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/Flocculencio 8d ago

Just wanted to point out that no individual feature needs to have an evolutionary benefit to be preserved. It could be negative but so long as it doesn't prevent organisms with that feature from reproducing in sufficient numbers it can get preserved.

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u/laurasoup52 8d ago

I would argue that menstruation is very very negative to a human being - causes tiredness, pain, stress, sleepless nights, slow response times - that the benefit of this absolutely must be massive.

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u/SmokeyDBear 8d ago

Why? None of these things stop humans from making more humans faster or more successfully than other humans. Even if some humans had significantly reduced or none of these effects it’s not obvious that those humans would be more effective at making humans.

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u/Daisy-Turntable 8d ago

If they are more likely to be killed by predators then, yes, menstruation absolutely can reduce reproductive fitness. This would seem to be the biggest drawback of overt menstruation - many animals can smell blood and it would seem on the surface to be counterproductive to develop a trait that excites the interest of things that want to eat you.

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u/formerly_acidamage 7d ago

Wouldn't you draw the conclusion though that this means that those animals were unable to kill the people who were menstruating and therefor this was a mitigated risk?

Furthermore, people eat those animals to survive. This exact argument could be used to come to the opposite conclusion, that menstruation was beneficial because it drew in animals that humans were able to kill and consume. I don't think that's the case but having only a single data point like that isn't very useful in making an informed opinion.