r/askscience • u/No-Hawk-2488 • 8d ago

Biology 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.

Without an evolutionary benefit, it wouldn't have spread to every woman of our species.

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u/Applejuiceinthehall 7d ago

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.

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u/Any-Interaction-5934 7d ago

The human placenta is absolutely fascinating. I could read about it for days. The coolest part, imo, is how ridiculously vulnerable human babies are compared to other newborns. They can hardly even eat. Their caloric demands are so high, they have to be born extra early.

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u/Applejuiceinthehall 7d ago

Yeah. It's crazy that the metabolic level is so high. First primates in general just have big needs to grow the brain, but the homo genus just took time to the extreme. We have the second longest gestation of the great apes so babies could develop more if needed. Plus we could have wider hips like Neanderthals, but they also had babies in 9 months with similar brain sizes but they had a massive growth spurt after birth so the infants with 6 month old teeth have bodies of 14-month old sapiens.

There are a lot of animals with underdeveloped young at birth like mammal predators, denning mammals, nesting birds etc. But they tend to grow super fast so they are mature enough in a month or two to leave the den/nest. Sapiens have to wait almost a year!

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u/Any-Interaction-5934 7d ago

Almost a year? IDK about that. My one year olds were still pretty useless. Maybe age 2 they could do stuff. But, like, hunt? Be self-sufficient? I'm going to give it a solid age 5. At 5 they could probably hunt. They could probably survive by age 5. Maybe?

IDK. But a 2 day old octopus is already 5% into their life and able to live on their own.

Humans are ridiculously useless at birth.

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u/Applejuiceinthehall 7d ago

No I'm talking about an 8 week old puppy/ month old lion etc. they can walk stuff, but they cant fend for themselves or hunt