Menstruation (mammal)

{{see also|Estrous cycle}}

Females of most mammal species advertise fertility to males with visual behavioral cues, pheromones, or both.{{cite encyclopedia | title = Estrus | encyclopedia = Britannica Online | url = http://www.britannica.com/EBchecked/topic/193725/estrus | access-date = 5 October 2008}} This period of advertised fertility is known as oestrus, "estrus" or heat. In species that experience estrus, females are generally only receptive to copulation while they are in heat (dolphins are an exception).{{cite web | last = Mikkelson | first = David P. | name-list-style = vanc | title = Buried Pleasure | work = Snopes.com | date = 29 June 2007 | url = http://www.snopes.com/critters/wild/pleasure.asp | access-date = 5 October 2008}}, which references:

:{{cite book | last = Diamond | first = Jared M. | name-list-style = vanc |title=Why is sex fun?: the evolution of human sexuality |publisher=HarperCollins |location=London |year=1997 |isbn=978-0-465-03127-6}} In the estrous cycles of most placentals, if no fertilization takes place, the uterus reabsorbs the endometrium. This breakdown of the endometrium without vaginal discharge is sometimes called covert menstruation.{{cite journal | vauthors = Profet M | title = Menstruation as a defense against pathogens transported by sperm | journal = The Quarterly Review of Biology | volume = 68 | issue = 3 | pages = 335–86 | date = September 1993 | pmid = 8210311 | doi = 10.1086/418170 | s2cid = 23738569 }} Overt menstruation (where there is blood flow from the vagina) occurs primarily in humans and close evolutionary relatives such as chimpanzees. Some species, such as domestic dogs, experience small amounts of vaginal bleeding while approaching heat;{{Cite book|title=Pathways to Pregnancy and Parturition| vauthors = Senger PL |publisher=Current Conceptions, Inc.|year=2012|isbn=978-0-9657648-3-4|location=Redmon, OR|pages=146}} this discharge has a different physiologic cause than menstruation.{{cite web|title=Canine False Pregnancy and Female Reproduction |publisher=Mar Vista Animal Medical Center |date=2 February 2008 |url=http://marvistavet.com/html/body_canine_false_pregnancy.html |access-date=5 October 2008 |url-status=dead |archive-url=https://web.archive.org/web/20081015180639/http://www.marvistavet.com/html/body_canine_false_pregnancy.html |archive-date=15 October 2008 }}

{{see also|concealed ovulation}}

A few mammals do not experience obvious, visible signs of fertility (concealed ovulation). In humans, studies show that both males and females can detect the fertility of females through hormonal signaling and alterations in scent (fertility awareness), but some research suggests that behavioral clues may be needed to consciously assess fertility.Studies that found women are more attractive to men when fertile:

:{{cite journal | vauthors = Roberts SC, Havlicek J, Flegr J, Hruskova M, Little AC, Jones BC, Perrett DI, Petrie M | title = Female facial attractiveness increases during the fertile phase of the menstrual cycle | journal = Proceedings. Biological Sciences | volume = 271 | issue = Suppl 5 | pages = S270–2 | date = August 2004 | pmid = 15503991 | pmc = 1810066 | doi = 10.1098/rsbl.2004.0174 }}

:{{cite journal | vauthors = Miller G, Tybur JM, Jordan BD | title = Ovulatory cycle effects on tip earnings by lap dancers: economic evidence for human estrus? | journal = Evolution and Human Behavior | volume = 28 | issue = 6 | pages = 375–381 |date=June 2007 | url = http://www.unm.edu/~gfmiller/cycle_effects_on_tips.pdf | access-date = 21 January 2008 | doi = 10.1016/j.evolhumbehav.2007.06.002| citeseerx = 10.1.1.154.8176 }}Study that found male sexual behavior is impacted more by social behavior than somatic processes:

:{{cite journal | vauthors = Bullivant SB, Sellergren SA, Stern K, Spencer NA, Jacob S, Mennella JA, McClintock MK | title = Women's sexual experience during the menstrual cycle: identification of the sexual phase by noninvasive measurement of luteinizing hormone | journal = Journal of Sex Research | volume = 41 | issue = 1 | pages = 82–93 | date = February 2004 | pmid = 15216427 | doi = 10.1080/00224490409552216 | s2cid = 40401379 | url = https://dx.doi.org/10.1080/00224490409552216 }}

