View source for Biology:Menstruation (mammal)

{{Short description|Periodic shedding of the uterine lining}}
'''Menstruation'''  is the shedding of the uterine lining (endometrium).  It occurs on a regular basis in uninseminated<ref>Gras, Lyn, et al. "The incidence of chromosomal aneuploidy in stimulated and unstimulated (natural) uninseminated human oocytes." Human Reproduction 7.10 (1992): 1396-1401.</ref> sexually reproductive-age [[Biology:Female|female]]s of certain mammal species.

Although there is some disagreement in definitions between sources, menstruation is generally considered to be limited to [[Biology:Primate|primate]]s. It is common in [[Biology:Simian|simian]]s (Old World monkeys, New World monkeys, and [[Biology:Ape|ape]]s), but completely lacking in strepsirrhine primates and possibly weakly present in [[Biology:Tarsier|tarsier]]s. Beyond primates, it is known only in [[Biology:Bat|bat]]s, the elephant shrew, and the [[Biology:Spiny mouse|spiny mouse]] species ''[[Biology:Cairo spiny mouse|Acomys cahirinus]]''.<ref>{{cite journal | journal = Journal of Molecular Endocrinology | volume = 61 | issue = 1 | pages = R25–R41 | date = July 2018 | doi = 10.1530/JME-17-0278 | title = A missing piece: the spiny mouse and the puzzle of menstruating species | vauthors = Bellofiore N, Cousins F, Temple-Smith P, Dickinson H, and Evans J | pmid = 29789322 | url = https://jme.bioscientifica.com/view/journals/jme/61/1/JME-17-0278.xml| doi-access = free }}</ref><ref>{{cite journal |author = 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 }}</ref><ref>{{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 | year = 2007 | pmid = 18046752 | doi = 10.1002/ajpa.20734 | s2cid = 44416632 | doi-access = free }}</ref><ref>{{cite book|url=http://www.uwyo.edu/wjm/repro/menstrua.htm|title=Is menstruation obsolete?|year=1999| first1 = Elsimar M. | last1 = Coutinho  | first2 = Sheldon J. | last2 = Segal | name-list-style= vanc |publisher=Oxford University Press}}</ref><ref>{{cite web | url = http://www.eshre.eu/binarydata.aspx?type=doc&sessionId=d3g32gezrjsijs554mxn33fb/syllabus_course4%5B1%5D.pdf | title = Course 4:Implantation | first1 = Paul | last1 = Bischof | first2 = Marie | last2 = Cohen | name-list-style = vanc | publisher = European Society of Human Reproduction and Embryology | access-date = 2018-12-04 | archive-url = https://web.archive.org/web/20190125073414/http://www.eshre.eu/binarydata.aspx?type=doc&sessionId=d3g32gezrjsijs554mxn33fb%2Fsyllabus_course4%5B1%5D.pdf | archive-date = 2019-01-25 | url-status = dead }}</ref> Overt menstruation (where there is bleeding from the [[Biology:Uterus|uterus]] through the [[Biology:Vagina|vagina]]) is found primarily in humans and close relatives such as [[Biology:Pan (genus)|chimpanzees]].<ref name="strassmann">{{cite journal | vauthors = Strassmann BI | title = The evolution of endometrial cycles and menstruation | journal = The Quarterly Review of Biology | volume = 71 | issue = 2 | pages = 181–220 | date = June 1996 | pmid = 8693059 | doi = 10.1086/419369 | s2cid = 6207295 }}</ref>

Females of other species of [[Biology:Placentalia|placental mammal]] undergo estrous cycles, in which the endometrium is completely reabsorbed by the animal (covert menstruation) at the end of its [[Biology:Biological life cycle|reproductive cycle]].<ref>{{cite journal | journal =  Biology of Reproduction | volume = 102 | issue = 6 | pages = 1160–1169 | date = June 2020 | doi = 10.1093/biolre/ioaa029 | title = Characteristics of the endometrium in menstruating species: lessons learned from the animal kingdom | vauthors = Catalini L, Fedder J | pmid = 32129461 | pmc = 7253787 | url =  https://academic.oup.com/biolreprod/article/102/6/1160/5775593| doi-access = free }}</ref> Many zoologists regard this as different from a "true" menstrual cycle. Female domestic animals used for breeding—for example dogs, pigs, cattle, or horses—are monitored for physical signs of an estrous cycle period, which indicates that the animal is ready for insemination.

