mesotherm

Mesotherms have two basic characteristics:{{cite journal|last1=Grady|title=Evidence for mesothermy in dinosaurs|journal=Science|date=2014|volume=344|issue=6189|pages=1268–1272|doi=10.1126/science.1253143|display-authors=etal|pmid=24926017|bibcode=2014Sci...344.1268G |s2cid=9806780 }}

1. Elevation of body temperature via metabolic production of heat.
2. Weak or absent metabolic control of a particular body temperature.

The first trait distinguishes mesotherms from ectotherms, the second from endotherms. For instance, endotherms, when cold, will generally resort to shivering or metabolizing brown fat to maintain a constant body temperature, leading to higher metabolic rates. A mesotherm, however, will experience lower body temperatures and lower metabolic rates as ambient temperature drops.{{cite journal |last1=Bernal |display-authors=etal |year=2001 |title=Review: Analysis of the evolutionary convergence for high performance swimming in lamnid sharks and tunas |journal=Comparative Biochemistry and Physiology A |volume=129 |issue=2–3 |pages=695–726 |doi=10.1016/S1095-6433(01)00333-6 |pmid=11423338}} In addition, mesotherm body temperatures tend to rise as body size increases (a phenomenon known as gigantothermy{{cite journal |last1=Paladino |display-authors=etal |year=1990 |title=Metabolism of leatherback turtles, gigantothermy, and thermoregulation of dinosaurs |journal=Nature |volume=344 |issue=6269 |pages=858–860 |doi=10.1038/344858a0 |bibcode=1990Natur.344..858P |s2cid=4321764}}), unlike endotherms. This reflects the lower surface area to volume ratio in large animals, which reduces rates of heat loss.

While extant mesotherms are relatively rare, good examples include tuna, lamnid sharks (e.g., the great white shark), the leatherback sea turtle, some species of bee,{{cite web |title=Temperature regulation |quote=bumblebees are warm-blooded insects. |website=bumblebee.org |url=http://www.bumblebee.org/bodyTempReg.htm}} naked mole rats, hyraxes, and echidnas.

Historically, the same word was used by de Candolle to describe plants that require a moderate degree of heat for successful growth.{{cite book |last=Allaby |first=Michael |year=2004 |title=A Dictionary of Ecology |edition=4th |publisher=Oxford University Press |isbn=9780198608912 |url=https://archive.org/details/dictionaryofplan0000unse_a2q0 |url-access=registration |via=Internet Archive (archive.org)}} In his scheme, a mesotherm plant grew in regions where the warmest month had a mean temperature greater than {{convert|22|C|0|abbr=on}} and the coldest month had a mean temperature of at least {{convert|6|C|abbr=on}}.

The thermoregulatory status of dinosaurs has long been debated, and is still an active area of research. The term 'mesothermy' was originally coined{{cite book|last1=Sampson|first1=Scott|title=Dinosaur Odyssey: Fossil Threads in the Web of Life|publisher=University of California Press|pages=175–192}} to advocate for an intermediate status of non-avian dinosaur thermoregulation, between endotherms and ectotherms. A more technical definition was provided by Grady et al,{{cite journal|last1=Grady|title=Evidence for mesothermy in dinosaurs|journal=Science|date=2014|volume=344|issue=6189|pages=1268–1272|doi=10.1126/science.1253143|display-authors=etal|pmid=24926017|bibcode=2014Sci...344.1268G |s2cid=9806780 }} who argued for dinosaur mesothermy on the basis of their intermediate growth rates, and the empirical relationship between growth, metabolism and thermoregulation in extant vertebrates.

This viewpoint was challenged by D'Emic,{{cite journal|last1=D'Emic|first1=Michael|title=Comment on "Evidence for mesothermy in dinosaurs"|date=2015|volume=348|issue=6238|pages=982|doi=10.1126/science.1260061|pmid=26023130|journal=Science|bibcode=2015Sci...348..982D |doi-access=free}} who argued that because growth rates are sensitive to seasonal variation in resources, dinosaur maximum growth rates were underestimated by Grady et al. Adjusting dinosaur rates upwards by a factor of two, D'Emic found dinosaurs to grow similarly to mammals, and thus were likely endothermic. However, sensitivity to seasonal variation in resources should be true for all vertebrates. If all vertebrate taxa were similarly adjusted, the relative differences in rates does not change.{{cite journal|last1=Grady|title=Response to Comments on "Evidence for mesothermy in dinosaurs"|journal=Science|volume=348|issue=6238|pages=982|doi=10.1126/science.1260299|display-authors=etal|pmid=26023132|year=2015|bibcode=2015Sci...348R.982G |doi-access=free}} Dinosaurs remain intermediate growers and good candidates for mesothermy.{{cite journal | url=https://www.science.org/doi/full/10.1126/science.1260299 | doi=10.1126/science.1260299 | title=Response to Comments on "Evidence for mesothermy in dinosaurs" | date=2015 | last1=Grady | first1=John M. | last2=Enquist | first2=Brian J. | last3=Dettweiler-Robinson | first3=Eva | last4=Wright | first4=Natalie A. | last5=Smith | first5=Felisa A. | journal=Science | volume=348 | issue=6238 | page=982 | pmid=26023132 | bibcode=2015Sci...348R.982G }}

Nonetheless, the dinosaur mesothermy hypothesis requires further support to be confirmed. Fossil oxygen isotopes, which can reveal an organism's body temperature, should be particularly informative. Recently, a study of theropod and sauropod isotopes{{cite journal|last1=Eagle|title=Isotopic ordering in eggshells reflects body temperatures and suggests differing thermophysiology in two Cretaceous dinosaurs|journal=Nature Communications|date=2015|volume=6|doi=10.1038/ncomms9296|display-authors=etal|pages=8296|pmid=26462135|bibcode=2015NatCo...6.8296E |doi-access=free}} offered some support for dinosaur mesothermy. Feathered theropods are probably the best candidates for dinosaur endothermy, yet the examined theropods had relatively low body temperatures {{convert|32.0|C|F|abbr=on}}. Large sauropods had higher body temperatures {{convert|37.0|C|F|abbr=on}}, which may be reflective of mesothermic gigantothermy.{{cite journal | url=https://www.cell.com/current-biology/fulltext/S0960-9822(24)00525-6 | doi=10.1016/j.cub.2024.04.051 | title=Early Jurassic origin of avian endothermy and thermophysiological diversity in dinosaurs | date=2024 | last1=Chiarenza | first1=Alfio Alessandro | last2=Cantalapiedra | first2=Juan L. | last3=Jones | first3=Lewis A. | last4=Gamboa | first4=Sara | last5=Galván | first5=Sofía | last6=Farnsworth | first6=Alexander J. | last7=Valdes | first7=Paul J. | last8=Sotelo | first8=Graciela | last9=Varela | first9=Sara | journal=Current Biology | volume=34 | issue=11 | pages=2517–2527.e4 | pmid=38754424 | bibcode=2024CBio...34.2517C | doi-access=free }} Future isotopic analysis of small, juvenile dinosaurs will better resolve this question.{{Citation needed|date=June 2023}}

• Heterothermy
• Gigantothermy
• Poikilotherm
• {{section link|Argentine black and white tegu|Endothermic behavior}}

{{Reflist|2}}

Category:Thermoregulation