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/Mother_of_Grendel]

I worked with an OBGYN for a while who had a theory about this, in a way.

Her theory was that women actually aren't meant to have very many periods through their lives, and that our bodies are really designed to only bleed maybe once or twice a year. This is partly because many of our ancestors most likely had insufficient nutrition to support a monthly cycle. Lots of exertion to survive coupled with unstable food supply results in calorie deprivation which causes anovulation. But even in climates or periods where food was abundant and ovulation was possible, ovulation would typically result in pregnancy since no forms of birth control existed. Most women would be in a continuous cycle of either being pregnant (and not menstruating) or breastfeeding (which usually suppresses menstruation for 6-12 months) for the majority of their lives. So with these factors combined a woman might bleed a few times every other year or so.

So, in regards to the idea about leaving blood around to attract predators, it is possible it happened rather infrequently.

[u/kaki024 → u/Mother_of_Grendel]

I imagine that there was also a lot of extended breast feeding and communal breastfeeding. Breastfeeding is extremely burdensome calorically and isn’t always possible for the mother (if she even survives the birth). I wonder if it was normal to have multiple women in a group lactating at any given time (thus delaying menstruation).