the scrotum which is typically posterior to the penis but is often anterior in marsupials. Female mammals generally have a vulva (clitoris and labia) on the outside, while the internal system contains paired oviducts, one or two uteri, one or two cervices and a vagina.{{cite book|last1=Bacha Jr.|first1=William J.|last2=Bacha|first2=Linda M.|title= Color Atlas of Veterinary Histology |publisher = Wiley |year = 2012|page=308|access-date = 28 November 2023 |isbn= 978-1-11824-364-0|url = https://books.google.com/books?id=08BOg2b7zRgC&pg=PA308}}{{cite book|last1=Cooke|first1=Fred|last2=Bruce|first2=Jenni|title= The Encyclopedia of Animals: A Complete Visual Guide |publisher = University of California Press |year = 2004|page=79| access-date = 28 November 2023 |isbn= 978-0-52024-406-1|url = https://books.google.com/books?id=2V1tHqi4hLEC&pg=PA79}} Marsupials have two lateral vaginas and a medial vagina. The "vagina" of monotremes is better understood as a "urogenital sinus". The uterine systems of placentals can vary between a duplex, where there are two uteri and cervices which open into the vagina, a bipartite, where two uterine horns have a single cervix that connects to the vagina, a bicornuate, which consists where two uterine horns that are connected distally but separate medially creating a Y-shape, and a simplex, which has a single uterus.{{cite book| vauthors = Maxwell KE |year=2013|title=The Sex Imperative: An Evolutionary Tale of Sexual Survival|publisher=Springer|pages=112–113|isbn=978-1-4899-5988-1|url=https://books.google.com/books?id=dnf1BwAAQBAJ}}{{cite book| vauthors = Vaughan TA, Ryan JP, Czaplewski NJ |year= 2011 |title= Mammalogy|publisher=Jones & Bartlett Publishers|page=387 |isbn= 978-0-03-025034-7 }}{{rp|220–221, 247}}

File:Dendrolagus matschiei 1.jpg with young in pouch]]

The ancestral condition for mammal reproduction is the birthing of relatively undeveloped young, either through direct vivipary or a short period as soft-shelled eggs. This is likely due to the fact that the torso could not expand due to the presence of epipubic bones. The oldest demonstration of this reproductive style is with Kayentatherium, which produced undeveloped perinates, but at much higher litter sizes than any modern mammal, 38 specimens.{{cite journal | vauthors = Hoffman EA, Rowe TB | title = Jurassic stem-mammal perinates and the origin of mammalian reproduction and growth | journal = Nature | volume = 561 | issue = 7721 | pages = 104–108 | date = September 2018 | pmid = 30158701 | doi = 10.1038/s41586-018-0441-3 | bibcode = 2018Natur.561..104H| s2cid = 205570021 }} Most modern mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypus and the four species of echidna, lay eggs. The monotremes have a sex-determination system different from that of most other mammals.{{cite journal | vauthors = Wallis MC, Waters PD, Delbridge ML, Kirby PJ, Pask AJ, Grützner F, Rens W, Ferguson-Smith MA, Graves JA | display-authors = 6 | title = Sex determination in platypus and echidna: autosomal location of SOX3 confirms the absence of SRY from monotremes | journal = Chromosome Research | volume = 15 | issue = 8 | pages = 949–959 | year = 2007 | pmid = 18185981 | doi = 10.1007/s10577-007-1185-3 | s2cid = 812974 }} In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.{{cite journal | vauthors = Marshall Graves JA | title = Weird animal genomes and the evolution of vertebrate sex and sex chromosomes | journal = Annual Review of Genetics | volume = 42 | pages = 565–586 | year = 2008 | pmid = 18983263 | doi = 10.1146/annurev.genet.42.110807.091714 | url = https://www.mnf.uni-greifswald.de/fileadmin/Zoologisches_Museum/Hildebrandt/Dokumente/graves08.pdf | url-status = dead | archive-url = http://webarchive.nationalarchives.gov.uk/20120904084145/http%3A//www.mnf.uni%2Dgreifswald.de/fileadmin/Zoologisches_Museum/Hildebrandt/Dokumente/graves08.pdf | archive-date = 4 September 2012 | access-date = 25 January 2024 }}

Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses. Marsupials have a short gestation period, typically shorter than its estrous cycle and generally giving birth to a number of undeveloped newborns that then undergo further development; in many species, this takes place within a pouch-like sac, the marsupium, located in the front of the mother's abdomen. This is the plesiomorphic condition among viviparous mammals; the presence of epipubic bones in all non-placentals prevents the expansion of the torso needed for full pregnancy.{{cite book| vauthors = Power ML, Schulkin J |year=2013|title=The Evolution Of The Human Placenta|publisher=Johns Hopkins University Press|url={{Google books|plainurl=yes|id=xfffGC3hjPoC|page=1891}}|pages=1890–1891|location=Baltimore|isbn=978-1-4214-0643-5|oclc=940749490}} Even non-placental eutherians probably reproduced this way. The placentals give birth to relatively complete and developed young, usually after long gestation periods.{{cite book|chapter-url={{Google books|plainurl=yes|id=dK-D6HMSOQIC|page=6}}|location=Chicago| vauthors = Sally M |title=Mammals|chapter=Mammal Behavior and Lifestyle|year=2005|publisher=Raintree|page=6|isbn=978-1-4109-1050-9|oclc=53476660}} They get their name from the placenta, which connects the developing fetus to the uterine wall to allow nutrient uptake.{{cite book|url={{Google books|plainurl=yes|id=23s9DAAAQBAJ|page=288}} | vauthors = Verma PS, Pandey BP |year=2013|title=ISC Biology Book I for Class XI|publisher=S. Chand and Company|page=288|location=New Delhi|isbn=978-81-219-2557-0}} In placentals, the epipubic is either completely lost or converted into the baculum; allowing the torso to be able to expand and thus birth developed offspring.

The mammary glands of mammals are specialised to produce milk, the primary source of nutrition for newborns. The monotremes branched early from other mammals and do not have the teats seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.{{cite journal | vauthors = Oftedal OT | title = The mammary gland and its origin during synapsid evolution | journal = Journal of Mammary Gland Biology and Neoplasia | volume = 7 | issue = 3 | pages = 225–252 | date = July 2002 | pmid = 12751889 | doi = 10.1023/a:1022896515287 | s2cid = 25806501 }} Compared to placental mammals, the milk of marsupials changes greatly in both production rate and in nutrient composition, due to the underdeveloped young. In addition, the mammary glands have more autonomy allowing them to supply separate milks to young at different development stages.{{cite book| vauthors = Krockenberger A |year=2006 |title= Marsupials | chapter = Lactation| veditors = Dickman CR, Armati PJ, Hume ID |page=109|publisher=Cambridge University Press |isbn=978-1-139-45742-2}} Lactose is the main sugar in placental milk while monotreme and marsupial milk is dominated by oligosaccharides.{{cite book| vauthors = Schulkin J, Power ML |year=2016|title=Milk: The Biology of Lactation|publisher=Johns Hopkins University Press|page=66|isbn=978-1-4214-2042-4}} Weaning is the process in which a mammal becomes less dependent on their mother's milk and more on solid food.{{cite book | vauthors = Thompson KV, Baker AJ, Baker AM |year=2010|title=Wild Mammals in Captivity Principles and Techniques for Zoo Management|publisher=University of Chicago Press| chapter = Paternal Care and Behavioral Development in Captive Mammals | veditors = Kleiman DG, Thompson KV, Baer CK |page=374|isbn=978-0-226-44011-8|edition=2nd}}

Nearly all mammals are endothermic ("warm-blooded"). Most mammals also have hair to help keep them warm. Like birds, mammals can forage or hunt in weather and climates too cold for ectothermic ("cold-blooded") reptiles and insects. Endothermy requires plenty of food energy, so mammals eat more food per unit of body weight than most reptiles.{{cite book| vauthors = Campbell NA, Reece JB |year=2002 |title=Biology |edition=6th |publisher=Benjamin Cummings |page=[https://archive.org/details/biologyc00camp/page/845 845]|isbn=978-0-8053-6624-2|oclc=47521441|url=https://archive.org/details/biologyc00camp/page/845}} Small insectivorous mammals eat prodigious amounts for their size. A rare exception, the naked mole-rat produces little metabolic heat, so it is considered an operational poikilotherm.{{cite journal| vauthors = Buffenstein R, Yahav S |year=1991|title=Is the naked mole-rat Hererocephalus glaber an endothermic yet poikilothermic mammal?|journal=Journal of Thermal Biology|volume=16|issue=4|pages=227–232|doi=10.1016/0306-4565(91)90030-6|bibcode=1991JTBio..16..227B }} Birds are also endothermic, so endothermy is not unique to mammals.{{cite book|chapter-url={{Google books|plainurl=yes| id=Af7IwQWJoCMC|page=218}}|location=Cambridge| vauthors = Schmidt-Nielsen K, Duke JB |year=1997|title=Animal Physiology: Adaptation and Environment|chapter=Temperature Effects|edition=5th|page=218|isbn=978-0-521-57098-5|oclc=35744403 |publisher=Cambridge University Press }}

{{See also|Life expectancy|Maximum life span}}

Among mammals, species maximum lifespan varies significantly (for example the shrew has a lifespan of two years, whereas the oldest bowhead whale is recorded to be 211 years).{{cite journal | vauthors = Lorenzini A, Johnson FB, Oliver A, Tresini M, Smith JS, Hdeib M, Sell C, Cristofalo VJ, Stamato TD | display-authors = 6 | title = Significant correlation of species longevity with DNA double strand break recognition but not with telomere length | journal = Mechanisms of Ageing and Development | volume = 130 | issue = 11–12 | pages = 784–792 | year = 2009 | pmid = 19896964 | pmc = 2799038 | doi = 10.1016/j.mad.2009.10.004 }} Although the underlying basis for these lifespan differences is still uncertain, numerous studies indicate that the ability to repair DNA damage is an important determinant of mammalian lifespan. In a 1974 study by Hart and Setlow,{{cite journal | vauthors = Hart RW, Setlow RB | title = Correlation between deoxyribonucleic acid excision-repair and life-span in a number of mammalian species | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 71 | issue = 6 | pages = 2169–2173 | date = June 1974 | pmid = 4526202 | pmc = 388412 | doi = 10.1073/pnas.71.6.2169 | bibcode = 1974PNAS...71.2169H | doi-access = free }} it was found that DNA excision repair capability increased systematically with species lifespan among seven mammalian species. Species lifespan was observed to be robustly correlated with the capacity to recognise DNA double-strand breaks as well as the level of the DNA repair protein Ku80. In a study of the cells from sixteen mammalian species, genes employed in DNA repair were found to be up-regulated in the longer-lived species.{{cite journal | vauthors = Ma S, Upneja A, Galecki A, Tsai YM, Burant CF, Raskind S, Zhang Q, Zhang ZD, Seluanov A, Gorbunova V, Clish CB, Miller RA, Gladyshev VN | display-authors = 6 | title = Cell culture-based profiling across mammals reveals DNA repair and metabolism as determinants of species longevity | journal = eLife | volume = 5 | date = November 2016 | pmid = 27874830 | pmc = 5148604 | doi = 10.7554/eLife.19130 | doi-access = free }} The cellular level of the DNA repair enzyme poly ADP ribose polymerase was found to correlate with species lifespan in a study of 13 mammalian species.{{cite journal | vauthors = Grube K, Bürkle A | title = Poly(ADP-ribose) polymerase activity in mononuclear leukocytes of 13 mammalian species correlates with species-specific life span | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 89 | issue = 24 | pages = 11759–11763 | date = December 1992 | pmid = 1465394 | pmc = 50636 | doi = 10.1073/pnas.89.24.11759 | bibcode = 1992PNAS...8911759G | doi-access = free }} Three additional studies of a variety of mammalian species also reported a correlation between species lifespan and DNA repair capability.{{cite journal | vauthors = Francis AA, Lee WH, Regan JD | title = The relationship of DNA excision repair of ultraviolet-induced lesions to the maximum life span of mammals | journal = Mechanisms of Ageing and Development | volume = 16 | issue = 2 | pages = 181–189 | date = June 1981 | pmid = 7266079 | doi = 10.1016/0047-6374(81)90094-4 | s2cid = 19830165 }}{{cite journal | vauthors = Treton JA, Courtois Y | title = Correlation between DNA excision repair and mammalian lifespan in lens epithelial cells | journal = Cell Biology International Reports | volume = 6 | issue = 3 | pages = 253–260 | date = March 1982 | pmid = 7060140 | doi = 10.1016/0309-1651(82)90077-7 | doi-broken-date = 4 April 2025 }}{{cite journal | vauthors = Maslansky CJ, Williams GM | title = Ultraviolet light-induced DNA repair synthesis in hepatocytes from species of differing longevities | journal = Mechanisms of Ageing and Development | volume = 29 | issue = 2 | pages = 191–203 | date = February 1985 | pmid = 3974310 | doi = 10.1016/0047-6374(85)90018-1 | s2cid = 23988416 }}

{{Main|Animal locomotion}}

{{Main|Terrestrial locomotion}}

File:Muybridge race horse animated.gif. Photographs by Eadweard Muybridge, 1887]]

