Menstruation is a peculiar phenomenon that women go through on a roughly monthly cycle, and it’s not immediately obvious from an evolutionary standpoint why they do it. It’s wasteful — they are throwing away a substantial amount of blood and tissue. It seems hazardous; ancestrally, in a world full of predators and disease, leaving a blood trail or filling a delicate orifice with dying tissue seems like a bad idea. And as many women can tell you, it’s uncomfortable, awkward, and sometimes debilitating. So why, evolution, why?
One assumption some people might make is that that is just the way mammalian reproduction works. This isn’t true! Most mammals do not menstruate — they do not cycle their uterine linings, but instead only build up a thickened endometrium if fertilization occurs, which looks much more efficient. Of the mammals, only most primates, a few bats, and elephant shrews are among the lucky animals that menstruate, and as you can see from the phylogeny, the scattered diversity of menstruating mammals implies that the trait was not present ancestrally — we primates acquired it relatively late.
Apparently, there are various hypotheses suggested by biologists, but PZ Myers discusses a new one:
A new paper by Emera, Romero, and Wagner suggests an interesting new idea. They turn the question around: menstruation isn’t the phenomenon to be explained, decidualization, the production of a thickened endometrial lining, is the key process.
All mammals prepare a specialized membrane for embryo implantation, the difference is that most mammals exhibit triggered decidualization, where the fertilized embryo itself instigates the thickening, while most primates have spontaneous decidualization (SD), which occurs even in the absence of a fertilized embryo. You can, for instance, induce menstruation in mice. By scratching the mouse endometrium, they will go through a pseudopregnancy and build up a thickened endometrial lining that will be shed when progesterone levels drop. So the reason mice don’t menstruate isn’t that they lack a mechanism for shedding the endometrial lining…it’s that they don’t build it up in the first place unless they’re actually going to use it.
So the question is, why do humans have spontaneous decidualization?
The answer that Emera suggests is entirely evolutionary, and involves maternal-fetal conflict. The mother and fetus have an adversarial relationship: mom’s best interest is to survive pregnancy to bear children again, and so her body tries to conserve resources for the long haul. The fetus, on the other hand, benefits from wresting as much from mom as it can, sometimes to the mother’s detriment.
I won't get into the details here. You can check out Pharyngula yourself for that, if you want. But the basic idea is that menstruation is a "consequence of self-defense" - basically, a way for a woman to defend herself from her own fetus! Neat, huh?
In evolutionary terms, a woman doesn't have quite the same interests as her fetus. For the most part, yes. But not entirely so. And as Myers points out, men and women don't have identical interests - in evolutionary terms, note - either:
The maternal-fetal conflict is also a conflict between males and females: it is in the man’s reproductive interests to have his genes propagated in any one pregnancy, while it is in the woman’s reproductive interests to bail out and try again if conditions aren’t optimal for any one pregnancy. This conflict is also played out in culture, as well as genetics — pro-choice is a pro-woman strategy, anti-abortion is a pro-man position. Sometimes, politics is a reflection of an evolutionary struggle, too.
I hope I don't need to point out that this isn't about applying "good" and "bad" labels to this kind of evolutionary conflict. It just is. This is strictly a matter of biology, of differing interests in an evolutionary sense. If this hypothesis is confirmed through additional research, it will just explain why women menstruate, nothing more.
But it's a neat idea, isn't it?
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