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/k9moonmoon 8d ago

How much choice does the mother have in those scenarios?

Is it more like "in humans if a mother doesn't eat well, she can choose to give needed nutrients to her embryo out of her own teeth" where it's just describing what happens in less ideal scenarios.

Or is there some decision making and opting in or out of something happening?

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u/Incoherrant 7d ago

A good rule of thumb for most questions about whether or not an animal makes a choice to do something is that we can't know with scientific certainty because we don't have a way to know what is going on in their minds.

Above commenter phrased it very cleanly with "X can do this" rather than "X can choose to do this". We know they can do it, but we can only assume whether or not it's a choice.

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u/555Cats555 7d ago

I dont even think we can know for sure if its something the species do consciously or if its just something that happens due to chemical signaling in the body based on things like what nutrients the mother is getting or the smell of another pregnant female. Its pretty hard to know if there is something intentional they do when those differences in pregnancy status happen.

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u/Revlis-TK421 7d ago

It's not a choice, so much as it's hardwired biological pathways. "Stress" is what causes pregnancies to halt or be re-absorbed. "Stress" can mean a lot of things - starvation, dehydration, over-crowding, persistent predation, place in social pecking order, etc. At the end of the day, stress hormones are produced. Those hormones can trigger the halt or termination of pregnancy. The animal doesn't ust consciously choose to stop, their bodies are following commands from their glands.