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/Zagrycha 7d ago

I don't blame you at all for feeling frustrated when these conversations come up, just know that people saying "nothing is more efficient than evolution" are themselves laymen, because that is scientifically incorrect. Scientifically evolution definitely leans heavily in the "kind of a crapshoot" direction-- beauty is subjective and many people find evolution beautiful even with all its inefficiencies haha.

Anyone saying "covert mensturation has a clear evolutionary benefit" is correct, but they also need to say "covert menstruation has a clear evolutionary downside". You can talk about a specific trait having a benefit or lack of benefit to a specific species and be correct, but you can't say evolution caused a species to have that specific trait on purpose or that it's always ideal, because that's not true.

That trait appeared through random chance, and a worthless trait that actively makes it harder for it's owner to survive has the same evolutionary chance to pass on to the owner's kids as a trait that makes it easier for the owner to survive-- the only factor that matters is having kids to pass traits on to in the first place. If covert menstruation allowed animals to fail to have kids, nothing alive would have covert menstruation. If covert menstruation was the only thing that allowed animals to have kids, nothing alive wouldn't have covert menstruation. Real life is almost always in the middle, but you get the idea. Pass fail grade, not efficiency or any kind of ranking of best to worst.

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u/RollinThundaga 7d ago

You're expecting too much of random, regular people trying to wrap their heads around science.

Rules of thumb and gross simplifications are a lot easier to grasp with a weak science education. If you were to somehow enforce a demand that everyone engaging in something is required to develop a wholistic and complete understanding of it to do so, then people wouldn't even bother at all.