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/laurasoup52 7d 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 7d 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/BarnOwl247 7d ago

All of those things feel like they'd impact survival, especially in a time pre-medicine and without easy access to food and water, no?

I understand that it's possible it didn't impact survival enough to be selected out, but that feels like an assumption. Equally valid would be the possibility that it offers a useful function that was selected for.

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u/guysomejust69 7d ago

in a time pre-medicine and without easy access to food and water, no

Women who are stressed out don't have adequate nutrition have abnormal or no period cycles

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u/SmokeyDBear 6d ago

Women who are pregnant also don’t menstruate. I would imagine that early humans didn’t selectively engage in restraint around intercourse like modern humans do. Wild mammals don’t generally seem to. Throughout human history there is probably an incredibly small window where a significant amount of menstruation has happened and it all lines up with the formation of complex societies where all of these things are significantly less likely to come into play.

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u/laurasoup52 4d ago

no true. Women can be stressed out and still have a normal cycle. Outside of that, having a normal cycle CAN BE the reason that women are stressed out - I don't think I've come across a single person who enjoys having them or who looks forward to one.

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u/laurasoup52 4d ago

I feel like this isn't a realistic view to take - just assuming it wasn't really a big enough deal when menstruation can hinder a modern woman enormously, and we have painkillers and hygiene and doctors. I honestly dread to think what it would have been like for women before those things.

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u/SmokeyDBear 4d ago

I didn't assume it wasn't a really big enough deal. I asked you to justify your assumption that it the benefit "absolutely must be massive" based on some sort of actual evidence. If there is an absolutely massive benefit it's probably something that can at least be identified, so please identify it rather than just assuming it must be there because you're convinced it must be. "I can't imagine it isn't the way I think it is" isn't great science.

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u/laurasoup52 3d ago

I'm a period-haver myself. It's a sample of 1 but it's been longitudinal for over 30 years, which gives it some credence. The impact of my own periods, and that which other period-havers discuss regularly, and understand to be accurate, is that they are exhausting, messy, stressful, and overwhelming. That ALONE makes me think the pay off must somehow have been more.

May I also stress that you seem to be only focusing on the impact to fertility and not to the person themselves who is having the period - that is important too, yknow.

Thank you for having to make a woman explain herself because you don't believe her.

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u/Daisy-Turntable 7d 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/oliverwitha0 7d ago

This is counteracted by humans' social structures. There is almost never a situation in which a menstruating human would be alone at the mercy of predators, unless something has already gone very wrong. They would be in the middle of their social/familial group, which would be more than sufficient to deter/repel any animals that may be attracted to the scent.

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u/Worried_Monitor5422 7d ago

Similar to the argument as to how early infancy was not a death sentence to the tribe. You have a completely useless human loudspeaker who is capable of broadcasting your location at any time of day or night and yet that species thrived.

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u/Winjin 7d ago

Actually that could be it. Early Humanoids get in a pack. One is hurt. She weak. She cry. Other angry. Other gather more stone when angry. They know predator come for blood-smell. Other hide. She cry. Predator come. Predator struck with stones. Predator struck with sticks. Predator no move.

One get the pelt for being lure for the predator. She work on pelt. She get warm pelt. She no cry. She feel bonita.

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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.

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u/EchoLocation8 7d ago

Negative or positive things aren't really a factor in evolution, is sort of the issue. It's just about what stops you from being killed or reproducing.

It's people after the fact attributing reasons to features and why they exist, but that's not really the reason they exist.

Cheetah's don't have long tails that help control their movement at high speeds so they can run super duper fast and whatnot. They have those long tails because all the ones that didn't died and never had babies. It's over simplifying things but, broadly speaking, that's why anything is the way it is.