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When I watch wilderness specials for more than a few minutes, I notice a familiar pattern: predators are depicted as being alerted by the scent of blood. Wounded animals seem to make the best prey and predators seem to have a heightened awareness for the smell of blood.

It would seem that it would then be a disadvantage to evolutionary survival for female organisms to menstruate. How do females of a species not attract predators to the signal of menstrual blood?

Do female organisms in the wild actually menstruate? If so, is there a difference in the composition of menses that does not alert predators? How would this not at the very least give predators an idea of the general vicinity of the female?

If wild animals excrete their menses, do they have ways in which they are able to mask the evidence of its release or are they somehow internalizing the tissue so that they are not released?

If female organisms in the wild do menstruate, then why is this not a survival disadvantage?

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    $\begingroup$ Most mammals reabsorb the endometrium (estrous). I believe only primates actually menstruate. $\endgroup$ – canadianer Sep 28 '14 at 8:55
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    $\begingroup$ In addition to the rarity of menstruation in animals, it's worth noting that menses are only about 50% actual blood, the remainder being tissue and mucus. The smell is probably distinct and unlikely to be confused with wound blood. A menstruating female is also neither pregnant nor nursing very small offspring, and is therefore more resistant to predators. Why follow a scent signal to a prey species that's both healthy and adult? That's the opposite of what you want to do as a predator. $\endgroup$ – Resonating Sep 28 '14 at 18:46
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Thanks to the other answer for pointing me in the right direction with some references. It seems that two biologists in the early 1990s had a back-and-forth over this topic in The Quarterly Review of Biology.1,2

A statement of the problem:

The function of menstruation is a central enigma of mammalian, and especially primate, reproductive physiology. Each cycle the uterus builds a glandular epithelium with a high secretory capacity and an elaborate microvasculature, only to reabsorb or void it with the menses if implantation does not oc-cur. Why does the endometrium not maintain a steady state of readiness for implantation by the blastocyst? What is the selective advantage of cyclical regeneration and regression?1

One hypothesis2

In 1993, Margie Profet hypothesized that menstruation is primarily advantageous as a defense against pathogens transported by sperm. During mammalian insemination, she stated, bacteria from the male and female genitalia cling to sperm tails and are transported to the uterus. Menstruation, triggered by the abrupt constriction (causing necrosis of endometrial lining) followed by dilation (causing shedding of said necrotic tissue), forces loose not only the endometrial lining but sperm-borne pathogens contained therein. In addition to shedding the uterine tissue harboring these bacteria, menstruation delivers immune cells to the uterine cavity, triggered by the mechanical pressure exerted via the shedding of endometrial lining.

In this 50 page (!) article, Profet also points out that other mammals with covert menstruation take advantage of some of these same benefits, as the immunological stimulation includes phagocytosis of pathogens by cells that may be expelled in cervical/vaginal mucous. Further, proestrous bleeding (i.e. that preceding ovulation, 180 degrees off from the human cycle), which occurs in some mammals (domestic dog, coyote, etc), seems to serve a similar function.

If not eliminated prior to estrus, these tenacious pathogens might otherwise attach to incoming sperm during estrus and ascend the oviducts.

As to why humans and other primates are subject to overt bleeding in contrast to other mammals, she lays out a theoretical argument that selection pressure to menstruate should increase with decreasing per-cycle “fecundability” (the probability of pregnancy) because copulation not followed by bleeding risks infection. In contrast to, e.g., mice (98% fecundability), humans have some of the lowest fecundability rates, primarily due to “concealed” ovulation which de-couples copulation and ovulation. (Contrast this to most mammals whose females have a defined estrous period during which they are receptive to male sexual advances; the data for the existence of such a period in humans are controversial, and sexual behavior is driven primarily by “higher” social, environmental, and cognitive cues.)

A different view1

In response/objection to this paper, Beverly Strassman in the same journal published an article outlining an alternative hypothesis about the potential adaptive value of menstruation. She proposed a hypothesis based on energy conservation:

[T]he uterine endometrium is shed/resorbed whenever implantation fails because cyclical regression and renewal is energetically less costly than maintaining the endometrium in the metabolically active state required for implantation.

Her research determined that, in its regressed state (i.e. s/p endometrial shedding), oxygen consumption in the endometrial tissue is markedly declined, nearly sevenfold. Although this would seem to be a trivial component of whole-body metabolism, she argues otherwise:

Metabolic rate is at least 7% lower, on average, during the follicular phase than during the luteal phase in women, which signifies an estimated energy savings of 53 MJ over four cycles, or nearly six days worth of food.

Her argument shares with Profer’s the implication that mammals with lower fecundability are more likely to menstruate. In those with high probability of pregnancy at every cycle, the energetic benefits of ridding the body of the need to support endometrial lining (relevant only in the absence of pregnancy), are decreased. Her hypothesis also predicts the larger volume of menstrual shedding in humans and chimps in contrast to other mammals related to the increased uterine size relative to the female body (a ratio that is largely determined by the especially large head of the human fetus).

Conclusion

As noted in the comments, only primates have overt menstrual bleeding. The adaptive benefits have been variously explained, and it is probable that there is more than one operative explanatory model. Whatever the small decrement in survivalship incurred by the intermittent and uncontrolled depositing of blood in the wild, it is apparently outweighed in these species by the processes described above.


References and Notes
1. Strassmann, Beverly I. The Evolution of Endometrial Cycles and Menstruation The Quarterly Review of Biology, June 1996; Vol. 71, No. 2 pp 181-220.

2. Profet, Margie. Menstruation as a Defense Against Pathogens Transported by Sperm. The Quarterly Review of Biology, Sept 1993; Vol. 68, No. 3 pp. 335-386.

Note: Your comparison to urine and feces is an interesting one. Perhaps a vaginal sphincter would be a good idea…..were it not for men and babies….

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    $\begingroup$ Another view is that high host of pregnancy (e.g. brain development) is driving menstruation (it is worth to remove substandard zygotes): aeon.co/magazine/science/… and quora.com/…. This way it explains why it makes sense for primates, but not - other mammals. $\endgroup$ – Piotr Migdal Oct 1 '14 at 9:19
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It is a very nice question.

From wikipedia:

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. 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.

This other wiki article says:

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. Humans do, in fact, reabsorb about two-thirds of the endometrium each cycle. Strassmann asserts that overt menstruation occurs not because it is beneficial in itself. Rather, the fetal development of these species requires a more developed endometrium, one which is too thick to reabsorb completely. Strassman correlates species that have overt menstruation to those that have a large uterus relative to the adult female body size

I just cite two wiki articles but it is probably worth going through some more reading in order to give a more complete overview of the selection pressure underlying the evolution of menstruation in mammals and especially in primates. In the first paragraph above, wiki cites 5 articles. From the title of one of those, it seems that menstruation is also a defense against pathogens transported in sperm. I hope someone will give a better answer than mine.

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protected by Chris Mar 25 '15 at 10:40

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