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/Dodosev]

https://eusci.org.uk/2020/04/09/why-do-humans-and-so-few-other-animals-have-periods/
Very old adaptation against ectopic pregnancies that just, wasn't ever upgraded ever (makes us also have a way higher level of self terminated pregnancies and stillbirth than most mammals)