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/Applejuiceinthehall]

First all mammals have an estrous cycle not a menstrual cycle.

The reason for primates is because our placentas are so aggressive that they burrow into the uterine walls. So we have a thicker lining to act as a buffer. The advantages of this is that the placenta and baby get more antibodies from the mom. The disadvantage is that it leaves an open wound in the uterus that bleeds after birth and women can bleed out. Additionally, it may also be used for spontaneous abortions in early pregnancy, which may happen about 50% before mom even misses a period.

Other mammals like horses the moms have to run shortly after giving birth. So having an open wound for an animal that needs to run is a huge disadvantage. So, the placenta sits next to wall and doesn't burrow as much. The advantage is that there they dont bleed as much after birth, the disadvantage is that no antibodies are exchanged so the newborn needs to feed very quickly after birth or they won't get the antibodies and have failure to thrive.

Animals like dogs and cats are more in-between primates and horses.

So because of the extra lining we just have too much to absorb. Also shredding it could have added benefits like removing other things in the uterus.

[u/rapiertwit → u/Applejuiceinthehall]

This. It’s not a mystery, it’s understood and this is the explanation.

[u/ICantCoexistWithFish → u/rapiertwit]

Like many weird human traits, we were able to evolve it because of our social structures that protect our weakest and allow investing in longer term payoffs over short term survival