Most vertebrates—the amphibians, the reptiles and some mammals such as humans and bears—are plantigrade, walking on the whole of the underside of the foot. Many mammals, such as cats and dogs, are digitigrade, walking on their toes, the greater stride length allowing more speed. Some animals such as horses are unguligrade, walking on the tips of their toes. This even further increases their stride length and thus their speed.{{cite book|url={{Google books |plainurl=yes |id=LaaLNfY6gB8C |page=3}}| vauthors = Walker WF, Homberger DG |author-link2= Dominique G. Homberger|year=1998|title=Anatomy and Dissection of the Fetal Pig|edition=5th|location=New York|publisher=W. H. Freeman and Company|page=3|isbn=978-0-7167-2637-1|oclc=40576267}} A few mammals, namely the great apes, are also known to walk on their knuckles, at least for their front legs. Giant anteaters{{cite journal | vauthors = Orr CM | title = Knuckle-walking anteater: a convergence test of adaptation for purported knuckle-walking features of African Hominidae | journal = American Journal of Physical Anthropology | volume = 128 | issue = 3 | pages = 639–658 | date = November 2005 | pmid = 15861420 | doi = 10.1002/ajpa.20192 }} and platypuses{{cite journal | vauthors = Fish FE, Frappell PB, Baudinette RV, MacFarlane PM | title = Energetics of terrestrial locomotion of the platypus Ornithorhynchus anatinus | journal = The Journal of Experimental Biology | volume = 204 | issue = Pt 4 | pages = 797–803 | date = February 2001 | doi = 10.1242/jeb.204.4.797 | pmid = 11171362 | bibcode = 2001JExpB.204..797F | hdl = 2440/12192 | url = https://jeb.biologists.org/cgi/reprint/204/4/797.pdf | access-date = 25 January 2024 | archive-date = 14 March 2024 | archive-url = https://web.archive.org/web/20240314100116/https://jeb.biologists.org/cgi/reprint/204/4/797.pdf | url-status = live }} are also knuckle-walkers. Some mammals are bipeds, using only two limbs for locomotion, which can be seen in, for example, humans and the great apes. Bipedal species have a larger field of vision than quadrupeds, conserve more energy and have the ability to manipulate objects with their hands, which aids in foraging. Instead of walking, some bipeds hop, such as kangaroos and kangaroo rats.{{cite journal|url=http://www.philosophistry.com/static/bipedalism.html|vauthors=Dhingra P|year=2004|title=Comparative Bipedalism – How the Rest of the Animal Kingdom Walks on two legs|journal=Anthropological Science|volume=131|issue=231|access-date=11 March 2017|archive-date=21 April 2021|archive-url=https://web.archive.org/web/20210421085055/https://philosophistry.com/static/bipedalism.html|url-status=live}}{{cite journal | vauthors = Alexander RM | title = Bipedal animals, and their differences from humans | journal = Journal of Anatomy | volume = 204 | issue = 5 | pages = 321–330 | date = May 2004 | pmid = 15198697 | pmc = 1571302 | doi = 10.1111/j.0021-8782.2004.00289.x }}

Animals will use different gaits for different speeds, terrain and situations. For example, horses show four natural gaits, the slowest horse gait is the walk, then there are three faster gaits which, from slowest to fastest, are the trot, the canter and the gallop. Animals may also have unusual gaits that are used occasionally, such as for moving sideways or backwards. For example, the main human gaits are bipedal walking and running, but they employ many other gaits occasionally, including a four-legged crawl in tight spaces.{{cite journal| vauthors = Dagg AI |author-link=Anne Innis Dagg|year=1973|title=Gaits in Mammals|journal=Mammal Review|volume=3|issue=4|pages=135–154|doi=10.1111/j.1365-2907.1973.tb00179.x|bibcode=1973MamRv...3..135D }} Mammals show a vast range of gaits, the order that they place and lift their appendages in locomotion. Gaits can be grouped into categories according to their patterns of support sequence. For quadrupeds, there are three main categories: walking gaits, running gaits and leaping gaits.{{cite book| vauthors = Roberts TD |title=Understanding Balance: The Mechanics of Posture and Locomotion|url={{Google books|plainurl=yes|id=o8RvD3X8ur8C|page=211}}|location=San Diego|year=1995|publisher= Nelson Thornes|isbn=978-1-56593-416-0|page=211|oclc=33167785}} Walking is the most common gait, where some feet are on the ground at any given time, and found in almost all legged animals. Running is considered to occur when at some points in the stride all feet are off the ground in a moment of suspension.

{{Main|Arboreal locomotion}}

File:Brachiating Gibbon (Some rights reserved).jpgs are very good brachiators because their elongated limbs enable them to easily swing and grasp on to branches.]]

Arboreal animals frequently have elongated limbs that help them cross gaps, reach fruit or other resources, test the firmness of support ahead and, in some cases, to brachiate (swing between trees).{{cite book| vauthors = Cartmill M |year=1985|chapter=Climbing|title=Functional Vertebrate Morphology| veditors = Hildebrand M, Bramble DM, Liem KF, Wake DB |pages=73–88|location=Cambridge|publisher=Belknap Press|isbn=978-0-674-32775-7|oclc=11114191}} Many arboreal species, such as tree porcupines, silky anteaters, spider monkeys, and possums, use prehensile tails to grasp branches. In the spider monkey, the tip of the tail has either a bare patch or adhesive pad, which provides increased friction. Claws can be used to interact with rough substrates and reorient the direction of forces the animal applies. This is what allows squirrels to climb tree trunks that are so large to be essentially flat from the perspective of such a small animal. However, claws can interfere with an animal's ability to grasp very small branches, as they may wrap too far around and prick the animal's own paw. Frictional gripping is used by primates, relying upon hairless fingertips. Squeezing the branch between the fingertips generates frictional force that holds the animal's hand to the branch. However, this type of grip depends upon the angle of the frictional force, thus upon the diameter of the branch, with larger branches resulting in reduced gripping ability. To control descent, especially down large diameter branches, some arboreal animals such as squirrels have evolved highly mobile ankle joints that permit rotating the foot into a 'reversed' posture. This allows the claws to hook into the rough surface of the bark, opposing the force of gravity. Small size provides many advantages to arboreal species: such as increasing the relative size of branches to the animal, lower center of mass, increased stability, lower mass (allowing movement on smaller branches) and the ability to move through more cluttered habitat. Size relating to weight affects gliding animals such as the sugar glider.{{cite journal| vauthors = Vernes K |year=2001|title=Gliding Performance of the Northern Flying Squirrel (Glaucomys sabrinus) in Mature Mixed Forest of Eastern Canada|journal=Journal of Mammalogy|volume=82|issue=4|pages=1026–1033|doi=10.1644/1545-1542(2001)082<1026:GPOTNF>2.0.CO;2|s2cid=78090049 |doi-access=free}} Some species of primate, bat and all species of sloth achieve passive stability by hanging beneath the branch. Both pitching and tipping become irrelevant, as the only method of failure would be losing their grip.

{{Main|Aerial locomotion}}

File:Israeli Bats - 26 September 2015.webms flying]]

Bats are the only mammals that can truly fly. They fly through the air at a constant speed by moving their wings up and down (usually with some fore-aft movement as well). Because the animal is in motion, there is some airflow relative to its body which, combined with the velocity of the wings, generates a faster airflow moving over the wing. This generates a lift force vector pointing forwards and upwards, and a drag force vector pointing rearwards and upwards. The upwards components of these counteract gravity, keeping the body in the air, while the forward component provides thrust to counteract both the drag from the wing and from the body as a whole.{{cite web|url=https://blogs.bu.edu/biolocomotion/2011/10/16/bats-the-only-flying-mammal/|title=Bats – the only flying mammals|vauthors=Barba LA|work=Bio-Aerial Locomotion|date=October 2011|access-date=20 May 2016|archive-date=14 May 2016|archive-url=https://web.archive.org/web/20160514085336/http://blogs.bu.edu/biolocomotion/2011/10/16/bats-the-only-flying-mammal/|url-status=live}}

The wings of bats are much thinner and consist of more bones than those of birds, allowing bats to manoeuvre more accurately and fly with more lift and less drag.{{cite web|url=https://www.sciencedaily.com/releases/2007/01/070118161402.htm|title=Bats In Flight Reveal Unexpected Aerodynamics|year=2007|website=ScienceDaily|access-date=12 July 2016|archive-date=19 December 2019|archive-url=https://web.archive.org/web/20191219210807/https://www.sciencedaily.com/releases/2007/01/070118161402.htm|url-status=live}}{{cite journal | vauthors = Hedenström A, Johansson LC | title = Bat flight: aerodynamics, kinematics and flight morphology | journal = The Journal of Experimental Biology | volume = 218 | issue = Pt 5 | pages = 653–663 | date = March 2015 | pmid = 25740899 | doi = 10.1242/jeb.031203 | bibcode = 2015JExpB.218..653H | s2cid = 21295393 | url = https://jeb.biologists.org/content/jexbio/218/5/653.full.pdf | access-date = 25 January 2024 | archive-date = 25 January 2024 | archive-url = https://web.archive.org/web/20240125191350/https://jeb.biologists.org/content/jexbio/218/5/653.full.pdf | url-status = live }} By folding the wings inwards towards their body on the upstroke, they use 35% less energy during flight than birds.{{cite web|url=https://www.sciencedaily.com/releases/2012/04/120411084133.htm|title=Bats save energy by drawing in wings on upstroke|website=ScienceDaily|access-date=12 July 2016|year=2012|archive-date=31 May 2021|archive-url=https://web.archive.org/web/20210531050233/https://www.sciencedaily.com/releases/2012/04/120411084133.htm|url-status=live}} The membranes are delicate, ripping easily; however, the tissue of the bat's membrane is able to regrow, such that small tears can heal quickly.{{cite book|url={{Google books|plainurl=yes|id=XS9y642cjvMC|page=14}}| vauthors = Karen T |year=2008 |title=Hanging with Bats: Ecobats, Vampires, and Movie Stars|publisher=University of New Mexico Press|location=Albuquerque|page=14|isbn=978-0-8263-4403-8|oclc=191258477}} The surface of their wings is equipped with touch-sensitive receptors on small bumps called Merkel cells, also found on human fingertips. These sensitive areas are different in bats, as each bump has a tiny hair in the center, making it even more sensitive and allowing the bat to detect and collect information about the air flowing over its wings, and to fly more efficiently by changing the shape of its wings in response.{{cite journal | vauthors = Sterbing-D'Angelo S, Chadha M, Chiu C, Falk B, Xian W, Barcelo J, Zook JM, Moss CF | display-authors = 6 | title = Bat wing sensors support flight control | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 108 | issue = 27 | pages = 11291–11296 | date = July 2011 | pmid = 21690408 | pmc = 3131348 | doi = 10.1073/pnas.1018740108 | bibcode = 2011PNAS..10811291S | doi-access = free }}

{{Multiple image|align=right|image1=Wombat3.jpg|image2=ScalopusAquaticus.jpg|total_width=400|footer=Semi-fossorial wombat (left) vs. fully fossorial eastern mole (right)}}

{{See also|Fossorial|Burrow}}

A fossorial (from Latin fossor, meaning "digger") is an animal adapted to digging which lives primarily, but not solely, underground. Some examples are badgers, and naked mole-rats. Many rodent species are also considered fossorial because they live in burrows for most but not all of the day. Species that live exclusively underground are subterranean, and those with limited adaptations to a fossorial lifestyle sub-fossorial. Some organisms are fossorial to aid in temperature regulation while others use the underground habitat for protection from predators or for food storage.Damiani, R, 2003, Earliest evidence of cynodont burrowing, The Royal Society Publishing, Volume 270, Issue 1525

Fossorial mammals have a fusiform body, thickest at the shoulders and tapering off at the tail and nose. Unable to see in the dark burrows, most have degenerated eyes, but degeneration varies between species; pocket gophers, for example, are only semi-fossorial and have very small yet functional eyes, in the fully fossorial marsupial mole, the eyes are degenerated and useless, Talpa moles have vestigial eyes and the Cape golden mole has a layer of skin covering the eyes. External ears flaps are also very small or absent. Truly fossorial mammals have short, stout legs as strength is more important than speed to a burrowing mammal, but semi-fossorial mammals have cursorial legs. The front paws are broad and have strong claws to help in loosening dirt while excavating burrows, and the back paws have webbing, as well as claws, which aids in throwing loosened dirt backwards. Most have large incisors to prevent dirt from flying into their mouth.{{cite journal|jstor=2455381|vauthors=Shimer HW|year=1903|title=Adaptations to Aquatic, Arboreal, Fossorial and Cursorial Habits in Mammals. III. Fossorial Adaptations|journal=The American Naturalist|volume=37|number=444|pages=819–825|doi=10.1086/278368|bibcode=1903ANat...37..819S |s2cid=83519668|url=https://zenodo.org/record/1431331|access-date=23 August 2020|archive-date=9 April 2023|archive-url=https://web.archive.org/web/20230409004021/https://zenodo.org/record/1431331|url-status=live}}

Many fossorial mammals such as shrews, hedgehogs, and moles were classified under the now obsolete order Insectivora.{{cite journal | vauthors = Stanhope MJ, Waddell VG, Madsen O, de Jong W, Hedges SB, Cleven GC, Kao D, Springer MS | display-authors = 6 | title = Molecular evidence for multiple origins of Insectivora and for a new order of endemic African insectivore mammals | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 95 | issue = 17 | pages = 9967–9972 | date = August 1998 | pmid = 9707584 | pmc = 21445 | doi = 10.1073/pnas.95.17.9967 | doi-access = free | bibcode = 1998PNAS...95.9967S }}

{{Main|Aquatic locomotion|Marine mammal|Aquatic mammal}}

File:Living-on-the-Edge-Settlement-Patterns-by-the-Symbiotic-Barnacle-Xenobalanus-globicipitis-on-Small-pone.0127367.s001.ogvs swimming]]

Fully aquatic mammals, the cetaceans and sirenians, have lost their legs and have a tail fin to propel themselves through the water. Flipper movement is continuous. Whales swim by moving their tail fin and lower body up and down, propelling themselves through vertical movement, while their flippers are mainly used for steering. Their skeletal anatomy allows them to be fast swimmers. Most species have a dorsal fin to prevent themselves from turning upside-down in the water.{{cite journal | vauthors = Perry DA |year=1949 |title=The anatomical basis of swimming in Whales |journal=Journal of Zoology |volume=119 |issue=1 |pages=49–60 |doi=10.1111/j.1096-3642.1949.tb00866.x}}{{cite journal | vauthors = Fish FE, Hui CA |year=1991 |title=Dolphin swimming – a review |journal=Mammal Review |volume=21 |issue=4 |pages=181–195 |url=https://darwin.wcupa.edu/~biology/fish/pubs/pdf/1991MRDolphinswimming.pdf |archive-url=https://web.archive.org/web/20060829000617/http://darwin.wcupa.edu/~biology/fish/pubs/pdf/1991MRDolphinswimming.pdf |url-status=dead |archive-date=29 August 2006 |doi=10.1111/j.1365-2907.1991.tb00292.x }} The flukes of sirenians are raised up and down in long strokes to move the animal forward, and can be twisted to turn. The forelimbs are paddle-like flippers which aid in turning and slowing.{{cite book| vauthors = Marsh H |chapter-url= https://www.environment.gov.au/biodiversity/abrs/publications/fauna-of-australia/pubs/volume1b/57-ind.pdf |chapter=Chapter 57: Dugongidae |title=Fauna of Australia |volume=1 |publisher=Australian Government Publications |isbn=978-0-644-06056-1 |location=Canberra |year=1989 |oclc=27492815 |url-status=dead |archive-url=https://web.archive.org/web/20130511221756/http://www.environment.gov.au/biodiversity/abrs/publications/fauna-of-australia/pubs/volume1b/57-ind.pdf |archive-date=11 May 2013 }}

