Why do humans menstruate when most animals don’t?
In: Biology
Because we weren’t meant to live like this. Humans evolved constantly underfed and females were pregnant most of the time from maturity to death. This had a huge toll on their bodies and there was no extra energy for “periods”. Combined with not having a specific mating season, it was a good enough system. Now, women’s bodies have way more rescources and are rarely pregnant (compared to then) so this tiny backup mechanism in our biology to quickly flush out the gunk has become a huge painful part of women’s everyday lives. So it’s basically a side effect of our changed lifestyle and how society evolves faster than biology.
Essentially because procreation is so hard for humans. The foetus behaves more like a parasite than anything else, including having the ability to regulate the mother’s heart rate, temperature, and blood pressure. The body will also divert resources away from the mother and into the foetus: vitamins and minerals such as calcium which means many mothers come out of pregnancy with teeth and bone depletion and other insufficiencies. Then, if the mother is able to breastfeed with the amount and duration the body is designed to that’s another couple of years of diverted resources. So menstruation is about making sure only the best, most viable zygotes survive and in the best environment possible. Thats why chemical pregnancy (a loss caused by ineffective/insufficient implantation into the uterine wall, always before 6 weeks) is the most common loss. It’s hard to track because most women dont realise they’ve had one, it generally just manifests as a late period, but some estimates put the number as high as half of all conceptions, including all other miscarriages.
Short answer: We don’t know.
Long answer: There are some theories of why menstruation is only found in a few mammal species.
[Here](https://onlinelibrary.wiley.com/doi/full/10.1002/bies.201100099) are a few of them:
1. Menstruation evolved because it’s useful to clean the uterus from unused sperm.
2. Menstruation and the cyic regression are energetically more favorable than mantaining the endometrium in it’s active state.
3. Menstruation is “pre-conditioning” the uterus for the high level of inflammation and oxidatve stress during the placentation.
4. It’s a mechanical result after the spontaneous conversion of the cells inside the uterus (dezidualisation) which is normally induced by the embryo cells. However, it has beem observed that the dezidualisation is often induced spontaneously in higher primates during the pregnancy to force an abort of defect embryos.
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Part of the reason could be that humans are fertile year round while many species have a small window where mating is possible.
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I also read about a theory once that early humans had advantages if the could have sex all year round and not just to specific times (like dogs for example), because you could use sex to “buy” food or other goods. So this trait got passed along.
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