Why do women menstruate?

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.

Phylogeny showing the distribution of menstruation in placental mammals and the inferred states of ancestral lineages. Menstruating species/lineages are colored in pink, non- menstruating species/lineages in black. Species in which the character state is not known are not colored, and lineages of equivocal state are represented with black lines. Monodelphis represents the outgroup. Inference of ancestral states was performed in MacClade 4 by the parsimony method. Note that there is strong evidence for three independent originations of menstruation among placental mammals.

I suppose we could blame The Curse on The Fall, but then this phylogeny would suggest that Adam and Eve were part of a population of squirrel-like proto-primates living in the early Paleocene. That’s rather unbiblical, though, and what did the bats and elephant shrews do to deserve this?

There are many explanations floating around. One is that it’s a way to flush out nasty pathogens injected into the reproductive tract by ejaculating males — but that phenomenon is ubiquitous, so you have to wonder why only a few species bother. Another explanation is that it’s more efficient to get rid of the endometrium when not using it, than to maintain it indefinitely; but this is a false distinction, because other mammals don’t maintain the endometrium, they just build it up in response to fertilization. And finally, another reason is that humans have rather agressive embryos that implant deeply and intimately with the mother’s tissues, and menstruation “preconditions” the uterine lining to cope with the stress. There is, unfortunately, no evidence that menstruation provides any boost to the ‘toughness’ of the uterus at all.

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?

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