Semi-aquatic mammals, like pinnipeds, have two pairs of flippers on the front and back, the fore-flippers and hind-flippers. The elbows and ankles are enclosed within the body.{{cite book | vauthors = Berta A | title = Return to the Sea: The Life and Evolutionary Times of Marine Mammals | chapter = Pinniped Diversity: Evolution and Adaptations | publisher = University of California Press | date = 2012 | isbn = 978-0-520-27057-2 | pages = 62–64}}{{cite journal | vauthors = Fish FE, Hurley J, Costa DP | title = Maneuverability by the sea lion Zalophus californianus: turning performance of an unstable body design | journal = The Journal of Experimental Biology | volume = 206 | issue = Pt 4 | pages = 667–674 | date = February 2003 | pmid = 12517984 | doi = 10.1242/jeb.00144 | doi-access = free | bibcode = 2003JExpB.206..667F }} Pinnipeds have several adaptions for reducing drag. In addition to their streamlined bodies, they have smooth networks of muscle bundles in their skin that may increase laminar flow and make it easier for them to slip through water. They also lack arrector pili, so their fur can be streamlined as they swim. They rely on their fore-flippers for locomotion in a wing-like manner similar to penguins and sea turtles.{{Cite journal | vauthors = Fish FE |title=Transitions from drag-based to lift-based propulsion in mammalian swimming |doi=10.1093/icb/36.6.628 |journal=Integrative and Comparative Biology |volume=36 |issue=6 |pages=628–641 |year=1996|doi-access=free }} Fore-flipper movement is not continuous, and the animal glides between each stroke. Compared to terrestrial carnivorans, the fore-limbs are reduced in length, which gives the locomotor muscles at the shoulder and elbow joints greater mechanical advantage; the hind-flippers serve as stabilizers.{{cite book| vauthors = Riedman M |year=1990|title=The Pinnipeds: Seals, Sea Lions, and Walruses| url = https://archive.org/details/pinnipedssealsse0000ried | url-access = registration |publisher=University of California Press|isbn=978-0-520-06497-3|oclc=19511610}} Other semi-aquatic mammals include beavers, hippopotamuses, otters and platypuses.{{cite journal | vauthors = Fish FE | title = Biomechanics and energetics in aquatic and semiaquatic mammals: platypus to whale | journal = Physiological and Biochemical Zoology | volume = 73 | issue = 6 | pages = 683–698 | year = 2000 | pmid = 11121343 | doi = 10.1086/318108 | url = https://darwin.wcupa.edu/~biology/fish/pubs/pdf/2000PBZ-PlatToWhale.pdf | url-status = dead | citeseerx = 10.1.1.734.1217 | s2cid = 49732160 | archive-url = https://web.archive.org/web/20160804111726/http://darwin.wcupa.edu/~biology/fish/pubs/pdf/2000PBZ-PlatToWhale.pdf | archive-date = 4 August 2016 }} Hippos are very large semi-aquatic mammals, and their barrel-shaped bodies have graviportal skeletal structures,{{cite book | vauthors = Eltringham SK |year=1999|title=The Hippos|chapter=Anatomy and Physiology|location= London|publisher=T & AD Poyser Ltd|page=8|isbn=978-0-85661-131-5|oclc=42274422}} adapted to carrying their enormous weight, and their specific gravity allows them to sink and move along the bottom of a river.{{cite magazine|title=Hippopotamus Hippopotamus amphibius|magazine=National Geographic|access-date= 30 April 2016|url= https://animals.nationalgeographic.com/animals/mammals/hippopotamus/|archive-url= https://web.archive.org/web/20141125041546/http://animals.nationalgeographic.com/animals/mammals/hippopotamus/|archive-date= 25 November 2014|url-status=dead}}

{{Further|Animal communication|Animal echolocation}}

File:Monkey & Baby.JPGs use at least four distinct alarm calls for different predators.]]

Many mammals communicate by vocalising. Vocal communication serves many purposes, including in mating rituals, as warning calls,{{cite journal | vauthors = Zuberbühler K |title=Predator-specific alarm calls in Campbell's monkeys, Cercopithecus campbelli |journal=Behavioral Ecology and Sociobiology |volume=50 |issue=5 |year=2001 |pages=414–442 |jstor=4601985 |doi=10.1007/s002650100383|bibcode=2001BEcoS..50..414Z |s2cid=21374702 }} to indicate food sources, and for social purposes. Males often call during mating rituals to ward off other males and to attract females, as in the roaring of lions and red deer.{{cite journal | vauthors = Slabbekoorn H, Smith TB | title = Bird song, ecology and speciation | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 357 | issue = 1420 | pages = 493–503 | date = April 2002 | pmid = 12028787 | pmc = 1692962 | doi = 10.1098/rstb.2001.1056 }} The songs of the humpback whale may be signals to females;{{cite book | vauthors = Bannister JL |chapter=Baleen Whales (Mysticetes) |pages=80–89 |title=Encyclopedia of Marine Mammals |edition=2nd | veditors = Perrin WF, Würsig B, Thewissen JG |editor3-link = Hans Thewissen |year=2008 |publisher=Academic Press |isbn=978-0-12-373553-9 |chapter-url={{Google books |plainurl=yes |id=2rkHQpToi9sC |page=1007}}}} they have different dialects in different regions of the ocean.{{cite journal | vauthors = Scott N |year=2002 |title=Creatures of Culture? Making the Case for Cultural Systems in Whales and Dolphins |journal=BioScience |volume=52 |issue=1 |pages=9–14 |doi=10.1641/0006-3568(2002)052[0009:COCMTC]2.0.CO;2|s2cid=86121405 |doi-access=free }} Social vocalisations include the territorial calls of gibbons, and the use of frequency in greater spear-nosed bats to distinguish between groups.{{cite journal | vauthors = Boughman JW | title = Vocal learning by greater spear-nosed bats | journal = Proceedings. Biological Sciences | volume = 265 | issue = 1392 | pages = 227–233 | date = February 1998 | pmid = 9493408 | pmc = 1688873 | doi = 10.1098/rspb.1998.0286 }} The vervet monkey gives a distinct alarm call for each of at least four different predators, and the reactions of other monkeys vary according to the call. For example, if an alarm call signals a python, the monkeys climb into the trees, whereas the eagle alarm causes monkeys to seek a hiding place on the ground.{{cite journal |vauthors=Seyfarth RM, Cheney DL, Marler P |title=Vervet Monkey Alarm Calls: Semantic communication in a Free-Ranging Primate |journal=Animal Behaviour |volume=28 |issue=4 |pages=1070–1094 |year=1980 |doi=10.1016/S0003-3472(80)80097-2 |s2cid=53165940 |url=https://www.researchgate.net/publication/223576319 |access-date=22 September 2018 |archive-date=12 September 2019 |archive-url=https://web.archive.org/web/20190912082142/https://www.researchgate.net/publication/223576319_Vervet_Monkey_Alarm_Calls_Semantic_Communication_In_A_Free-Ranging_Primate |url-status=live }} Prairie dogs similarly have complex calls that signal the type, size, and speed of an approaching predator.{{cite web |url=https://www.cbc.ca/news/technology/prairie-dogs-language-decoded-by-scientists-1.1322230 |title=Prairie dogs' language decoded by scientists |date=21 June 2013 |publisher=CBC News |access-date=20 May 2015 |archive-date=22 May 2015 |archive-url=https://web.archive.org/web/20150522024501/http://www.cbc.ca/news/technology/prairie-dogs-language-decoded-by-scientists-1.1322230 |url-status=live }} Elephants communicate socially with a variety of sounds including snorting, screaming, trumpeting, roaring and rumbling. Some of the rumbling calls are infrasonic, below the hearing range of humans, and can be heard by other elephants up to {{convert|6|mi}} away at still times near sunrise and sunset.{{cite magazine | vauthors = Mayell H |title=Elephants Call Long-Distance After-Hours |url=https://news.nationalgeographic.com/news/2004/03/0303_040303_elephants.html |archive-url=https://web.archive.org/web/20040305154249/http://news.nationalgeographic.com/news/2004/03/0303_040303_elephants.html |url-status=dead |archive-date=5 March 2004 |magazine=National Geographic |access-date=15 November 2016 |date=3 March 2004}}

file:Killer whale.ogg

Mammals signal by a variety of means. Many give visual anti-predator signals, as when deer and gazelle stot, honestly indicating their fit condition and their ability to escape,{{cite book | title=Animal Signals | publisher=Oxford University Press | vauthors = Smith JM, Harper D | year=2003 | author1-link=John Maynard Smith| pages=61–63 |url={{Google books |plainurl=yes |id=SUA51MeG1lcC |page=61}}|isbn=978-0-19-852684-1 |oclc=54460090 |series=Oxford Series in Ecology and Evolution}}{{cite journal|title=Stotting in Thomson's gazelles: an honest signal of condition |url=https://webs.wofford.edu/moellerjf/Animal%20Behavior%202011/stotting%20in%20gazelles.pdf | vauthors = FitzGibbon CD, Fanshawe JH |journal=Behavioral Ecology and Sociobiology |year=1988 |volume=23 |issue=2 |pages=69–74 |doi=10.1007/bf00299889 |bibcode=1988BEcoS..23...69F |s2cid=2809268 |url-status=dead |archive-url=https://web.archive.org/web/20140225073844/http://webs.wofford.edu/moellerjf/Animal%20Behavior%202011/stotting%20in%20gazelles.pdf |archive-date=25 February 2014 }} or when white-tailed deer and other prey mammals flag with conspicuous tail markings when alarmed, informing the predator that it has been detected.{{cite journal | vauthors = Bildstein KL |title=Why White-Tailed Deer Flag Their Tails |journal=The American Naturalist |date=May 1983 |volume=121 |issue=5 |pages=709–715 |jstor=2460873 |doi=10.1086/284096|bibcode=1983ANat..121..709B |s2cid=83504795 }} Many mammals make use of scent-marking, sometimes possibly to help defend territory, but probably with a range of functions both within and between species.{{cite journal | vauthors = Gosling LM | title = A reassessment of the function of scent marking in territories. | journal = Zeitschrift für Tierpsychologie | date = January 1982 | volume = 60 | issue = 2 | pages = 89–118 | url = https://www.researchgate.net/publication/230317056 | doi = 10.1111/j.1439-0310.1982.tb00492.x | bibcode = 1982Ethol..60...89G | access-date = 12 October 2019 | archive-date = 27 March 2018 | archive-url = https://web.archive.org/web/20180327084413/https://www.researchgate.net/profile/Leonard_Gosling/publication/230317056_A_Reassessment_of_the_Function_of_Scent_Marking_in_Territories/links/59dcd8b4a6fdcca56e35e24c/A-Reassessment-of-the-Function-of-Scent-Marking-in-Territories.pdf | url-status = live }}{{cite journal | vauthors = Zala SM, Potts WK, Penn DJ | title = Scent-marking displays provide honest signals of health and infection. | journal = Behavioral Ecology | date = March 2004 | volume = 15 | issue = 2 | pages = 338–344 | doi = 10.1093/beheco/arh022 | hdl = 10.1093/beheco/arh022 | hdl-access = free }}{{cite journal | vauthors = Johnson RP |title=Scent Marking in Mammals |journal=Animal Behaviour |date=August 1973 |volume=21 |issue=3 |pages=521–535 |doi=10.1016/S0003-3472(73)80012-0}} Microbats and toothed whales including oceanic dolphins vocalise both socially and in echolocation.{{cite journal | vauthors = Schevill WE, McBride AF | year=1956 | title=Evidence for echolocation by cetaceans | journal=Deep-Sea Research | volume=3 | issue=2 | pages=153–154 | doi=10.1016/0146-6313(56)90096-x |bibcode=1956DSR.....3..153S}}{{Cite book | vauthors = Wilson W, Moss C |year=2004 |title=Echolocation in Bats and Dolphins | veditors = Thomas J |page=22 |publisher=Chicago University Press |isbn=978-0-226-79599-7 |oclc=50143737}}{{cite book |url={{Google books |plainurl=yes |id=Q3MIsrPDA5EC |page=front}} | vauthors = Au WW |year=1993 |title=The Sonar of Dolphins |publisher=Springer-Verlag |isbn=978-3-540-97835-0 |oclc=26158593}}

File:Short-beaked echidna in suburban-Sydney backyard.webm foraging for insects]]

To maintain a high constant body temperature is energy expensive—mammals therefore need a nutritious and plentiful diet. While the earliest mammals were probably predators, different species have since adapted to meet their dietary requirements in a variety of ways. Some eat other animals—this is a carnivorous diet (and includes insectivorous diets). Other mammals, called herbivores, eat plants, which contain complex carbohydrates such as cellulose. An herbivorous diet includes subtypes such as granivory (seed eating), folivory (leaf eating), frugivory (fruit eating), nectarivory (nectar eating), gummivory (gum eating) and mycophagy (fungus eating). The digestive tract of an herbivore is host to bacteria that ferment these complex substances, and make them available for digestion, which are either housed in the multichambered stomach or in a large cecum. Some mammals are coprophagous, consuming feces to absorb the nutrients not digested when the food was first ingested.{{rp|131–137}} An omnivore eats both prey and plants. Carnivorous mammals have a simple digestive tract because the proteins, lipids and minerals found in meat require little in the way of specialised digestion. Exceptions to this include baleen whales who also house gut flora in a multi-chambered stomach, like terrestrial herbivores.{{cite journal | vauthors = Sanders JG, Beichman AC, Roman J, Scott JJ, Emerson D, McCarthy JJ, Girguis PR |author7-link= Peter Girguis | title = Baleen whales host a unique gut microbiome with similarities to both carnivores and herbivores | journal = Nature Communications | volume = 6 | pages = 8285 | date = September 2015 | pmid = 26393325 | pmc = 4595633 | doi = 10.1038/ncomms9285 | bibcode = 2015NatCo...6.8285S }}

The size of an animal is also a factor in determining diet type (Allen's rule). Since small mammals have a high ratio of heat-losing surface area to heat-generating volume, they tend to have high energy requirements and a high metabolic rate. Mammals that weigh less than about {{convert|18|oz|g+lb}} are mostly insectivorous because they cannot tolerate the slow, complex digestive process of an herbivore. Larger animals, on the other hand, generate more heat and less of this heat is lost. They can therefore tolerate either a slower collection process (carnivores that feed on larger vertebrates) or a slower digestive process (herbivores).{{cite journal |url=https://www.abdn.ac.uk/energetics-research/publications/pdf_docs/86.pdf |vauthors=Speaksman JR |year=1996 |title=Energetics and the evolution of body size in small terrestrial mammals |journal=Symposia of the Zoological Society of London |number=69 |pages=69–81 |access-date=31 May 2016 |archive-date=2 June 2021 |archive-url=https://web.archive.org/web/20210602065723/https://www.abdn.ac.uk/energetics-research/publications/pdf_docs/86.pdf |url-status=dead }} Furthermore, mammals that weigh more than {{convert|18|oz|g+lb}} usually cannot collect enough insects during their waking hours to sustain themselves. The only large insectivorous mammals are those that feed on huge colonies of insects (ants or termites).

