A comparative perspective
The term “menstruation” customarily refers to externally visible bleeding. In fact, wet-nosed primates (lemurs + lorises) completely lack bleeding at the end of the cycle, as is only to be expected because the placenta does not invade the inner lining of the womb (endometrium). They contrast starkly with dry-nosed primates (tarsiers and higher primates), which all have a highly invasive placenta with maternal blood directly bathing the outermost fetal membrane. But there is no convincing evidence for menstruation in tarsiers, so it is seemingly restricted to monkeys, apes and humans. Even among them its expression is very variable: weak to moderate in New World monkeys, moderate in Old World monkeys and gibbons, clearly evident in great apes. But women have by far the most pronounced menstrual bleeding. So any proposed explanation of menstruation should account for its variation and, especially, remarkably heavy blood loss in women. In fact, body size is probably involved because menstrual bleeding generally increases as primates get bigger, from microscopically detectable menstruation in diminutive marmosets to obvious blood flow in chimpanzees. However, humans lie within the size range of great apes, so their particularly heavy menstruation requires additional explanation.
It is also important to note that menstruation resembling that in higher primates is exceedingly rare among other mammals. True menstruation has been reliably demonstrated only for a few bat species and for elephant-shrews. So the selection pressure driving evolution of menstruation must have been rather special.
Shedding of the uterine lining, accompanied by bleeding, occurs as progesterone declines at the end of the cycle, but it is unclear why. The traditional explanation, which I uncritically accepted as a zoology student, is this: During the luteal phase following ovulation, various changes occur in the endometrium. Blood vessels markedly proliferate and enlarge and the endometrium notably thickens. These changes prepare the womb for initial development of the placenta if conception occurs. Otherwise, the luteal phase ends in menstruation, long regarded as nothing more than shedding of redundant tissue. However, this does not explain the restricted occurrence of menstruation, as many non-primates have a highly invasive placenta that should similarly require advance preparation of the womb. Moreover, the particularly heavy menstrual bleeding of women remains unexplained.
In 1993, Margie Profet offered a radical explanation for the evolution of menstruation that attracted considerable uncritical media attention despite her non-biological background: Menstruation protects the female reproductive tract against pathogens in semen. At first, this notion seems appealing, as germs carried on sperms or in seminal fluid could seriously threaten the womb and oviducts. However, sperm-borne germs are presumably widespread among mammals, so menstruation should be equally widespread. Profet single-mindedly set out to show that menstruation is indeed universal among mammals, taking any report — however obscure — of bleeding in the female tract as evidence of “menstruation”. In itself, the basic notion of germs on sperms is unproblematic, but there is plenty of evidence that counter-adaptations in the vagina and the neck of the womb provide protection. A core problem is that Profet’s hypothesis does not explain the particularly heavy menstrual bleeding of women. In fact, a widely discussed unusual feature is that human copulation can occur virtually throughout the cycle, so up to a month could elapse before the next menstruation, making her hypothesis scarcely logical. The ultimate example of closing the stable door after the horse has bolted!
Currently, there is no widely accepted explanation for the evolution of menstruation, but a number of competing suggestions. Anthropologist Beverly Strassmann proposed that menstruation is an energy-saving adaptation. Cyclical changes in the thickness of the endometrium, universal among mammals, are particularly evident in higher primates. By the end of the luteal phase, energy consumption of the human endometrium is seven-fold higher than its starting level. Strassmann suggests that maintaining an expanded endometrium is more costly than regenerating it in each cycle, so bleeding occurs as a side effect when blood volume is too great for efficient absorption. Strassmann’s perspective on menstruation is greatly enhanced by her long-term fieldwork with the Dogon in Mali, leading her to note a further weakness of the germs-on-sperms hypothesis: In societies without contraception, menstruation occurs only rarely among women during their reproductive years. An average Dogon woman has about nine pregnancies resulting in live births, and menstruation is especially rare during prime childbearing years. Nonetheless, Strassmann’s hypothesis does not really explain why human menstrual blood loss greatly exceeds that in any other higher primate.
In 2009, gynecologist Jan Brosens and colleagues presented an ingenious new explanation. Recognizing that true menstruation is limited to higher primates and just a few other mammals, they noted that both menstruation and pregnancy are inflammatory conditions. Accordingly, they proposed that menstruation preconditions the womb to respond to deep invasion of maternal tissues during pregnancy. This proposal has the advantage that particularly heavy blood loss during menstruation can be linked to the particularly invasive nature of the human placenta. But it is difficult to see how this hypothesis can be tested.
Another tentative possibility deserves consideration: In certain bats with true menstruation, sperms are stored in the female reproductive tract. So perhaps menstruation might flush out ageing sperms before the next ovarian cycle begins. Given that sperm storage demonstrably occurs in crypts in the neck of the womb in humans, there may be some connection here. Heavy menstruation is perhaps linked to extreme extension of copulation throughout the human cycle.
The bottom line is that we still await a conclusive evolutionary explanation for human menstruation. But surely women everywhere deserve to know why they are burdened with this monthly curse.
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To see more about the evolution of human reproduction, do please check out my new website with the following link: http://robertdmartin.wix.com/how-we-do-it. Among other things, the site provides information on the 1-hour TV documentary The Nature of Sex that was inspired by my book How We Do It and is about to be launched (dedicated FaceBook page: https://www.facebook.com/sex.and.nature)