Orangutans also lack visible signs of impending ovulation.{{cite web | last = Knott | first = Cheryl | name-list-style = vanc | title = Orangutans: Reproduction and Life History | work = Gunung Palung Orangutan Project | publisher = Harvard University | year = 2003 | url = http://www.fas.harvard.edu/~gporang/orangutans.html | access-date = 5 October 2008|archive-url = https://web.archive.org/web/20080421175238/http://www.fas.harvard.edu/~gporang/orangutans.html |archive-date = April 21, 2008|url-status=dead}} Also, it has been said that the extended estrus period of the bonobo (reproductive-age females are in heat for 75% of their menstrual cycle){{cite book | last1 = Lanting | first1 = Frans | last2 = de Waal | first2 = F. B. M. | name-list-style = vanc |title=Bonobo: the forgotten ape |publisher=University of California Press |location=Berkeley |year=1997 |pages=107 |isbn=978-0-520-20535-2 |url=http://www.serpentfd.org/a/dewall1997.html |access-date=5 September 2007 }} has a similar effect to the lack of a "heat" in human females.{{cite journal | last = Stanford | first = Craig B. | name-list-style = vanc | title = The Brutal Ape vs. the Sexy Ape? The Make-Love-Not-War Ape | journal = American Scientist | volume = 88 | issue = 2 | pages = 110 | date = March–April 2000 | doi = 10.1511/2000.2.110}}

Most female mammals have an estrous cycle, yet only ten primate species, four bat species, the elephant shrew, and one known species of spiny mouse have a menstrual cycle.{{cite web|url=http://scienceblogs.com/pharyngula/2011/12/21/why-do-women-menstruate/|title=Why do women menstruate?|year=2011|work=ScienceBlogs|archive-url=https://web.archive.org/web/20130127131136/http://scienceblogs.com/pharyngula/2011/12/21/why-do-women-menstruate/|archive-date=27 January 2013|url-status=live|access-date=15 January 2013|df=dmy-all}}{{cite journal | vauthors = Bellofiore N, Ellery SJ, Mamrot J, Walker DW, Temple-Smith P, Dickinson H | title = First evidence of a menstruating rodent: the spiny mouse (Acomys cahirinus) | journal = American Journal of Obstetrics and Gynecology | volume = 216 | issue = 1 | pages = 40.e1–40.e11 | date = January 2017 | pmid = 27503621 | doi = 10.1016/j.ajog.2016.07.041| s2cid = 88779 }} As these groups are not closely related, it is likely that four distinct evolutionary events have caused menstruation to arise.{{cite journal | vauthors = Emera D, Romero R, Wagner G | title = The evolution of menstruation: a new model for genetic assimilation: explaining molecular origins of maternal responses to fetal invasiveness | journal = BioEssays | volume = 34 | issue = 1 | pages = 26–35 | date = January 2012 | pmid = 22057551 | pmc = 3528014 | doi = 10.1002/bies.201100099 }}

There are varying views on evolution of overt menstruation in humans and related species, and the evolutionary advantages in losing blood associated with dismantling the uterine lining rather than absorbing it, as most mammals do.{{cite journal | vauthors = Martin RD | title = The evolution of human reproduction: A primatological perspective | journal = American Journal of Physical Anthropology | volume = 134 | issue = S45 | pages = 59–84 | date = 28 November 2007 | doi = 10.1002/ajpa.20734 | pmid = 18046752 | s2cid = 44416632 | doi-access = free }} The reason is likely related to differences in the ovulation process.

Most female placentals have a uterine lining that builds up when the animal begins ovulation, and later further increases in thickness and blood flow after a fertilized egg has successfully implanted. This final process of thickening is known as decidualization, and is usually triggered by hormones released by the embryo. In humans, decidualization happens spontaneously at the beginning of each menstrual cycle, triggered by hormonal signals from the ovaries. For this reason, the human uterine lining becomes fully thickened during each cycle as a defense to trophoblast penetration of the endometrial wall,{{cite journal | vauthors = Gellersen B, Brosens JJ | title = Cyclic Decidualization of the Human Endometrium in Reproductive Health and Failure | journal = Endocrine Reviews | volume = 35 | issue = 6 | pages = 851–905 | date = 1 December 2014 | doi = 10.1210/er.2014-1045 | pmid = 25141152 | doi-access = free }} regardless of whether an egg becomes fertilized or successfully implants in the uterus. This produces more unneeded material per cycle than in non-menstruating mammals, which may explain why the extra material is not simply reabsorbed as done by those species. In essence, menstruating animals treat every cycle as a possible pregnancy by thickening the protective layer around the endometrial wall, while non-menstruating placental mammals do not begin the pregnancy process until a fertilized egg has implanted in the uterine wall.{{medcn|date=March 2021}}