{{Multiple image|image1=Blacky.jpg|image2=A polar bear (Ursus maritimus) scavenging a narwhal whale (Monodon monoceros) carcass - journal.pone.0060797.g001-A.png|footer=The hypocarnivorous American black bear (Ursus americanus) vs. the hypercarnivorous polar bear (Ursus maritimus)|direction=vertical}}

Some mammals are omnivores and display varying degrees of carnivory and herbivory, generally leaning in favour of one more than the other. Since plants and meat are digested differently, there is a preference for one over the other, as in bears where some species may be mostly carnivorous and others mostly herbivorous.{{cite journal| vauthors = Sacco T, van Valkenburgh B |year=2004|title=Ecomorphological indicators of feeding behaviour in the bears (Carnivora: Ursidae)|journal=Journal of Zoology|volume=263|issue=1|pages=41–54|doi=10.1017/S0952836904004856}} They are grouped into three categories: mesocarnivory (50–70% meat), hypercarnivory (70% and greater of meat), and hypocarnivory (50% or less of meat). The dentition of hypocarnivores consists of dull, triangular carnassial teeth meant for grinding food. Hypercarnivores, however, have conical teeth and sharp carnassials meant for slashing, and in some cases strong jaws for bone-crushing, as in the case of hyenas, allowing them to consume bones; some extinct groups, notably the Machairodontinae, had sabre-shaped canines.{{cite journal | vauthors = Van Valkenburgh B | title = Deja vu: the evolution of feeding morphologies in the Carnivora | journal = Integrative and Comparative Biology | volume = 47 | issue = 1 | pages = 147–163 | date = July 2007 | pmid = 21672827 | doi = 10.1093/icb/icm016 | doi-access = free }}

Some physiological carnivores consume plant matter and some physiological herbivores consume meat. From a behavioural aspect, this would make them omnivores, but from the physiological standpoint, this may be due to zoopharmacognosy. Physiologically, animals must be able to obtain both energy and nutrients from plant and animal materials to be considered omnivorous. Thus, such animals are still able to be classified as carnivores and herbivores when they are just obtaining nutrients from materials originating from sources that do not seemingly complement their classification.{{cite journal| vauthors = Singer MS, Bernays EA |year=2003| title=Understanding omnivory needs a behavioral perspective |journal=Ecology |volume=84 |issue=10 |pages=2532–2537 |doi=10.1890/02-0397|bibcode=2003Ecol...84.2532S |url=https://www.researchgate.net/publication/228805019}} For example, it is well documented that some ungulates such as giraffes, camels, and cattle, will gnaw on bones to consume particular minerals and nutrients.{{Cite journal| vauthors = Hutson JM, Burke CC, Haynes G |date=1 December 2013|title=Osteophagia and bone modifications by giraffe and other large ungulates|journal=Journal of Archaeological Science|volume=40|issue=12|pages=4139–4149|doi=10.1016/j.jas.2013.06.004|bibcode=2013JArSc..40.4139H }} Also, cats, which are generally regarded as obligate carnivores, occasionally eat grass to regurgitate indigestible material (such as hairballs), aid with haemoglobin production, and as a laxative.{{cite web|url=https://www.petmd.com/cat/wellness/evr_ct_eating_grass|title=Why Do Cats Eat Grass?|publisher=Pet MD|access-date=13 January 2017|archive-date=10 December 2016|archive-url=https://web.archive.org/web/20161210193104/http://www.petmd.com/cat/wellness/evr_ct_eating_grass|url-status=live}}

Many mammals, in the absence of sufficient food requirements in an environment, suppress their metabolism and conserve energy in a process known as hibernation.{{cite journal | vauthors = Geiser F | title = Metabolic rate and body temperature reduction during hibernation and daily torpor | journal = Annual Review of Physiology | volume = 66 | pages = 239–274 | year = 2004 | pmid = 14977403 | doi = 10.1146/annurev.physiol.66.032102.115105 | s2cid = 22397415 }} In the period preceding hibernation, larger mammals, such as bears, become polyphagic to increase fat stores, whereas smaller mammals prefer to collect and stash food.{{cite journal | vauthors = Humphries MM, Thomas DW, Kramer DL | title = The role of energy availability in Mammalian hibernation: a cost-benefit approach | journal = Physiological and Biochemical Zoology | volume = 76 | issue = 2 | pages = 165–179 | year = 2003 | pmid = 12794670 | doi = 10.1086/367950 | s2cid = 14675451 }} The slowing of the metabolism is accompanied by a decreased heart and respiratory rate, as well as a drop in internal temperatures, which can be around ambient temperature in some cases. For example, the internal temperatures of hibernating Arctic ground squirrels can drop to {{cvt|-2.9|C}}; however, the head and neck always stay above {{cvt|0|C}}.{{cite journal | vauthors = Barnes BM | title = Freeze avoidance in a mammal: body temperatures below 0 degree C in an Arctic hibernator | journal = Science | volume = 244 | issue = 4912 | pages = 1593–1595 | date = June 1989 | pmid = 2740905 | doi = 10.1126/science.2740905 | bibcode = 1989Sci...244.1593B }} A few mammals in hot environments aestivate in times of drought or extreme heat, for example the fat-tailed dwarf lemur (Cheirogaleus medius).{{cite book|chapter-url={{Google books|plainurl=yes|id=jukgezrNdCsC|page=99}}| veditors = Navas CA, Carvalho JE | vauthors = Fritz G |year=2010|title=Aestivation: Molecular and Physiological Aspects|volume=49|publisher=Springer-Verlag|pages=95–113|isbn=978-3-642-02420-7|chapter=Aestivation in Mammals and Birds|doi=10.1007/978-3-642-02421-4|series=Progress in Molecular and Subcellular Biology}}

{{excerpt|Drinking|In other land mammals}}

{{See also|Animal cognition}}

In intelligent mammals, such as primates, the cerebrum is larger relative to the rest of the brain. Intelligence itself is not easy to define, but indications of intelligence include the ability to learn, matched with behavioural flexibility. Rats, for example, are considered to be highly intelligent, as they can learn and perform new tasks, an ability that may be important when they first colonise a fresh habitat. In some mammals, food gathering appears to be related to intelligence: a deer feeding on plants has a brain smaller than a cat, which must think to outwit its prey.{{cite book | veditors = Wilson DE, Burnie D | title=Animal: The Definitive Visual Guide to the World's Wildlife| pages=86–89| publisher=DK Publishing| edition=| year =2001| isbn=978-0-7894-7764-4| oclc=46422124 }}

File:A Bonobo at the San Diego Zoo "fishing" for termites.jpg fishing for termites with a stick]]

Tool use by animals may indicate different levels of learning and cognition. The sea otter uses rocks as essential and regular parts of its foraging behaviour (smashing abalone from rocks or breaking open shells), with some populations spending 21% of their time making tools.{{cite journal | vauthors = Mann J, Patterson EM | title = Tool use by aquatic animals | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 368 | issue = 1630 | pages = 20120424 | date = November 2013 | pmid = 24101631 | pmc = 4027413 | doi = 10.1098/rstb.2012.0424 }} Other tool use, such as chimpanzees using twigs to "fish" for termites, may be developed by watching others use tools and may even be a true example of animal teaching.{{cite book| vauthors = Raffaele P |year=2011 |title=Among the Great Apes: Adventures on the Trail of Our Closest Relatives |publisher=Harper |page=83 |isbn=978-0-06-167184-5 |location=New York |oclc=674694369}} Tools may even be used in solving puzzles in which the animal appears to experience a "Eureka moment".{{cite book |url={{Google books|plainurl=yes|id=IIR8CgAAQBAJ}} | vauthors = Köhler W |year=1925 |publisher=Liveright |title=The Mentality of Apes |isbn=978-0-87140-108-3|oclc=2000769}} Other mammals that do not use tools, such as dogs, can also experience a Eureka moment.{{cite journal | vauthors = McGowan RT, Rehn T, Norling Y, Keeling LJ | title = Positive affect and learning: exploring the "Eureka Effect" in dogs | journal = Animal Cognition | volume = 17 | issue = 3 | pages = 577–587 | date = May 2014 | pmid = 24096703 | doi = 10.1007/s10071-013-0688-x | s2cid = 15216926 }}

Brain size was previously considered a major indicator of the intelligence of an animal. Since most of the brain is used for maintaining bodily functions, greater ratios of brain to body mass may increase the amount of brain mass available for more complex cognitive tasks. Allometric analysis indicates that mammalian brain size scales at approximately the {{frac|2|3}} or {{frac|3|4}} exponent of the body mass. Comparison of a particular animal's brain size with the expected brain size based on such allometric analysis provides an encephalisation quotient that can be used as another indication of animal intelligence.{{cite journal | vauthors = Karbowski J | title = Global and regional brain metabolic scaling and its functional consequences | journal = BMC Biology | volume = 5 | issue = 18 | pages = 18 | date = May 2007 | pmid = 17488526 | pmc = 1884139 | doi = 10.1186/1741-7007-5-18 | bibcode = 2007arXiv0705.2913K | arxiv = 0705.2913 | doi-access = free }} Sperm whales have the largest brain mass of any animal on earth, averaging {{convert|8000|cm3}} and {{convert|7.8|kg}} in mature males.{{cite journal | vauthors = Marino L | title = Cetacean brains: how aquatic are they? | journal = Anatomical Record | volume = 290 | issue = 6 | pages = 694–700 | date = June 2007 | pmid = 17516433 | doi = 10.1002/ar.20530 | s2cid = 27074107 | url = https://animalstudiesrepository.org/cgi/viewcontent.cgi?article=1006&context=anatom | doi-access = free | access-date = 5 October 2019 | archive-date = 20 March 2020 | archive-url = https://web.archive.org/web/20200320082257/https://animalstudiesrepository.org/cgi/viewcontent.cgi?article=1006&context=anatom | url-status = live }}

Self-awareness appears to be a sign of abstract thinking. Self-awareness, although not well-defined, is believed to be a precursor to more advanced processes such as metacognitive reasoning. The traditional method for measuring this is the mirror test, which determines if an animal possesses the ability of self-recognition.{{cite journal | vauthors = Gallop GG | title = Chimpanzees: self-recognition | journal = Science | volume = 167 | issue = 3914 | pages = 86–87 | date = January 1970 | pmid = 4982211 | doi = 10.1126/science.167.3914.86 | bibcode = 1970Sci...167...86G | s2cid = 145295899 }} Mammals that have passed the mirror test include Asian elephants (some pass, some do not);{{cite journal | vauthors = Plotnik JM, de Waal FB, Reiss D | title = Self-recognition in an Asian elephant | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 103 | issue = 45 | pages = 17053–17057 | date = November 2006 | pmid = 17075063 | pmc = 1636577 | doi = 10.1073/pnas.0608062103 | url = https://www.emory.edu/LIVING_LINKS/publications/articles/Plotnik_etal_2006.pdf | bibcode = 2006PNAS..10317053P | doi-access = free | access-date = 25 January 2024 | archive-date = 25 January 2024 | archive-url = https://web.archive.org/web/20240125191350/https://www.emory.edu/LIVING_LINKS/publications/articles/Plotnik_etal_2006.pdf | url-status = live }} chimpanzees;{{cite journal | vauthors = Robert S |title=Ontogeny of mirror behavior in two species of great apes |journal=American Journal of Primatology |volume=10 |issue=2 |pages=109–117 |year=1986 |doi=10.1002/ajp.1350100202 |pmid=31979488|s2cid=85330986 }} bonobos;{{cite journal| vauthors = Walraven V, van Elsacker L, Verheyen R |year=1995 |title=Reactions of a group of pygmy chimpanzees (Pan paniscus) to their mirror images: evidence of self-recognition |journal=Primates |volume=36 |pages=145–150 |doi=10.1007/bf02381922|s2cid=38985498 }} orangutans;{{cite book|chapter-url={{Google books |plainurl=yes |id=5ywsREEZ-j4C|page=150}} | vauthors = Leakey R |year=1994 |chapter=The Origin of the Mind |title=The Origin Of Humankind |location=New York |publisher=BasicBooks |page=150 |isbn=978-0-465-05313-1 |oclc=30739453}} humans, from 18 months (mirror stage);{{cite book | vauthors = Archer J |year=1992 |title=Ethology and Human Development |publisher=Rowman & Littlefield |pages=215–218 |isbn=978-0-389-20996-6 |oclc=25874476 |url={{Google books |plainurl=yes |id=QDT27k5envcC|page=217}}}} common bottlenose dolphins;{{efn|Decreased latency to approach the mirror, repetitious head circling and close viewing of the marked areas were considered signs of self-recognition since they do not have arms and cannot touch the marked areas.}}{{cite book |title=Self-awareness in Animals and Humans: Developmental Perspectives | vauthors = Marten K, Psarakos S |chapter=Evidence of self-awareness in the bottlenose dolphin (Tursiops truncatus) | veditors = Parker ST, Mitchell R, Boccia M |pages=361–379 |year=1995|location=Cambridge|publisher=Cambridge University Press |isbn=978-0-521-44108-7 |oclc=28180680}} orcas;{{cite journal | vauthors = Delfour F, Marten K | title = Mirror image processing in three marine mammal species: killer whales (Orcinus orca), false killer whales (Pseudorca crassidens) and California sea lions (Zalophus californianus) | journal = Behavioural Processes | volume = 53 | issue = 3 | pages = 181–190 | date = April 2001 | pmid = 11334706 | doi = 10.1016/s0376-6357(01)00134-6 | s2cid = 31124804 }} and false killer whales.

