Menstruation (mammal)

For menstruation in humans, see menstruation.

Menstruation in mammals is the shedding of the uterine lining (endometrium). It occurs on a regular basis in reproductive-age females of certain mammal species.

Though there is some disagreement in definitions between sources, menstruation is generally considered to be limited to primates. Overt menstruation (where there is bleeding from the uterus through the vagina) is found primarily in humans and close evolutionary relatives such as chimpanzees.^[1] It is common in simians including Old World monkeys and apes and New World monkey, but variably expressed in prosimians, being completely lacking in strepsirrhine primates and possibly weakly present in tarsiers. Outside the primates it is known only in bats and the elephant shrew, an insectivore.^[2][3][4][5]

Females of other species of placental mammal undergo estrous cycles, in which the endometrium is completely reabsorbed by the animal (covert menstruation) at the end of its reproductive cycle. 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

See also: Estrous cycle

Females of most mammal species advertise fertility to males with visual behavioral cues, pheromones, or both.^[6] This period of advertised fertility is known as oestrus, "estrus" or heat.^[6] In species that experience estrus, females are generally only receptive to copulation while they are in heat^[6] (dolphins are an exception).^[7] In the estrous cycles of most placental mammals, if no fertilization takes place, the uterus reabsorbs the endometrium. This breakdown of the endometrium without vaginal discharge is sometimes called covert menstruation.^[8] Overt menstruation (where there is blood flow from the vagina) occurs primarily in humans and close evolutionary relatives such as chimpanzees.^[1] Some species, such as domestic dogs, experience small amounts of vaginal bleeding while in heat; this discharge has a different physiologic cause than menstruation.^[9]

Concealed ovulation

See also: concealed ovulation

A few mammals do not experience obvious, visible signs of fertility (concealed ovulation). In humans, while women can be taught to recognize their own level of fertility (fertility awareness), whether men can detect fertility in women is debated; recent studies have given conflicting results.^[10][11]

Orangutans also lack visible signs of impending ovulation.^[12] 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) ^[13] has a similar effect to the lack of a "heat" in human females.^[14]

Evolution

All female placental mammals have a uterine lining that builds up when the animal is fertile, but is dismantled (menstruated) when the animal is infertile. Some anthropologists have questioned the energy cost of rebuilding the endometrium every fertility cycle. However, anthropologist Beverly Strassmann has proposed that the energy savings of not having to continuously maintain the uterine lining more than offsets energy cost of having to rebuild the lining in the next fertility cycle, even in species such as humans where much of the lining is lost through bleeding (overt menstruation) rather than reabsorbed (covert menstruation).^[1][15] However, even in humans, much of it is re-absorbed.

Many have questioned the evolution of overt menstruation in humans and related species, speculating on what advantage there could be to losing blood associated with dismantling the endometrium rather than absorbing it, as most mammals do.

Animal menstrual cycles

Among animals that have menstrual cycles, these are frequently different from the 29.53 day lunar cycle; the average cycle length in orangutans is the closest to that of humans, 28 days,^[12] while the cycle for chimpanzees is 35 days^[16] and the cycle for sheep is just 16 days.^[17]

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 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 different ways of injecting or feeding hormones to livestock. They are costly, and have variable success rates. ^[18]

Average length (days) of estrus and estrous cycles:^[19]

Species	Estrus	Cycle
Mouse, rat	0.5	4
Hamster	1	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
• Breeding pair
• Reproduction
• Animal husbandry

External links

• Beef Cattle Production
• Mare's Estrous Cycle
• The Canine Estrous Cycle
• Managing the Sow and Gilt Estrous Cycle

Footnotes

1. ^ ^{a b c} Strassmann BI (1996). "The evolution of endometrial cycles and menstruation". Q Rev Biol 71 (2): 181–220. doi:10.1086/419369. PMID 8693059. 
2. ^ PMID 8210311
3. ^ PMID 18046752
4. ^ Elsimar M. Coutinho and Sheldon J. Segal (1999). Is menstruation obsolete?. Oxford University Press. http://www.uwyo.edu/wjm/repro/menstrua.htm. 
5. ^ Paul Bischof and Marie Cohen. [1.pdf "Course 4:Implantation"]. European Society of Human Reproduction and Embryology. http://www.eshre.eu/binarydata.aspx?type=doc&sessionId=d3g32gezrjsijs554mxn33fb/syllabus_course4[1].pdf. 
6. ^ ^{a b c} "Estrus". Britannica Online. http://www.britannica.com/EBchecked/topic/193725/estrus. Retrieved 5 October 2008. 
7. ^ Mikkelson, Barbara and David P. (29 June 2007). "Buried Pleasure". Snopes.com. http://www.snopes.com/critters/wild/pleasure.asp. Retrieved 5 October 2008. , which references:

   Diamond, Jared M. (1997). Why is sex fun?: the evolution of human sexuality. London: HarperCollins. ISBN 0-465-03127-7. 