For this reason, it is speculated that menstruation is a side effect of spontaneous decidualization, which evolved in some placental mammals due to its advantages over non-spontaneous decidualization. Spontaneous decidualization allows for more maternal control in the maternal-fetal conflict by increasing selectivity over the implanted embryo. This may be necessary in humans and other primates, due to the abnormally large number of genetic disorders in these species.{{Cite journal |last1=Teklenburg |first1=Gijs |last2=Salker |first2=Madhuri |last3=Molokhia |first3=Mariam |last4=Lavery |first4=Stuart |last5=Trew |first5=Geoffrey |last6=Aojanepong |first6=Tepchongchit |last7=Mardon |first7=Helen J. |last8=Lokugamage |first8=Amali U. |last9=Rai |first9=Raj |last10=Landles |first10=Christian |last11=Roelen |first11=Bernard A. J. |last12=Quenby |first12=Siobhan |last13=Kuijk |first13=Ewart W. |last14=Kavelaars |first14=Annemieke |last15=Heijnen |first15=Cobi J. |date=2010-04-21 |editor-last=Vitzthum |editor-first=Virginia J. |title=Natural Selection of Human Embryos: Decidualizing Endometrial Stromal Cells Serve as Sensors of Embryo Quality upon Implantation |journal=PLOS ONE |language=en |volume=5 |issue=4 |pages=e10258 |doi=10.1371/journal.pone.0010258 |pmid=20422011 |pmc=2858159 |bibcode=2010PLoSO...510258T |issn=1932-6203 |doi-access=free }} Since most aneuploidy events result in stillbirth or miscarriage, there is an evolutionary advantage to ending the pregnancy early, rather than nurturing a fetus that will later miscarry. There is evidence to show that some abnormalities in the developing embryo can be detected by endometrial stromal cells in the uterus, but only upon differentiation into decidual cells. This triggers epigenetic changes that prevent formation of the placenta, which prevents the embryo from implanting and leaves it to be removed in the next menstruation.{{better source|date=March 2021}}{{Cite web|url=https://medicalxpress.com/news/2013-09-choosy-uterus-insight-embryos-implant.html|title=The 'choosy uterus': New insight into why embryos do not implant|website=medicalxpress.com|language=en-us|access-date=2020-01-26}} This failsafe mode is not possible in species where decidualization is controlled by hormonal triggers from the embryo. This is sometimes referred to as the choosy uterus theory, and it is theorized that this positive outweighs the negative impacts of menstruation in species with high aneuploidy rates and hence a high number of 'doomed' embryos.{{medcn|date=March 2021}}

File:Menstruating Species.pngs; blue, Chiroptera; orange, Afrotheria; yellow, Rodentia.{{cite journal | vauthors = Bellofiore N, Cousins F, Temple-Smith P, Dickinson H, Evans J | title = A missing piece: the spiny mouse and the puzzle of menstruating species | language = en-US | journal = Journal of Molecular Endocrinology | volume = 61 | issue = 1 | pages = R25–R41 | date = July 2018 | pmid = 29789322 | doi = 10.1530/jme-17-0278 | doi-access = free }}{{cite journal | vauthors = Zhang X, Zhu C, Lin H, Yang Q, Ou Q, Li Y, Chen Z, Racey P, Zhang S, Wang H | title = Wild fulvous fruit bats (Rousettus leschenaulti) exhibit human-like menstrual cycle | journal = Biology of Reproduction | volume = 77 | issue = 2 | pages = 358–64 | date = August 2007 | pmid = 17494915 | doi = 10.1095/biolreprod.106.058958 | citeseerx = 10.1.1.1013.9590 | s2cid = 22138847 }}]]

The female will ovulate spontaneously and be receptive to the male for breeding (express estrus) at regular biologically defined intervals. The female is receptive to males only while experiencing estrus.

For breeding livestock, there are a number of advantages to be gained by finding methods to induce ovulation on a planned schedule, and thus synchronize the estrus cycle between many female animals. If animals can be bred on the same schedule, it increases convenience for the livestock owner, since the young animals will be at the same stage of development. Also, if artificial insemination (AI) is used for breeding, the AI technician's time can be used more efficiently, by breeding several females at the same time. In order to induce estrus, a variety of techniques have been tried in recent years, involving both more natural, and more hormonal based methods.[https://journals.co.za/content/animalia/3/4/AJA03027104_166 Synchronized mating and lambing in spring-bred Merino sheep: the use of progestogen-impregnated intravaginal sponges and teaser rams] ({{lang|af|Met opsomming in Afrikaans}}) ({{lang|fr|Avec resume en francais}}) G. L. HUNTER, P. C. BELONJE and C. H. VAN NIEKERK, Department of Agricultural Technical Services, Stellenbosch, Agroanimalia 3,133-140 (1971) Different ways of injecting or feeding hormones to livestock are costly, and have variable success rates.{{cite web|title=Estrous|url=http://www.uwyo.edu/wjm/repro/estrous.htm|publisher=University of Wyoming|access-date=September 15, 2011}}

Average length (days) of estrus and estrous cycles:

• Estrous cycle
• Breeding pair
• Reproduction
• Animal husbandry

{{Reflist|2}}

• {{cite web |first1=Rick |last1=Rasby |first2=Rosemary |last2=Vinton |url=http://beef.unl.edu/learning/estrous.shtml |title=Estrous Cycle Learning Module}}
• {{cite web |first=Jos |last=Mottershead |url=http://www.equine-reproduction.com/articles/estrous.htm |title=The Mare's Estrous Cycle}}
• {{cite web |first1=Jerry |last1=May |first2=Ron |last2=Bates |url=http://www.thepigsite.com/articles/2090/managing-the-sow-and-gilt-estrous-cycle |title=Managing the Sow and Gilt Estrous Cycle}}

Category:Animal breeding

Category:Animal physiology

Category:Zoology

Category:Menstrual cycle

Category:Mammal female reproductive system