{{Main| Social animal}}

File:Borneo elephants.png

Eusociality is the highest level of social organisation. These societies have an overlap of adult generations, the division of reproductive labour and cooperative caring of young. Usually insects, such as bees, ants and termites, have eusocial behaviour, but it is demonstrated in two rodent species: the naked mole-rat{{cite journal | vauthors = Jarvis JU | title = Eusociality in a mammal: cooperative breeding in naked mole-rat colonies | journal = Science | volume = 212 | issue = 4494 | pages = 571–573 | date = May 1981 | pmid = 7209555 | doi = 10.1126/science.7209555 | bibcode = 1981Sci...212..571J | jstor = 1686202 | s2cid = 880054 }} and the Damaraland mole-rat.{{cite journal | vauthors = Jacobs DS, Bennett NC, Jarvis JU, Crowe TM | year=1991 | title= The colony structure and dominance hierarchy of the Damaraland mole-rat, Cryptomys damarensis (Rodentia: Bathyergidae) from Namibia | journal=Journal of Zoology | volume=224 | issue=4 | pages=553–576 | doi=10.1111/j.1469-7998.1991.tb03785.x }}

Presociality is when animals exhibit more than just sexual interactions with members of the same species, but fall short of qualifying as eusocial. That is, presocial animals can display communal living, cooperative care of young, or primitive division of reproductive labour, but they do not display all of the three essential traits of eusocial animals. Humans and some species of Callitrichidae (marmosets and tamarins) are unique among primates in their degree of cooperative care of young.{{cite book| vauthors = Hardy SB | year=2009| title=Mothers and Others: The Evolutionary Origins of Mutual Understanding| publisher=Belknap Press of Harvard University Press| pages=92–93| url={{Google books| plainurl=yes| id=dsiksDFQPDsC| page=92}}| location=Boston}} Harry Harlow set up an experiment with rhesus monkeys, presocial primates, in 1958; the results from this study showed that social encounters are necessary in order for the young monkeys to develop both mentally and sexually.{{cite journal | vauthors = Harlow HF, Suomi SJ | title = Social recovery by isolation-reared monkeys | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 68 | issue = 7 | pages = 1534–1538 | date = July 1971 | pmid = 5283943 | pmc = 389234 | doi = 10.1073/pnas.68.7.1534 | bibcode = 1971PNAS...68.1534H | doi-access = free }}

A fission–fusion society is a society that changes frequently in its size and composition, making up a permanent social group called the "parent group". Permanent social networks consist of all individual members of a community and often varies to track changes in their environment. In a fission–fusion society, the main parent group can fracture (fission) into smaller stable subgroups or individuals to adapt to environmental or social circumstances. For example, a number of males may break off from the main group in order to hunt or forage for food during the day, but at night they may return to join (fusion) the primary group to share food and partake in other activities. Many mammals exhibit this, such as primates (for example orangutans and spider monkeys),{{cite journal | vauthors = van Schaik CP | title = The socioecology of fission–fusion sociality in Orangutans | journal = Primates; Journal of Primatology | volume = 40 | issue = 1 | pages = 69–86 | date = January 1999 | pmid = 23179533 | doi = 10.1007/BF02557703 | s2cid = 13366732 }} elephants,{{cite journal | vauthors = Archie EA, Moss CJ, Alberts SC | title = The ties that bind: genetic relatedness predicts the fission and fusion of social groups in wild African elephants | journal = Proceedings. Biological Sciences | volume = 273 | issue = 1586 | pages = 513–522 | date = March 2006 | pmid = 16537121 | pmc = 1560064 | doi = 10.1098/rspb.2005.3361 }} spotted hyenas,{{cite journal| vauthors = Smith JE, Memenis SK, Holekamp KE | title=Rank-related partner choice in the fission–fusion society of the spotted hyena (Crocuta crocuta)| journal=Behavioral Ecology and Sociobiology| year=2007| volume=61| issue=5| pages=753–765| doi=10.1007/s00265-006-0305-y| bibcode=2007BEcoS..61..753S| s2cid=24927919| url=https://www.mills.edu/academics/faculty/bio/jesmith/partner.pdf| url-status=dead| archive-url=https://web.archive.org/web/20140425002800/http://www.mills.edu/academics/faculty/bio/jesmith/partner.pdf| archive-date=25 April 2014}} lions,{{cite journal | vauthors = Matoba T, Kutsukake N, Hasegawa T | title = Head rubbing and licking reinforce social bonds in a group of captive African lions, Panthera leo | journal = PLOS ONE | volume = 8 | issue = 9 | pages = e73044 | year = 2013 | pmid = 24023806 | pmc = 3762833 | doi = 10.1371/journal.pone.0073044 | bibcode = 2013PLoSO...873044M | veditors = Hayward M | doi-access = free }} and dolphins.{{cite journal | vauthors = Krützen M, Barré LM, Connor RC, Mann J, Sherwin WB | title = 'O father: where art thou?' – Paternity assessment in an open fission–fusion society of wild bottlenose dolphins (Tursiops sp.) in Shark Bay, Western Australia | journal = Molecular Ecology | volume = 13 | issue = 7 | pages = 1975–1990 | date = July 2004 | pmid = 15189218 | doi = 10.1111/j.1365-294X.2004.02192.x | bibcode = 2004MolEc..13.1975K | s2cid = 4510393 }}

Solitary animals defend a territory and avoid social interactions with the members of its species, except during breeding season. This is to avoid resource competition, as two individuals of the same species would occupy the same niche, and to prevent depletion of food.{{cite book| url={{Google books | plainurl=yes | id=Nb21BwAAQBAJ| page=114}} | vauthors = Martin C | year=1991 | title=The Rainforests of West Africa: Ecology – Threats – Conservation | publisher=Springer | doi=10.1007/978-3-0348-7726-8| isbn=978-3-0348-7726-8}} A solitary animal, while foraging, can also be less conspicuous to predators or prey.{{cite journal | vauthors = le Roux A, Cherry MI, Gygax L | title=Vigilance behaviour and fitness consequences: comparing a solitary foraging and an obligate group-foraging mammal| journal=Behavioral Ecology and Sociobiology | date=5 May 2009 | volume=63 | issue=8 | pages=1097–1107 | doi=10.1007/s00265-009-0762-1| bibcode=2009BEcoS..63.1097L| s2cid=21961356 }}

File:Fighting red kangaroos 2.jpgs "boxing" for dominance]]

In a hierarchy, individuals are either dominant or submissive. A despotic hierarchy is where one individual is dominant while the others are submissive, as in wolves and lemurs,{{cite journal | vauthors = Palagi E, Norscia I | title = The Season for Peace: Reconciliation in a Despotic Species (Lemur catta) | journal = PLOS ONE | volume = 10 | issue = 11 | pages = e0142150 | year = 2015 | pmid = 26569400 | pmc = 4646466 | doi = 10.1371/journal.pone.0142150 | bibcode = 2015PLoSO..1042150P | veditors = Samonds KE | doi-access = free }} and a pecking order is a linear ranking of individuals where there is a top individual and a bottom individual. Pecking orders may also be ranked by sex, where the lowest individual of a sex has a higher ranking than the top individual of the other sex, as in hyenas.{{cite journal| vauthors = East ML, Hofer H | year=2000 | title=Male spotted hyenas (Crocuta crocuta) queue for status in social groups dominated by females | journal=Behavioral Ecology | volume=12 | issue=15| pages=558–568 | doi=10.1093/beheco/12.5.558| doi-access=free }} Dominant individuals, or alphas, have a high chance of reproductive success, especially in harems where one or a few males (resident males) have exclusive breeding rights to females in a group.{{cite journal |author2-link=Joan Silk | vauthors = Samuels A, Silk JB, Rodman P | year=1984 | title=Changes in the dominance rank and reproductive behavior of male bonnet macaques (Macaca radiate)| journal=Animal Behaviour | volume=32 | issue=4 | pages=994–1003 | doi=10.1016/s0003-3472(84)80212-2| s2cid=53186523 }} Non-resident males can also be accepted in harems, but some species, such as the common vampire bat (Desmodus rotundus), may be more strict.{{cite journal | vauthors = Delpietro HA, Russo RG | year=2002 | title=Observations of the common vampire bat (Desmodus rotundus) and the hairy-legged vampire bat (Diphylla ecaudata) in captivity | journal=Mammalian Biology | volume=67 | issue=2 | pages=65–78 | doi=10.1078/1616-5047-00011| bibcode=2002MamBi..67...65D }}

Some mammals are perfectly monogamous, meaning that they mate for life and take no other partners (even after the original mate's death), as with wolves, Eurasian beavers, and otters.{{cite journal | vauthors = Kleiman DG | title = Monogamy in mammals | journal = The Quarterly Review of Biology | volume = 52 | issue = 1 | pages = 39–69 | date = March 1977 | pmid = 857268 | doi = 10.1086/409721 | s2cid = 25675086 }}{{cite journal | vauthors = Holland B, Rice WR | journal = Evolution; International Journal of Organic Evolution | volume = 52 | issue = 1 | pages = 1–7 | date = February 1998 | pmid = 28568154 | doi = 10.2307/2410914 | url = https://wolfweb.unr.edu/homepage/jaz/eecb752/lecture06/Holland%26Rice1998.pdf | jstor = 2410914 | title = Perspective: Chase-Away Sexual Selection: Antagonistic Seduction Versus Resistance | access-date = 8 July 2016 | archive-url = https://web.archive.org/web/20190608065427/https://wolfweb.unr.edu/homepage/jaz/eecb752/lecture06/Holland%26Rice1998.pdf | archive-date = 8 June 2019 | url-status = dead }} There are three types of polygamy: either one or multiple dominant males have breeding rights (polygyny), multiple males that females mate with (polyandry), or multiple males have exclusive relations with multiple females (polygynandry). It is much more common for polygynous mating to happen, which, excluding leks, are estimated to occur in up to 90% of mammals.{{cite journal | vauthors = Clutton-Brock TH | title = Mammalian mating systems | journal = Proceedings of the Royal Society of London. Series B, Biological Sciences | volume = 236 | issue = 1285 | pages = 339–372 | date = May 1989 | pmid = 2567517 | doi = 10.1098/rspb.1989.0027 | bibcode = 1989RSPSB.236..339C | s2cid = 84780662 | url = https://zenodo.org/record/8204699 }} Lek mating occurs when males congregate around females and try to attract them with various courtship displays and vocalisations, as in harbour seals.{{cite journal| vauthors = Boness DJ, Bowen D, Buhleier BM, Marshall GJ | year=2006 | title=Mating tactics and mating system of an aquatic-mating pinniped: the harbor seal, Phoca vitulina | journal=Behavioral Ecology and Sociobiology | volume=61 | issue=1 | pages=119–130 | doi=10.1007/s00265-006-0242-9 | bibcode=2006BEcoS..61..119B | s2cid=25266746 | url=https://www.researchgate.net/publication/226692255}}

All higher mammals (excluding monotremes) share two major adaptations for care of the young: live birth and lactation. These imply a group-wide choice of a degree of parental care. They may build nests and dig burrows to raise their young in, or feed and guard them often for a prolonged period of time. Many mammals are K-selected, and invest more time and energy into their young than do r-selected animals. When two animals mate, they both share an interest in the success of the offspring, though often to different extremes. Mammalian females exhibit some degree of maternal aggression, another example of parental care, which may be targeted against other females of the species or the young of other females; however, some mammals may "aunt" the infants of other females, and care for them. Mammalian males may play a role in child rearing, as with tenrecs, however this varies species to species, even within the same genus. For example, the males of the southern pig-tailed macaque (Macaca nemestrina) do not participate in child care, whereas the males of the Japanese macaque (M. fuscata) do.{{cite book|chapter-url={{Google books|plainurl=yes|id=ipjhBwAAQBAJ|page=1}}|chapter=Origins of Parental Care| vauthors = Klopfer PH |year=1981 | veditors = Gubernick DJ |title=Parental Care in Mammals|publisher=Plenum Press|location=New York|isbn=978-1-4613-3150-6|oclc=913709574}}

{{Main|Human uses of mammals}}

File:Lascaux painting.jpg cave painting of a variety of large mammals, Lascaux, {{circa|17,300}} years old]]

