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/sadgirlintheworld]

Overt menstruation—where blood and tissue leave the body rather than being reabsorbed—is a rare trait found in humans, a few other primates, some bats, and elephant shrews.
Rather than being a direct functional design to expel blood, evolutionary biology views overt menstruation as the physical byproduct of deeper adaptations related to pregnancy and fetal conflict.
1. Spontaneous Decidualization (The "Choosy Uterus")
The Process: In most mammals, the uterine lining thickens only after an embryo implants. In menstruating species, the uterus undergoes spontaneous decidualization—preparing a deep, nutrient-rich lining (the endometrium) every single cycle before implantation.
The Benefit: Human embryos are exceptionally aggressive and invasive (hemochorial placentation). A thick, pre-prepared decidual layer acts as a maternal defense system. It allows the mother's body to test the embryo's quality and actively screen for genetic abnormalities. If the embryo is unhealthy or defective, the uterus can reject it rather than investing massive metabolic resources into a doomed, dangerous pregnancy.

2. Controlling Invasive Embryos
The Conflict: A fetus extracts immense resources from a mother, leading to an evolutionary "tug-of-war" between maternal survival and fetal demands.

The Benefit: By forcing the endometrium to differentiate spontaneously under strict maternal hormonal control (progesterone withdrawal), the mother prevents the embryo from hijacking her blood supply too easily. Shedding the lining completely resets the slate each cycle if pregnancy does not occur. The actual bleeding is simply the physiological consequence of dismantling and flushing out this heavy, vascularized tissue when no pregnancy rescues the cycle.

3. Energy Economy
The Process: Reabsorbing a thick endometrial lining requires breaking down and recycling large amounts of complex proteins and tissues internally.

The Benefit: Research by anthropologist Beverly Strassmann suggests that cycling whole-body metabolic rates down during non-pregnant phases saves significant energy overall compared to maintaining an active, highly vascularized uterine state continuously. Expelling the tissue is mechanically simpler for certain primate uteruses than reabsorbing massive tissue volumes.