== Estrus and menstruation ==
Females of most mammal species advertise fertility to males with visual behavioral cues, pheromones, or both.<ref name=britannica>{{cite encyclopedia | title = Estrus | encyclopedia = Britannica Online | url = http://www.britannica.com/EBchecked/topic/193725/estrus | access-date = 5 October 2008}}</ref> This period of advertised fertility is known as ''oestrus'', "estrus" or ''heat''.<ref name=britannica/> In species that experience estrus, females are generally only receptive to [[Biology:Copulation (zoology)|copulation]] while they are in heat<ref name=britannica/> ([[Biology:Dolphin|dolphin]]s are an exception).<ref>{{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}}</ref> In the estrous cycles of most [[Biology:Eutheria|placental mammals]], if no fertilization takes place, the uterus reabsorbs the endometrium. This breakdown of the endometrium without [[Medicine:Menstruation|vaginal discharge]] is sometimes called ''covert menstruation''.<ref name=profet>{{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 }}</ref> Overt menstruation (where there is blood flow from the vagina) occurs primarily in humans and close evolutionary relatives such as chimpanzees.<ref name=strassmann/> Some species, such as domestic [[Biology:Dog|dog]]s, experience small amounts of vaginal bleeding while approaching heat;<ref>{{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}}</ref> this discharge has a different physiologic cause than menstruation.<ref>{{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 }}</ref>

===Concealed ovulation===
A few mammals do not experience obvious, visible signs of fertility ([[Philosophy:Concealed ovulation|concealed ovulation]]). In humans, while women can learn to recognize their own level of fertility (fertility awareness), whether men can detect fertility in women is debated; recent studies have given conflicting results.<ref>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 }}</ref><ref>Study that found male sexual behavior is not affected by female fertility:
:{{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 }}</ref>

[[Biology:Orangutan|Orangutan]]s also lack visible signs of impending ovulation.<ref name=knott>{{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}}</ref> Also, it has been said that the extended estrus period of the [[Biology:Bonobo|bonobo]] (reproductive-age females are in heat for 75% of their menstrual cycle) <ref>{{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 }}</ref> has a similar effect to the lack of a "heat" in human females.<ref name=stanford>{{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}}</ref>

===Evolution===
Most female mammals have an estrous cycle, yet only ten primate species, four [[Biology:Chiroptera|bat]] species, the elephant shrew, and one known species of [[Biology:Spiny mouse|spiny mouse]] have a menstrual cycle.<ref>{{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}}</ref><ref>{{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 }}</ref> As these groups are not closely related, it is likely that four distinct evolutionary events have caused menstruation to arise.<ref name="Emera_2012">{{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 }}</ref>

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.<ref>{{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 }}</ref> The reason is likely related to differences in the ovulation process.<ref name="Emera_2012" />

Most female [[Biology:Eutheria|placental mammals]] 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 [[Biology:Trophoblast|trophoblast]] penetration of the endometrial wall,<ref>{{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 }}</ref> 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 estrous 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.<ref name="Emera_2012" /> This may be necessary in humans and other primates, due to the abnormally large number of genetic disorders in these species. <ref name=":0">{{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 }}</ref> Since most [[Medicine:Aneuploidy|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 [[Biology:Decidual cells|decidual cells]].<ref name=":0" />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}}<ref>{{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}}</ref> 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.png|thumb|400px|Phylogeny tree of menstruating and select non-menstruating mammals. Each color indicates a convergent evolutionary event: green, [[Biology:Primate|Primate]]s; blue, Chiroptera; orange, Afrotheria; yellow, Rodentia.<ref name="Emera_2012"/><ref>{{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 }}</ref><ref>{{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 }}</ref>]]

==Animal estrous cycles==
The female will ovulate spontaneously and be receptive to the male to be bred (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 [[Biology:Artificial insemination|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.<ref><big>[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]</big> ({{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)</ref> Different ways of injecting or feeding hormones to livestock are costly, and have variable success rates.<ref name="Estrous">{{cite web|title=Estrous|url=http://www.uwyo.edu/wjm/repro/estrous.htm|publisher=University of Wyoming|access-date=September 15, 2011}}</ref>

Average length (days) of estrus and estrous cycles:<ref name="Estrous"/> 
{| class="wikitable"
|-
! Species !! Estrus !! Cycle
|-
| Mouse, rat || 0.5 || 4
|-
| Hamster || 1 || 4
|-
| Rabbit || 2 || 4
|-
| Guinea pig || 0.5 || 16
|-
| Sheep || 2 || 17
|-
| Goat || 3 || 20
|-
| Cattle || 0.5 || 21
|-
| Pig || 2 || 21
|-
| Horse || 5 || 21
|-
| Elephant || 4 || 22
|-
| Red kangaroo || 3 || 35
|-
| Lion || 9 || 55
|-
| Dog || 7 || 60
|}

== See also ==
* Endometrium
* Estrous cycle
* [[Biology:Breeding pair|Breeding pair]]
* [[Biology:Reproduction|Reproduction]]
* Animal husbandry

== References ==
{{Reflist|2}}

== External links ==
* {{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 physiology]]
[[Category:Zoology]]

{{Sourceattribution|Menstruation (mammal)}}