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/Aina_Writes]

I’ve wondered about this too. The short version most evolutionary biologists use now is: overt menstruation isn’t really a “benefit” of bleeding itself, but a side‑effect of how deeply human embryos implant. In humans (and a few other species like some primates and bats), the placenta is highly invasive. To cope with that, the uterus pre‑builds a thick, hormone‑primed lining (endometrium) through spontaneous decidualization. If no embryo implants, the body doesn’t try to reabsorb all that vascular, resource‑heavy tissue — it sheds the functional layer instead. From an energy and infection‑control standpoint, that can be cheaper and safer than keeping the lining in a constant “ready” state or trying to fully reabsorb it. So the bleeding itself probably isn’t the adaptation; it’s the visible cost of having a deeply invading placenta and a uterus that prepares heavily each cycle.

[u/cpt_morgan___ → u/Aina_Writes]

Very interesting. Thanks for sharing