Non-human mammals play a wide variety of roles in human culture. They are the most popular of pets, with tens of millions of dogs, cats and other animals including rabbits and mice kept by families around the world.{{cite journal | vauthors = Murthy R, Bearman G, Brown S, Bryant K, Chinn R, Hewlett A, George BG, Goldstein EJ, Holzmann-Pazgal G, Rupp ME, Wiemken T, Weese JS, Weber DJ | display-authors = 6 | title = Animals in healthcare facilities: recommendations to minimize potential risks | journal = Infection Control and Hospital Epidemiology | volume = 36 | issue = 5 | pages = 495–516 | date = May 2015 | pmid = 25998315 | doi = 10.1017/ice.2015.15 |doi-access=free | s2cid = 541760 | url = https://www.cambridge.org/core/services/aop-cambridge-core/content/view/7086725BAB2AAA4C1949DA5B90F06F3B/S0899823X1500015Xa.pdf/div-class-title-animals-in-healthcare-facilities-recommendations-to-minimize-potential-risks-div.pdf |url-status=live |archive-url=https://web.archive.org/web/20231103092334/https://www.cambridge.org/core/services/aop-cambridge-core/content/view/7086725BAB2AAA4C1949DA5B90F06F3B/S0899823X1500015Xa.pdf/div-class-title-animals-in-healthcare-facilities-recommendations-to-minimize-potential-risks-div.pdf |archive-date=3 November 2023 }}{{cite web |last=The Humane Society of the United States |title=U.S. Pet Ownership Statistics |url=https://www.humanesociety.org/issues/pet_overpopulation/facts/pet_ownership_statistics.html |access-date=27 April 2012 |archive-date=7 April 2012 |archive-url=https://web.archive.org/web/20120407193941/http://www.humanesociety.org/issues/pet_overpopulation/facts/pet_ownership_statistics.html |url-status=dead }}{{cite web |last=USDA |title=U.S. Rabbit Industry profile |url=https://www.aphis.usda.gov/animal_health/emergingissues/downloads/RabbitReport1.pdf |access-date=10 July 2013 |archive-url=https://web.archive.org/web/20190807115557/http://www.aphis.usda.gov/animal_health/emergingissues/downloads/RabbitReport1.pdf |archive-date=7 August 2019 |url-status=dead }} Mammals such as mammoths, horses and deer are among the earliest subjects of art, being found in Upper Paleolithic cave paintings such as at Lascaux.{{Cite news |vauthors=McKie R |title=Prehistoric cave art in the Dordogne |url=https://www.theguardian.com/travel/2013/may/26/prehistoric-cave-art-dordogne |newspaper=The Guardian |access-date=9 November 2016 |date=26 May 2013 |archive-date=31 May 2021 |archive-url=https://web.archive.org/web/20210531050457/https://www.theguardian.com/travel/2013/may/26/prehistoric-cave-art-dordogne |url-status=live }} Major artists such as Albrecht Dürer, George Stubbs and Edwin Landseer are known for their portraits of mammals.{{cite news |vauthors=Jones J |title=The top 10 animal portraits in art |url=https://www.theguardian.com/artanddesign/jonathanjonesblog/2014/jun/27/top-10-animal-portraits-in-art |access-date=24 June 2016 |agency=The Guardian |date=27 June 2014 |archive-date=18 May 2016 |archive-url=https://web.archive.org/web/20160518105922/http://www.theguardian.com/artanddesign/jonathanjonesblog/2014/jun/27/top-10-animal-portraits-in-art |url-status=live }} Many species of mammals have been hunted for sport and for food; deer and wild boar are especially popular as game animals.{{cite web | title=Deer Hunting in the United States: An Analysis of Hunter Demographics and Behavior Addendum to the 2001 National Survey of Fishing, Hunting, and Wildlife-Associated Recreation Report 2001-6 | url=https://digitalmedia.fws.gov/cdm/ref/collection/document/id/292 | publisher=Fishery and Wildlife Service (US) | access-date=24 June 2016 | archive-date=13 August 2016 | archive-url=https://web.archive.org/web/20160813200949/http://digitalmedia.fws.gov/cdm/ref/collection/document/id/292 | url-status=live }}{{cite web | vauthors = Shelton L | work = The Natchez Democrat | title=Recreational Hog Hunting Popularity Soaring | url=https://www.grandviewoutdoors.com/big-game-hunting/recreational-hog-hunting-popularity-soaring/ | publisher=Grand View Outdoors | access-date=24 June 2016 | date=5 April 2014 | archive-url=https://web.archive.org/web/20171212193350/https://www.grandviewoutdoors.com/big-game-hunting/recreational-hog-hunting-popularity-soaring/ | archive-date=12 December 2017 | url-status=dead }}{{cite book | vauthors = Nguyen J, Wheatley R | title=Hunting For Food: Guide to Harvesting, Field Dressing and Cooking Wild Game | url={{Google books|plainurl=yes|id=3XN6CgAAQBAJ|page=6}} | year=2015 | publisher=F+W Media | isbn=978-1-4403-3856-4 | pages=6–77}} Chapters on hunting deer, wild hog (boar), rabbit, and squirrel. Mammals such as horses and dogs are widely raced for sport, often combined with betting on the outcome.{{cite encyclopedia |title=Horse racing | encyclopedia = The Encyclopædia Britannica |url= https://www.britannica.com/EBchecked/topic/272329/horse-racing |access-date=6 May 2014 |archive-url=https://web.archive.org/web/20131221033444/https://www.britannica.com/EBchecked/topic/272329/horse-racing |archive-date= 21 December 2013}}{{cite book | vauthors = Genders R |title=Encyclopaedia of Greyhound Racing |year=1981 |publisher=Pelham Books |isbn=978-0-7207-1106-6|oclc=9324926}} There is a tension between the role of animals as companions to humans, and their existence as individuals with rights of their own.{{cite journal |title=The Role of Animals in Human Society | vauthors = Plous S |date=1993 |doi=10.1111/j.1540-4560.1993.tb00906.x |journal=Journal of Social Issues |volume=49 |issue=1 |pages=1–9}} Mammals further play a wide variety of roles in literature,{{Cite news|vauthors=Fowler KJ|title=Top 10 books about intelligent animals|url=https://www.theguardian.com/books/2014/mar/26/top-10-books-intelligent-animals-watership-down-animal-farm|newspaper=The Guardian|access-date=9 November 2016|date=26 March 2014|archive-date=28 May 2021|archive-url=https://web.archive.org/web/20210528154902/https://www.theguardian.com/books/2014/mar/26/top-10-books-intelligent-animals-watership-down-animal-farm|url-status=live}}{{cite book | vauthors = Gamble N, Yates S |title=Exploring Children's Literature |year=2008 |publisher=Sage |isbn=978-1-4129-3013-0 |edition=2nd |location=Los Angeles |oclc=71285210}}{{cite web|title=Books for Adults|url=https://www.sealsitters.org/marine_mammals/reading.html|website=Seal Sitters|access-date=9 November 2016|archive-date=11 July 2016|archive-url=https://web.archive.org/web/20160711034305/http://www.sealsitters.org/marine_mammals/reading.html|url-status=live}} film,{{cite journal | vauthors = Paterson J |title=Animals in Film and Media |journal=Oxford Bibliographies |year=2013 |doi=10.1093/obo/9780199791286-0044}} mythology, and religion.{{cite book | vauthors = Johns C |date=2011 |title=Cattle: History, Myth, Art |publisher=The British Museum Press |isbn=978-0-7141-5084-0 |location=London |oclc=665137673}}{{cite book |title=Hayagrīva: The Mantrayānic Aspect of Horse-cult in China and Japan |publisher=Brill Archive |page=9 | vauthors = van Gulik RH }}{{cite web| vauthors = Grainger R |title=Lion Depiction across Ancient and Modern Religions |url=https://lionalert.org/page/Lion_Depiction_Across_Ancient_and_Modern_Religions |publisher=ALERT |access-date=6 November 2016 |date=24 June 2012 |url-status=dead |archive-url=https://web.archive.org/web/20160923134807/https://lionalert.org/page/Lion_Depiction_Across_Ancient_and_Modern_Religions |archive-date=23 September 2016 }}

{{See also|Livestock|Laboratory animal|Pack animal}}

File:Hand milking a cow at Cobbes Farm Museum.jpg have been kept for milk for thousands of years.]]

The domestication of mammals was instrumental in the Neolithic development of agriculture and of civilisation, causing farmers to replace hunter-gatherers around the world.{{efn|Diamond discussed this matter further in his 1997 book Guns, Germs, and Steel.{{cite book | vauthors = Diamond JM |author-link=Jared Diamond |year=1997 |title=Guns, Germs, and Steel: the Fates of Human Societies|chapter=Part 2: The rise and spread of food production |location=New York |publisher=W.W. Norton & Company|isbn=978-0-393-03891-0 |oclc=35792200 |chapter-url={{Google books|plainurl=yes |id=PWnWRFEGoeUC |page=176 }}}}}}{{cite journal | vauthors = Larson G, Burger J | title = A population genetics view of animal domestication | journal = Trends in Genetics | volume = 29 | issue = 4 | pages = 197–205 | date = April 2013 | pmid = 23415592 | doi = 10.1016/j.tig.2013.01.003 | url = https://www.palaeobarn.com/sites/domestication.org.uk/files/downloads/98.pdf | access-date = 9 November 2016 | archive-date = 8 June 2019 | archive-url = https://web.archive.org/web/20190608065300/http://www.palaeobarn.com/sites/domestication.org.uk/files/downloads/98.pdf | url-status = dead }} This transition from hunting and gathering to herding flocks and growing crops was a major step in human history. The new agricultural economies, based on domesticated mammals, caused "radical restructuring of human societies, worldwide alterations in biodiversity, and significant changes in the Earth's landforms and its atmosphere... momentous outcomes".{{cite journal | vauthors = Zeder MA | title = Domestication and early agriculture in the Mediterranean Basin: Origins, diffusion, and impact | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 105 | issue = 33 | pages = 11597–11604 | date = August 2008 | pmid = 18697943 | pmc = 2575338 | doi = 10.1073/pnas.0801317105 | bibcode = 2008PNAS..10511597Z | doi-access = free }}

Domestic mammals form a large part of the livestock raised for meat across the world. They include (2009) around 1.4 billion cattle, 1 billion sheep, 1 billion domestic pigs,{{Cite news |title=Graphic detail Charts, maps and infographics. Counting chickens |newspaper=The Economist |url=https://www.economist.com/blogs/dailychart/2011/07/global-livestock-counts |access-date=6 November 2016 |date=27 July 2011 |archive-date=15 July 2016 |archive-url=https://web.archive.org/web/20160715181213/http://www.economist.com/blogs/dailychart/2011/07/global-livestock-counts |url-status=live }}{{cite web |work=Cattle Today |url=https://cattle-today.com/ |title=Breeds of Cattle at CATTLE TODAY |publisher=Cattle-today.com |access-date=6 November 2016 |archive-date=15 July 2011 |archive-url=https://web.archive.org/web/20110715234745/https://cattle-today.com/ |url-status=live }} and (1985) over 700 million rabbits.{{cite web |vauthors=Lukefahr SD, Cheeke PR |title=Rabbit project development strategies in subsistence farming systems |url=https://www.fao.org/docrep/U4900T/u4900T0m.htm |publisher=Food and Agriculture Organization |access-date=6 November 2016 |archive-date=6 May 2016 |archive-url=https://web.archive.org/web/20160506105314/https://www.fao.org/docrep/U4900T/u4900T0m.htm |url-status=live }} Working domestic animals including cattle and horses have been used for work and transport from the origins of agriculture, their numbers declining with the arrival of mechanised transport and agricultural machinery. In 2004 they still provided some 80% of the power for the mainly small farms in the third world, and some 20% of the world's transport, again mainly in rural areas. In mountainous regions unsuitable for wheeled vehicles, pack animals continue to transport goods.{{cite book |vauthors=Pond WG |title=Encyclopedia of Animal Science |url=https://books.google.com/books?id=1SQl7Ao3mHoC&pg=PA248 |year=2004 |publisher=CRC Press |oclc=57033325 |isbn=978-0-8247-5496-9 |pages=248–250 |access-date=5 October 2018 |archive-date=23 January 2023 |archive-url=https://web.archive.org/web/20230123110032/https://books.google.com/books?id=1SQl7Ao3mHoC&pg=PA248 |url-status=live }} Mammal skins provide leather for shoes, clothing and upholstery. Wool from mammals including sheep, goats and alpacas has been used for centuries for clothing.{{cite book | vauthors = Braaten AW |title=Encyclopedia of Clothing and Fashion |year=2005 |volume=3 |publisher=Thomson Gale |isbn=978-0-684-31394-8 |oclc=963977000 |pages=[https://archive.org/details/encyclopediaofcl00vale/page/441 441–443] | veditors = Steele V |chapter=Wool |chapter-url=https://archive.org/details/encyclopediaofcl00vale/page/441 }}{{cite journal | vauthors = Quiggle C | title = Alpaca: An Ancient Luxury | journal = Interweave Knits | date =Fall 2000 | pages = 74–76 }}

File:Distribution of Mammals on Earth.png |first=Hannah |last=Ritchie |author-link=Hannah Ritchie}}]]

Mammals serve a major role in science as experimental animals, both in fundamental biological research, such as in genetics,{{cite web |title=Genetics Research |url=https://www.aht.org.uk/cms-display/genetics.html |publisher=Animal Health Trust |access-date=6 November 2016 |archive-url=https://web.archive.org/web/20171212193051/http://www.aht.org.uk/cms-display/genetics.html |archive-date=12 December 2017 |url-status=dead }} and in the development of new medicines, which must be tested exhaustively to demonstrate their safety.{{cite web |title=Drug Development |url=https://www.animalresearch.info/en/drug-development/ |publisher=Animal Research.info |access-date=6 November 2016 |archive-date=8 June 2016 |archive-url=https://web.archive.org/web/20160608124406/https://www.animalresearch.info/en/drug-development/ |url-status=live }} Millions of mammals, especially mice and rats, are used in experiments each year.{{cite web |title=EU statistics show decline in animal research numbers |url=https://speakingofresearch.com/2013/12/12/eu-statistics-show-decline-in-animal-research-numbers/ |publisher=Speaking of Research |year=2013 |access-date=6 November 2016 |archive-date=24 April 2019 |archive-url=https://web.archive.org/web/20190424155141/https://speakingofresearch.com/2013/12/12/eu-statistics-show-decline-in-animal-research-numbers/ |url-status=live }} A knockout mouse is a genetically modified mouse with an inactivated gene, replaced or disrupted with an artificial piece of DNA. They enable the study of sequenced genes whose functions are unknown.{{cite journal |vauthors=Pilcher HR |url=https://www.nature.com/news/1998/030512/full/news030512-17.html |title=It's a knockout |journal=Nature |date=2003 |access-date=6 November 2016 |doi=10.1038/news030512-17 |archive-date=10 November 2016 |archive-url=https://web.archive.org/web/20161110084106/http://www.nature.com/news/1998/030512/full/news030512-17.html |url-status=live |url-access=subscription }} A small percentage of the mammals are non-human primates, used in research for their similarity to humans.{{Cite web | url=https://www.ebra.org/ebrabulletin-the-supply-and-use-of-primates-in-the-eu_17.htm | title=The supply and use of primates in the EU | year=1996 | publisher=European Biomedical Research Association | archive-url=https://web.archive.org/web/20120117061036/http://www.ebra.org/ebrabulletin-the-supply-and-use-of-primates-in-the-eu_17.htm | archive-date=17 January 2012}}{{cite journal | vauthors = Carlsson HE, Schapiro SJ, Farah I, Hau J | title = Use of primates in research: a global overview | journal = American Journal of Primatology | volume = 63 | issue = 4 | pages = 225–237 | date = August 2004 | pmid = 15300710 | doi = 10.1002/ajp.20054 | s2cid = 41368228 }}{{Cite report| vauthors = Weatherall D |display-authors=etal |year=2006 |title=The use of non-human primates in research |location=London |publisher=Academy of Medical Sciences |url=https://www.acmedsci.ac.uk/images/project/nhpdownl.pdf |url-status=dead |archive-url=https://web.archive.org/web/20130323084639/http://www.acmedsci.ac.uk/images/project/nhpdownl.pdf |archive-date=23 March 2013 }}

Despite the benefits domesticated mammals had for human development, humans have an increasingly detrimental effect on wild mammals across the world. It has been estimated that the mass of all wild mammals has declined to only 4% of all mammals, with 96% of mammals being humans and their livestock now (see figure). In fact, terrestrial wild mammals make up only 2% of all mammals.{{Cite journal |vauthors=Ritchie H, Roser M |date=15 April 2021 |title=Biodiversity |url=https://ourworldindata.org/mammals |journal=Our World in Data |access-date=29 August 2021 |archive-date=11 December 2022 |archive-url=https://web.archive.org/web/20221211132929/https://ourworldindata.org/mammals |url-status=live }}{{cite journal | vauthors = Bar-On YM, Phillips R, Milo R | title = The biomass distribution on Earth | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 115 | issue = 25 | pages = 6506–6511 | date = June 2018 | pmid = 29784790 | pmc = 6016768 | doi = 10.1073/pnas.1711842115 | bibcode = 2018PNAS..115.6506B | doi-access = free }}

