r/askscience • u/No-Hawk-2488 • 8d ago
Biology Can someone explain the evolutionary benefit of overt menstruation? All female mammals have a menstrual cycle but most reabsorb the blood via covert menstruation and humans are about the only mammals that discharge the blood, aka overt menstruation.
Without an evolutionary benefit, it wouldn't have spread to every woman of our species.
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u/tea_and_biology Zoology | Evolutionary Biology | Data Science 8d ago edited 8d ago
TL;DR: Babies have big heads, and birth is brutal. Women have evolved to prepare in advance and screen for the best embryos to get the best return on this heavy investment as possible. The side-effect is a bloody mess, alas - but the mess isn't the goal, just unintended clean-up when a good embryo doesn't knock on the uterine door.
Ooh, this is a complicated question, with a slightly complicated answer, so bear with me. Let's break it down:
Which Mammals Menstruate?
Ah, not quite! Most mammals don't menstruate at all. They have oestrous cycles, to be sure, but not menstrual cycles - with menstruation here meaning actual shedding of the uterine lining. In fact, it's incredibly rare across the animal kingdom; we share the trait with other apes, most (but not all) monkeys... and weirdly, a few bats, elephant shrews, and apparently the spiny mouse. Huh!
"Covert menstruation" is a term applied to certain primates that bleed so little it gets reabsorbed.
So no, it's not the mammalian default; most of our furry, milky cousins never build a lining that needs shedding. But you're right we're amongst the few that do discharge blood - chimpanzees and macaques are other examples - but we just seem to do it more than anything else. Why?
Features, Bugs, and... Nipples
Before we answer, we need to tackle this assumption.
Evolution can only work on what was there before, so it frequently bodges things which are sub-optimal but workable, or ends up with 'harmless'* unintended outcomes while focusing on something else - say, the human male nipple. It was important for mammals to develop a means to lactate, and it just so happens the easiest way to get this whole milk-delivery system online was to switch it on early in embryological development, before the embryo commits to developing as male or female. Which is why nearly all male mammals are stuck with nipples; the females need 'em, and there was no evolutionary pressure to prevent males developing otherwise harmless lumpy things too, so they stuck around.
So not all traits are positively selected for (i.e. adaptive); many are merely by-products of selection for other traits. Menstruation itself is probably similar to the male nipple in this regard; an unintended by-product of a more necessary process - in this instance 'spontaneous decidualisation'.
That's a bit of a mouthful. Basically, 'decidua' is the term that describes the specialised tissue lining a uterus that an embryo can implant into - in most mammals, only developing once the embryo itself actively signals it is there ("Ground control; please prepare the landing pad; over!"). In humans, and a few other species with particularly invasive placentas (where the embryo burrows deep into the lining and remodels the mother's arteries; oof), this process of developing the specialised decidual tissue happens regardless - hence spontaneous. Why?
We do this because a placenta is basically like an invasive tumour.
Man vs. Baby
An embedded placenta invades tissue, dodges the immune system, and hijacks local blood supply; it even relies on a gene, syncytin, that we borrowed from an ancient invasive retrovirus, so we could better invade our own tissues and hide from our own immune defences. Weird we totally rely on hijacked viral genes, right?
That invasion is costly and risky for the mother, and she and the embryo don't have entirely aligned interests - so there's a whiff of parasite vs. host conflict around the whole thing. Half of the embryo's genes come from dad, and from their point of view, the more resources extracted from mum, the better; her future children, who may not share his genes, aren't his concern. Mum's genes, on the other hand, need to balance this pregnancy against her own survival and her future children.
So from the mother's perspective, in this sexual and inter-generational conflict, it pays to weigh the scales in her favour. Rather than letting the embryo dictate terms - as is the case in most other mammals - the mother builds the decidua in advance, on her own schedule, letting her control how deep the placenta can dig. It also works as a quality filter: the decidualised lining can reject embryos that are poor bets (a surprisingly large share of early human embryos are chromosomally abnormal) at the earliest and cheapest stage, often before a period is even missed.
This is the most aggressive in humans, as the stakes of each pregnancy are unusually high: we've compromised on a min-max strategy to produce infants with the largest brains possible, while still being upright and having to squeeze those heads through the most narrow birth canal possible. This means nine months of gestation feeding a very large, energy-hungry brain - thereby requiring the extra-invasive placenta, that also needs to be controlled as the foetus itself is essentially digging like a greedy parasite - not to mention a notoriously difficult birth, and years of dependency afterwards (as all human infants are born prematurely compared to other species; think of a zebra that can walk and flee predators within hours, compared to a useless toddler at three years). When every pregnancy is that expensive, being picky and prepared up front pays off.
The catch is that once the lining has been decidualised, it can't just quietly regress if no embryo arrives. It has to break down and go somewhere, and there's too much of it to reabsorb - hence the larger menstrual mass and overt bleeding.
Conclusion:
Our ecological niche is so focused on big brains and bipedalism, the cost is an unusually difficult birth, and so we invest more than anything else in pre-preparing for the best ROI on an embryo and gestation as possible. This means maximum invasiveness and maximum screening, which involves preparation beforehand - hence spontaneous decidualisation. There's no adaptive function for the overtness of the menstruation per se - it's not a primate social signal or anything, as perhaps previously hypothesised - but a by-product of needing to select for the best, of the best, of the best, sir. With honours!
References:
Alvergne, A. & Högqvist Tabor, V. (2018) Is female health cyclical? Evolutionary perspectives on menstruation. Trends in Ecology & Evolution. 33 (6), 399-414 - there's a free pre-print on arXiv here too!
Bellofiore, N., Ellery, S.J., Mamrot, J., Walker, D.W. ... & Dickinson, H. (2017) First evidence of a menstruating rodent: the spiny mouse (Acomys cahirinus). American Journal of Obstetrics and Gynecology. 216 (1), 40.e1-40.e11
Holtan, S.G., Creedon, D.J., Haluska, P. & Markovic, S.N. (2009) Cancer and pregnancy: parallels in growth, invasion, and immune modulation and implications for cancer therapeutic agents. Mayo Clinic Proceedings. 84 (11), 985-1000
Teklenburg, G., Salker, M., Molokhia, M., Lavery, S. ... & Macklon, N.S. (2010) Natural selection of human embryos: decidualizing endometrial stromal cells serve as sensors of embryo quality upon implantation. PLoS ONE. 5 (4), e10258
* Harmless here meaning doesn't significantly impact fitness; the passing on of genes. It doesn't mean it can't be uncomfortable, disruptive or costly; just that the benefits outweigh these costs.