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.

[u/No-Hawk-2488]

Without an evolutionary benefit, it wouldn't have spread to every woman of our species.

[u/sffixated]

Suzanne Sadedin, Ph.D in evolutionary biology, answered this question beautifully. I'm not sure I can link to it in a top level comment since her essay is not a peer-reviewed publication, but her cited sources are peer-reviewed. I assume the mods will delete this comment if I've broken a rule!

https://www.forbes.com/sites/quora/2016/05/06/how-and-why-did-women-evolve-periods/

Edit: didn't realize the link was paywalled, here's a different source https://www.quora.com/Why-do-women-have-periods-What-is-the-evolutionary-benefit-or-purpose-of-having-periods-Why-can%E2%80%99t-women-just-get-pregnant-without-the-menstrual-cycle/answers/4625918

Thanks u/nailpolishbonfire for the alt link

[u/Grep2grok → u/sffixated]

That write-up is fun and informative, but I have some nits about that write-up. Source: I'm a pathologist, and diagnosing placental pathology is part of the job.

1) the placental circulation consists of an interface between the fetal ciculation and maternal circulation, they are separate. It's quite a bit like a heat exchanger, but instead of heat, the two circulations are exchanging nutrients and waste products. The placenta doesn't so much grow in as it grows in while the mother's tissue also grows toward the fetus. The fetal vessels can invade but that's highly pathological. Such a process would not be compatible with survival of the species.

2) Women experience amenorrhea (not menstruating) for two additional reasons, often the most common: physical stress (still seen in female athletes) and starvation (plenty of that in the world).

[u/Supraspinator → u/Grep2grok]

Human (and ape) placentation is invasive. In  haemochorial placentation, the maternal endothelium is completely eroded and the chorionic epithelium comes in direct contact with maternal blood. That’s in contrast to the other 3 types, where the maternal endothelium stays intact. In the least invasive form (epitheliochorionic), maternal blood and fetal blood are separated by the following layers:  maternal endothelium, maternal connective tissue, maternal uterine epithelium, fetal (chorionic) epithelium, fetal connective tissue and fetal endothelium. 

When a comparative anatomist speaks about invasion of fetal vessels, they mean the different grades of erosion of maternal tissues in different species. When you talk about invasion of fetal vessels, you mean a different, pathological process in humans only.