{{Main|Hybrid (biology)}}

{{Multiple image

|image1=Equus quagga quagga, coloured.jpg

|image2=Rau Quagga on Devils Peak.jpg

|width=200

|footer=A true quagga, 1870 (left) vs. a bred-back quagga, 2014 (right)

}}

Hybrids are offspring resulting from the breeding of two genetically distinct individuals, which usually will result in a high degree of heterozygosity, though hybrid and heterozygous are not synonymous. The deliberate or accidental hybridising of two or more species of closely related animals through captive breeding is a human activity which has been in existence for millennia and has grown for economic purposes.{{cite book | vauthors = Price E |year=2008 |title=Principles and applications of domestic animal behavior: an introductory text |publisher=Cambridge University Press |url={{Google books|plainurl=yes|id=Ww07sIWTYAAC|page=228}}|location=Sacramento |isbn=978-1-84593-398-2|oclc=226038028}} Hybrids between different subspecies within a species (such as between the Bengal tiger and Siberian tiger) are known as intra-specific hybrids. Hybrids between different species within the same genus (such as between lions and tigers) are known as interspecific hybrids or crosses. Hybrids between different genera (such as between sheep and goats) are known as intergeneric hybrids.{{cite book|url={{Google books |plainurl=yes |id=O3M4qfxtGhIC|page=13}} |location=Heidelberg | vauthors = Taupitz J, Weschka M |year=2009|title=Chimbrids – Chimeras and Hybrids in Comparative European and International Research |publisher=Springer |page=13 |isbn=978-3-540-93869-9 |oclc=495479133}} Natural hybrids will occur in hybrid zones, where two populations of species within the same genera or species living in the same or adjacent areas will interbreed with each other. Some hybrids have been recognised as species, such as the red wolf (though this is controversial).{{cite journal |title=An account of the taxonomy of North American wolves from morphological and genetic analyses |year=2012 |journal=North American Fauna |volume=77 |page=2 |doi=10.3996/nafa.77.0001 |vauthors=Chambers SM, Fain SR, Fazio B, Amaral M |url=https://digital.library.unt.edu/ark:/67531/metadc700981/ |doi-access=free |access-date=12 October 2019 |archive-date=31 May 2021 |archive-url=https://web.archive.org/web/20210531050233/https://digital.library.unt.edu/ark:/67531/metadc700981/ |url-status=live }}

Artificial selection, the deliberate selective breeding of domestic animals, is being used to breed back recently extinct animals in an attempt to achieve an animal breed with a phenotype that resembles that extinct wildtype ancestor. A breeding-back (intraspecific) hybrid may be very similar to the extinct wildtype in appearance, ecological niche and to some extent genetics, but the initial gene pool of that wild type is lost forever with its extinction. As a result, bred-back breeds are at best vague look-alikes of extinct wildtypes, as Heck cattle are of the aurochs.{{cite book | vauthors = van Vuure T |title=Retracing the Aurochs – History, Morphology and Ecology of an extinct wild Ox |year=2005 |isbn=978-954-642-235-4 |publisher=Pensoft Publishers |oclc=940879282}}

Purebred wild species evolved to a specific ecology can be threatened with extinction{{cite journal | vauthors = Mooney HA, Cleland EE | title = The evolutionary impact of invasive species | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 98 | issue = 10 | pages = 5446–5451 | date = May 2001 | pmid = 11344292 | pmc = 33232 | doi = 10.1073/pnas.091093398 | bibcode = 2001PNAS...98.5446M | doi-access = free }} through the process of genetic pollution, the uncontrolled hybridisation, introgression genetic swamping which leads to homogenisation or out-competition from the heterosic hybrid species.{{cite journal | vauthors = Le Roux JJ, Foxcroft LC, Herbst M, MacFadyen S | title = Genetic analysis shows low levels of hybridization between African wildcats (Felis silvestris lybica) and domestic cats (F. s. catus) in South Africa | journal = Ecology and Evolution | volume = 5 | issue = 2 | pages = 288–299 | date = January 2015 | pmid = 25691958 | pmc = 4314262 | doi = 10.1002/ece3.1275| bibcode = 2015EcoEv...5..288L }} When new populations are imported or selectively bred by people, or when habitat modification brings previously isolated species into contact, extinction in some species, especially rare varieties, is possible.{{cite book| vauthors = Wilson A |title= Australia's state of the forests report |page=107 |year=2003}} Interbreeding can swamp the rarer gene pool and create hybrids, depleting the purebred gene pool. For example, the endangered wild water buffalo is most threatened with extinction by genetic pollution from the domestic water buffalo. Such extinctions are not always apparent from a morphological standpoint. Some degree of gene flow is a normal evolutionary process, nevertheless, hybridisation threatens the existence of rare species.{{cite journal |title= Extinction by Hybridization and Introgression | vauthors = Rhymer JM, Simberloff D |journal = Annual Review of Ecology and Systematics |date=November 1996 |volume= 27 | issue = 1 |pages= 83–109 |doi= 10.1146/annurev.ecolsys.27.1.83 | bibcode = 1996AnRES..27...83R }}{{cite book | vauthors = Potts BM | veditors = Barbour RC, Hingston AB |year=2001 |title=Genetic pollution from farm forestry using eucalypt species and hybrids: a report for the RIRDC/L&WA/FWPRDC Joint Venture Agroforestry Program |publisher= Rural Industrial Research and Development Corporation of Australia|isbn= 978-0-642-58336-9|oclc=48794104}}

{{See also|Holocene extinction}}

File:Extinctions Africa Austrailia NAmerica Madagascar.gif

The loss of species from ecological communities, defaunation, is primarily driven by human activity. This has resulted in empty forests, ecological communities depleted of large vertebrates.{{Cite book|title=Essentials of Conservation Biology | vauthors = Primack R |publisher=Sinauer Associates, Inc. Publishers |year=2014 |isbn=978-1-60535-289-3 |location=Sunderland, MA |pages=217–245 |edition=6th |oclc=876140621}}{{cite journal | vauthors = Vignieri S | title = Vanishing fauna. Introduction | journal = Science | volume = 345 | issue = 6195 | pages = 392–395 | date = July 2014 | pmid = 25061199 | doi = 10.1126/science.345.6195.392 | doi-access = free | bibcode = 2014Sci...345..392V }} In the Quaternary extinction event, the mass die-off of megafaunal variety coincided with the appearance of humans, suggesting a human influence. One hypothesis is that humans hunted large mammals, such as the woolly mammoth, into extinction.{{cite journal | vauthors = Burney DA, Flannery TF | title = Fifty millennia of catastrophic extinctions after human contact | journal = Trends in Ecology & Evolution | volume = 20 | issue = 7 | pages = 395–401 | date = July 2005 | pmid = 16701402 | doi = 10.1016/j.tree.2005.04.022 | url = https://www.anthropology.hawaii.edu/Fieldschools/Kauai/Publications/Publication%204.pdf | url-status = dead | archive-url = https://web.archive.org/web/20100610061434/http://www.anthropology.hawaii.edu/Fieldschools/Kauai/Publications/Publication%204.pdf | archive-date = 10 June 2010 }}{{cite book | vauthors = Diamond J | year=1984 |chapter=Historic extinctions: a Rosetta stone for understanding prehistoric extinctions |title=Quaternary extinctions: A prehistoric revolution | veditors = Martin PS, Klein RG | pages=824–862 | location=Tucson| publisher=University of Arizona Press | isbn=978-0-8165-1100-6|oclc=10301944}} The 2019 Global Assessment Report on Biodiversity and Ecosystem Services by IPBES states that the total biomass of wild mammals has declined by 82 per cent since the beginning of human civilisation.{{cite news |vauthors=Watts J |date=6 May 2019 |title=Human society under urgent threat from loss of Earth's natural life |url=https://www.theguardian.com/environment/2019/may/06/human-society-under-urgent-threat-loss-earth-natural-life-un-report |work=The Guardian |access-date=1 July 2019 |archive-date=14 June 2019 |archive-url=https://web.archive.org/web/20190614160705/https://www.theguardian.com/environment/2019/may/06/human-society-under-urgent-threat-loss-earth-natural-life-un-report |url-status=live }}{{cite news|vauthors=McGrath M|date=6 May 2019|title=Nature crisis: Humans 'threaten 1m species with extinction'|url=https://www.bbc.com/news/science-environment-48169783|work=BBC|access-date=1 July 2019|archive-date=30 June 2019|archive-url=https://web.archive.org/web/20190630044916/https://www.bbc.com/news/science-environment-48169783|url-status=live}} Wild animals make up just 4% of mammalian biomass on earth, while humans and their domesticated animals make up 96%.

Various species are predicted to become extinct in the near future,{{cite web | vauthors = Main D | url = https://www.livescience.com/41421-animals-threatened-with-extinction.html | title = 7 Iconic Animals Humans Are Driving to Extinction | work = Live Science | date = 22 November 2013 | access-date = 25 January 2024 | archive-date = 6 January 2023 | archive-url = https://web.archive.org/web/20230106233208/https://www.livescience.com/41421-animals-threatened-with-extinction.html | url-status = live }} among them the rhinoceros,{{cite web | url = https://blogs.scientificamerican.com/extinction-countdown/2011/10/25/poachers-drive-javan-rhino-to-extinction-in-vietnam/ | archive-url = https://web.archive.org/web/20150406103742/http://blogs.scientificamerican.com/extinction-countdown/2011/10/25/poachers-drive-javan-rhino-to-extinction-in-vietnam/ | archive-date = 6 April 2015 | title = Poachers Drive Javan Rhino to Extinction in Vietnam | vauthors = Platt JR | date = 25 October 2011 | publisher = Scientific American }} giraffes,{{Cite news |url=https://www.theguardian.com/environment/2016/dec/08/giraffe-red-list-vulnerable-species-extinction |title=Giraffes facing extinction after devastating decline, experts warn |vauthors=Carrington D |date=8 December 2016 |newspaper=The Guardian |access-date=4 February 2017 |archive-date=13 August 2021 |archive-url=https://web.archive.org/web/20210813122004/https://www.theguardian.com/environment/2016/dec/08/giraffe-red-list-vulnerable-species-extinction |url-status=live }} and species of primates{{cite journal | vauthors = Estrada A, Garber PA, Rylands AB, Roos C, Fernandez-Duque E, Di Fiore A, Nekaris KA, Nijman V, Heymann EW, Lambert JE, Rovero F, Barelli C, Setchell JM, Gillespie TR, Mittermeier RA, Arregoitia LV, de Guinea M, Gouveia S, Dobrovolski R, Shanee S, Shanee N, Boyle SA, Fuentes A, MacKinnon KC, Amato KR, Meyer AL, Wich S, Sussman RW, Pan R, Kone I, Li B | display-authors = 6 | title = Impending extinction crisis of the world's primates: Why primates matter | journal = Science Advances | volume = 3 | issue = 1 | pages = e1600946 | date = January 2017 | pmid = 28116351 | pmc = 5242557 | doi = 10.1126/sciadv.1600946 | bibcode = 2017SciA....3E0946E }} and pangolins.{{Cite news |url=https://www.telegraph.co.uk/news/earth/wildlife/11370277/Pangolins-why-this-cute-prehistoric-mammal-is-facing-extinction.html |archive-url=https://ghostarchive.org/archive/20220110/https://www.telegraph.co.uk/news/earth/wildlife/11370277/Pangolins-why-this-cute-prehistoric-mammal-is-facing-extinction.html |archive-date=10 January 2022 |url-access=subscription |url-status=live |title=Pangolins: why this cute prehistoric mammal is facing extinction| vauthors = Fletcher M |date=31 January 2015 |work=The Telegraph}}{{cbignore}} According to the WWF's 2020 Living Planet Report, vertebrate wildlife populations have declined by 68% since 1970 as a result of human activities, particularly overconsumption, population growth and intensive farming, which is evidence that humans have triggered a sixth mass extinction event.{{cite news |vauthors=Greenfield P |date=9 September 2020 |title=Humans exploiting and destroying nature on unprecedented scale – report |url=https://www.theguardian.com/environment/2020/sep/10/humans-exploiting-and-destroying-nature-on-unprecedented-scale-report-aoe |work=The Guardian |access-date=13 October 2020 |archive-date=21 October 2021 |archive-url=https://web.archive.org/web/20211021225045/https://www.theguardian.com/environment/2020/sep/10/humans-exploiting-and-destroying-nature-on-unprecedented-scale-report-aoe |url-status=live }}{{cite news |vauthors=McCarthy D |date=1 October 2020 |title=Terrifying wildlife losses show the extinction end game has begun – but it's not too late for change |url=https://www.independent.co.uk/voices/wildlife-loss-humans-population-agriculture-extinction-b738367.html |work=The Independent |access-date=13 October 2020 |archive-date=7 April 2023 |archive-url=https://web.archive.org/web/20230407003854/https://www.independent.co.uk/voices/wildlife-loss-humans-population-agriculture-extinction-b738367.html |url-status=live }} Hunting alone threatens hundreds of mammalian species around the world.{{cite web |url=https://www.science.org/content/article/people-are-hunting-primates-bats-and-other-mammals-extinction |title=People are hunting primates, bats, and other mammals to extinction |vauthors=Pennisi E |author-link=Elizabeth Pennisi |date=18 October 2016 |work=Science |access-date=3 February 2017 |archive-date=20 October 2021 |archive-url=https://web.archive.org/web/20211020025827/https://www.science.org/content/article/people-are-hunting-primates-bats-and-other-mammals-extinction |url-status=live }}{{cite journal | vauthors = Ripple WJ, Abernethy K, Betts MG, Chapron G, Dirzo R, Galetti M, Levi T, Lindsey PA, Macdonald DW, Machovina B, Newsome TM, Peres CA, Wallach AD, Wolf C, Young H | display-authors = 6 | title = Bushmeat hunting and extinction risk to the world's mammals | journal = Royal Society Open Science | volume = 3 | issue = 10 | pages = 160498 | date = October 2016 | pmid = 27853564 | pmc = 5098989 | doi = 10.1098/rsos.160498 | bibcode = 2016RSOS....360498R | hdl = 1893/24446 }} Scientists claim that the growing demand for meat is contributing to biodiversity loss as this is a significant driver of deforestation and habitat destruction; species-rich habitats, such as significant portions of the Amazon rainforest, are being converted to agricultural land for meat production.{{cite journal | vauthors = Williams M, Zalasiewicz J, Haff PK, Schwägerl C, Barnosky AD, Ellis EC |author-link5=Anthony David Barnosky |year=2015 |title=The Anthropocene Biosphere |journal=The Anthropocene Review |volume=2 |issue=3 |pages=196–219 |doi=10.1177/2053019615591020|bibcode=2015AntRv...2..196W |s2cid=7771527 }}{{cite web |url=https://www.science.org/content/article/meat-eaters-may-speed-worldwide-species-extinction-study-warns |title=Meat-eaters may speed worldwide species extinction, study warns |vauthors=Morell V |date=11 August 2015 |work=Science |access-date=3 February 2017 |archive-date=20 December 2016 |archive-url=https://web.archive.org/web/20161220105327/http://www.sciencemag.org/news/2015/08/meat-eaters-may-speed-worldwide-species-extinction-study-warns |url-status=live }}{{cite journal | vauthors = Machovina B, Feeley KJ, Ripple WJ | title = Biodiversity conservation: The key is reducing meat consumption | journal = The Science of the Total Environment | volume = 536 | pages = 419–431 | date = December 2015 | pmid = 26231772 | doi = 10.1016/j.scitotenv.2015.07.022 | bibcode = 2015ScTEn.536..419M }} Another influence is over-hunting and poaching, which can reduce the overall population of game animals,{{cite journal |vauthors=Redford KH |year=1992 |title=The empty forest |journal=BioScience |volume=42 |issue=6 |pages=412–422 |url=http://www.dse.ufpb.br/alexandre/Redford%201992%20-The%20empty%20forest.pdf |doi=10.2307/1311860 |jstor=1311860 |access-date=4 February 2017 |archive-date=28 February 2021 |archive-url=https://web.archive.org/web/20210228092214/http://www.dse.ufpb.br/alexandre/Redford%201992%20-The%20empty%20forest.pdf |url-status=live }} especially those located near villages,{{cite book| vauthors = Peres CA, Nascimento HS |title=Human Exploitation and Biodiversity Conservation |chapter=Impact of game hunting by the Kayapó of south-eastern Amazonia: implications for wildlife conservation in tropical forest indigenous reserves |volume=3 |issue=8 |year=2006 |pages=287–313 |publisher=Springer |isbn=978-1-4020-5283-5 |oclc=207259298}} as in the case of peccaries.{{cite journal | vauthors = Altrichter M, Boaglio G |title=Distribution and Relative Abundance of Peccaries in the Argentine Chaco: Associations with Human Factors | journal=Biological Conservation |volume=116 |issue=2 |year=2004 |pages=217–225 |doi=10.1016/S0006-3207(03)00192-7|bibcode=2004BCons.116..217A }} The effects of poaching can especially be seen in the ivory trade with African elephants.{{cite web |vauthors=Gobush K |title=Effects of Poaching on African elephants |url=https://conservationbiology.uw.edu/research-programs/effects-of-poaching-on-african-elephants/ |website=Center For Conservation Biology |publisher=University of Washington |access-date=12 May 2021 |archive-date=8 December 2021 |archive-url=https://web.archive.org/web/20211208132610/https://conservationbiology.uw.edu/research-programs/effects-of-poaching-on-african-elephants/ |url-status=live }} Marine mammals are at risk from entanglement from fishing gear, notably cetaceans, with discard mortalities ranging from 65,000 to 86,000 individuals annually.{{cite book |chapter-url=https://www.fao.org/docrep/003/t4890e/T4890E03.htm#ch1.1.10 |chapter=Bycatch of Marine Mammals |title=A global assessment of fisheries bycatch and discards |vauthors=Alverson DL, Freeburg MH, Murawski SA, Pope JG |year=1996 |orig-year=1994 |publisher=Food and Agriculture Organization of the United Nations |location=Rome |isbn=978-92-5-103555-9 |oclc=31424005 |access-date=25 January 2024 |archive-date=17 February 2019 |archive-url=https://web.archive.org/web/20190217074707/http://www.fao.org/docrep/003/T4890E/T4890E03.htm#ch1.1.10 |url-status=live }}