8. ^ Profet M (1993). "Menstruation as a defense against pathogens transported by sperm". Q Rev Biol 68 (3): 335–86. doi:10.1086/418170. PMID 8210311. 
9. ^ "Canine False Pregnancy and Female Reproduction". Mar Vista Animal Medical Center. 2 February 2008. http://marvistavet.com/html/body_canine_false_pregnancy.html. Retrieved 5 October 2008. 
10. ^ Studies that found women are more attractive to men when fertile:

    S.C. Roberts, J. Havlicek, J. Flegr, M. Hruskova, A.C. Little, B.C. Jones, D.I. Perrett and M. Petrie, M. (August 2004). "Female facial attractiveness increases during the fertile phase of the menstrual cycle". Proc.R.Soc.Lond.B (Suppl.) 271: S270–2. doi:10.1098/rsbl.2004.0174. PMC 1810066. PMID 15503991. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1810066. 

    Geoffrey Miller, Joshua M. Tybur and Brent D. Jordan (June 2007). "Ovulatory cycle effects on tip earnings by lap dancers: economic evidence for human estrus?" (PDF). Evolution and Human Behavior 28 (6): 375. doi:10.1016/j.evolhumbehav.2007.06.002. http://www.unm.edu/~gfmiller/cycle_effects_on_tips.pdf. Retrieved 21 January 2008. 

11. ^ Study that found male sexual behavior is not affected by female fertility:

    Susan B. Bullivant, Sarah A. Sellergren, Kathleen Stern, et al. (February 2004). "Women's sexual experience during the menstrual cycle: identification of the sexual phase by noninvasive measurement of luteinizing hormone". Journal of Sex Research 41 (1): 82–93. doi:10.1080/00224490409552216. PMID 15216427. http://www.findarticles.com/p/articles/mi_m2372/is_1_41/ai_n6032944. 

12. ^ ^{a b} Knott, Cheryl (2003). "Orangutans: Reproduction and Life History". Gunung Palung Orangutan Project. Harvard University. Archived from the original on April 21, 2008. http://web.archive.org/web/20080421175238/http://www.fas.harvard.edu/~gporang/orangutans.html. Retrieved 5 October 2008. 
13. ^ Lanting, Frans; Waal, F. B. M. de (1997). Bonobo: the forgotten ape. Berkeley: University of California Press. pp. 107. ISBN 0-520-20535-9. http://www.serpentfd.org/a/dewall1997.html. Retrieved 5 September 2007. 
14. ^ Stanford, Craig B. (March–April 2000). "The Brutal Ape vs. the Sexy Ape? The Make-Love-Not-War Ape". American Scientist 88 (2): 110. doi:10.1511/2000.2.110. 
15. ^ Kathleen O'Grady (2000). Is Menstruation Obsolete?. The Canadian Women's Health Network. http://www.cwhn.ca/resources/menstruation/obsolete.html. Retrieved 2007-01-21. 
16. ^ Lacreuse A, Chennareddi L, Gould KG, et al. (September 2008). "Menstrual cycles continue into advanced old age in the common chimpanzee (Pan troglodytes)". Biology of reproduction 79 (3): 407–12. doi:10.1095/biolreprod.108.068494. PMC 2547989. PMID 18495682. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2547989. 
17. ^ As cited by Adams, Cecil, "What's the link between the moon and menstruation?" (accessed 6 June 2006):
    Abell, George O.; Barry Singer (1983). Science and the Paranormal: Probing the Existence of the Supernatural. Scribner Book Company. ISBN 0-684-17820-6. 
18. ^ "Estrous". University of Wyoming. http://www.uwyo.edu/wjm/repro/estrous.htm. Retrieved September 15, 2011. 
19. ^ "Estrous". University of Wyoming. http://www.uwyo.edu/wjm/repro/estrous.htm. Retrieved September 15, 2011.