Attention is being given to endangered species globally, notably through the Convention on Biological Diversity, otherwise known as the Rio Accord, which includes 189 signatory countries that are focused on identifying endangered species and habitats.{{cite book | vauthors = Glowka L, Burhenne-Guilmin F, Synge HM, McNeely JA, Gündling L |title=IUCN environmental policy and law paper |series=Guide to the Convention on Biodiversity |year=1994 |publisher=International Union for Conservation of Nature |isbn=978-2-8317-0222-3 |oclc=32201845}} Another notable conservation organisation is the IUCN, which has a membership of over 1,200 governmental and non-governmental organisations.{{cite web |url=https://www.iucn.org/about |title=About IUCN |publisher=International Union for Conservation of Nature |access-date=3 February 2017 |date=3 December 2014 |archive-date=15 April 2020 |archive-url=https://web.archive.org/web/20200415031632/https://www.iucn.org/about/ |url-status=live }}

Recent extinctions can be directly attributed to human influences.{{cite journal | vauthors = Ceballos G, Ehrlich PR, Barnosky AD, García A, Pringle RM, Palmer TM | title = Accelerated modern human-induced species losses: Entering the sixth mass extinction | journal = Science Advances | volume = 1 | issue = 5 | pages = e1400253 | date = June 2015 | pmid = 26601195 | pmc = 4640606 | doi = 10.1126/sciadv.1400253 | bibcode = 2015SciA....1E0253C }}{{cite journal | vauthors = Dirzo R, Young HS, Galetti M, Ceballos G, Isaac NJ, Collen B | title = Defaunation in the Anthropocene | journal = Science | volume = 345 | issue = 6195 | pages = 401–406 | date = July 2014 | pmid = 25061202 | doi = 10.1126/science.1251817 | url = https://www.uv.mx/personal/tcarmona/files/2010/08/Science-2014-Dirzo-401-6-2.pdf | bibcode = 2014Sci...345..401D | s2cid = 206555761 | access-date = 25 January 2024 | archive-date = 7 August 2019 | archive-url = https://web.archive.org/web/20190807115621/https://www.uv.mx/personal/tcarmona/files/2010/08/Science-2014-Dirzo-401-6-2.pdf | url-status = live }} The IUCN characterises 'recent' extinction as those that have occurred past the cut-off point of 1500,{{cite journal | vauthors = Fisher DO, Blomberg SP | title = Correlates of rediscovery and the detectability of extinction in mammals | journal = Proceedings. Biological Sciences | volume = 278 | issue = 1708 | pages = 1090–1097 | date = April 2011 | pmid = 20880890 | pmc = 3049027 | doi = 10.1098/rspb.2010.1579 }} and around 80 mammal species have gone extinct since that time and 2015.{{cite book| vauthors = Ceballos G, Ehrlich AH, Ehrlich PR |year=2015|title=The Annihilation of Nature: Human Extinction of Birds and Mammals |location=Baltimore |publisher=Johns Hopkins University Press |isbn=978-1-4214-1718-9 |page=69}} Some species, such as the Père David's deer{{cite iucn |author=Jiang, Z. |author2=Harris, R.B. |date=2016 |title=Elaphurus davidianus |volume=2016 |page=e.T7121A22159785 |doi=10.2305/IUCN.UK.2016-2.RLTS.T7121A22159785.en |access-date=12 November 2021}} are extinct in the wild, and survive solely in captive populations. Other species, such as the Florida panther, are ecologically extinct, surviving in such low numbers that they essentially have no impact on the ecosystem.{{cite book |chapter-url={{Google books|plainurl=yes |id=hBntufCOxAsC |page=318}} | vauthors = McKinney ML, Schoch R, Yonavjak L |year=2013 |title=Environmental Science: Systems and Solutions|edition=5th|chapter=Conserving Biological Resources |publisher=Jones & Bartlett Learning|isbn=978-1-4496-6139-7|oclc=777948078}}{{rp|318}} Other populations are only locally extinct (extirpated), still existing elsewhere, but reduced in distribution,{{rp|75–77}} as with the extinction of grey whales in the Atlantic.{{Cite book |title=Encyclopedia of marine mammals |page=404 |year=2009 |publisher=Academic Press |isbn=978-0-12-373553-9 | vauthors = Perrin WF, Würsig BF, Thewissen JG | author-link3 = Hans Thewissen |oclc=455328678}}

{{div col|colwidth=22em}}

• List of mammal genera – living mammals
• List of mammalogists
• List of monotremes and marsupials
• List of placental mammals
• List of prehistoric mammals
• List of endangered mammals
• Lists of mammals by population size
• Lists of mammals by region
• Mammals described in the 2000s
• Mammals in culture
• Small mammal

{{div col end}}

{{Notelist}}

{{Reflist}}

• {{CCBYSASource|sourcepath=https://cnx.org/content/m46676/latest/?collection=col11496/latest|authors = Betts JF, Desaix P, Johnson E, Johnson JE, Korol O, Kruse D, Poe B, Wise JA, Womble M, Young KA|revision=729857320|sourcearticle=Anatomy and Physiology}}

{{Refbegin}}

• {{cite journal | vauthors = Brown WM | year = 2001 | title = Natural selection of mammalian brain components | journal = Trends in Ecology and Evolution | volume = 16 | issue = 9| pages = 471–473 | doi = 10.1016/S0169-5347(01)02246-7 | bibcode = 2001TEcoE..16..471B }}
• {{cite book|vauthors=McKenna MC, Bell SK|year=1997|title=Classification of Mammals Above the Species Level|publisher=Columbia University Press|location=New York|isbn=978-0-231-11013-6|oclc=37345734|url={{Google books|plainurl=yes|id=id=zS7FZkzIw-cC}}}}{{Dead link|date=July 2023 |bot=InternetArchiveBot |fix-attempted=yes }}
• {{cite book|vauthors=Nowak RM|year=1999|title=Walker's mammals of the world|publisher=Johns Hopkins University Press|location=Baltimore|edition=6th|isbn=978-0-8018-5789-8|oclc=937619124|url={{Google books|plainurl=yes| id=T37sFCl43E8C}}}}
• {{Cite journal | vauthors = Simpson GG | year = 1945 | title = The principles of classification and a classification of mammals | journal = Bulletin of the American Museum of Natural History | volume = 85 | pages = 1–350 }}
• {{cite journal | vauthors = Murphy WJ, Eizirik E, O'Brien SJ, Madsen O, Scally M, Douady CJ, Teeling E, Ryder OA, Stanhope MJ, de Jong WW, Springer MS | display-authors = 6 | title = Resolution of the early placental mammal radiation using Bayesian phylogenetics | journal = Science | volume = 294 | issue = 5550 | pages = 2348–2351 | date = December 2001 | pmid = 11743200 | doi = 10.1126/science.1067179 | bibcode = 2001Sci...294.2348M | s2cid = 34367609 }}
• {{cite journal | vauthors = Springer MS, Stanhope MJ, Madsen O, de Jong WW | title = Molecules consolidate the placental mammal tree | journal = Trends in Ecology & Evolution | volume = 19 | issue = 8 | pages = 430–438 | date = August 2004 | pmid = 16701301 | doi = 10.1016/j.tree.2004.05.006 | s2cid = 1508898 | url = https://www.zi.ku.dk/evolbiology/courses/ME04/7_9/springer200-phyl.pdf | access-date = 21 January 2005 | archive-date = 29 July 2016 | archive-url = https://web.archive.org/web/20160729033207/http://www.zi.ku.dk/evolbiology/courses/ME04/7_9/springer200-phyl.pdf | url-status = dead }}
• {{cite book| vauthors = Vaughan TA, Ryan JM, Capzaplewski NJ |year=2000|title=Mammalogy|edition=4th|publisher=Saunders College Publishing|location=Fort Worth, Texas|isbn=978-0-03-025034-7|oclc=42285340}}
• {{cite journal | vauthors = Kriegs JO, Churakov G, Kiefmann M, Jordan U, Brosius J, Schmitz J | title = Retroposed elements as archives for the evolutionary history of placental mammals | journal = PLOS Biology | volume = 4 | issue = 4 | pages = e91 | date = April 2006 | pmid = 16515367 | pmc = 1395351 | doi = 10.1371/journal.pbio.0040091 | doi-access = free }}
• {{cite book| vauthors = MacDonald DW, Norris S |year=2006|title=The Encyclopedia of Mammals|edition=3rd|publisher=Brown Reference Group|location=London|isbn=978-0-681-45659-4|oclc=74900519}}

{{Refend}}

{{Wikibooks|Dichotomous Key|Mammalia}}

{{EB1911 poster|Mammalia}}

• [https://www.mammaldiversity.org ASM Mammal Diversity Database] {{Webarchive|url=https://web.archive.org/web/20221225111803/https://www.mammaldiversity.org/explore.html |date=25 December 2022 }}
• [https://www.biodiversitymapping.org/mammals.htm Biodiversitymapping.org – All mammal orders in the world with distribution maps] {{Webarchive|url=https://web.archive.org/web/20160926130232/http://www.biodiversitymapping.org/mammals.htm |date=26 September 2016 }}
• [https://paleocene-mammals.de/ Paleocene Mammals] {{Webarchive|url=https://web.archive.org/web/20240203140812/http://www.paleocene-mammals.de/ |date=3 February 2024 }}, a site covering the rise of the mammals, paleocene-mammals.de
• [https://www.enchantedlearning.com/subjects/mammals/Evolution.shtml Evolution of Mammals] {{Webarchive|url=https://web.archive.org/web/20240125191350/https://www.enchantedlearning.com/subjects/mammals/Evolution.shtml |date=25 January 2024 }}, a brief introduction to early mammals, enchantedlearning.com
• [https://www.european-mammals.org/php/mapmaker.php European Mammal Atlas EMMA] {{Webarchive|url=https://web.archive.org/web/20240125191351/https://www.european-mammals.org/php/mapmaker.php |date=25 January 2024 }} from Societas Europaea Mammalogica, European-mammals.org
• [https://swfsc.noaa.gov/uploadedFiles/Divisions/PRD/Publications/Jeffersonetal93(14).pdf Marine Mammals of the World] {{Webarchive|url=https://web.archive.org/web/20190608065724/https://swfsc.noaa.gov/uploadedFiles/Divisions/PRD/Publications/Jeffersonetal93(14).pdf |date=8 June 2019 }} – An overview of all marine mammals, including descriptions, both fully aquatic and semi-aquatic, noaa.gov
• [https://www.mammalogy.org Mammalogy.org] {{Webarchive|url=https://web.archive.org/web/20200301013904/http://www.mammalogy.org/ |date=1 March 2020 }} The American Society of Mammalogists was established in 1919 for the purpose of promoting the study of mammals, and this website includes a mammal image library

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{{Mammals}}

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Category:Bathonian first appearances

Category:Extant Middle Jurassic first appearances

Category:Taxa named by Carl Linnaeus