Hey this is something I actually know about
Forgive any mistakes it’s been a while since I studied this but very very surface level explanation is that menstruation occurs in mammals that go through spontaneous decidualization. Decidualization meaning hormones tell the uterus cells to start thickening and a bunch of other stuff to support a pregnancy, spontaneous decidualization means this process occurs regardless of if there is actually a pregnancy.
most animals don’t menstruate because it’s a very costly process, building up a lot of stuff in the uterus and then expelling it is very wasteful and pain also creates vulnerability to predators. This is why menstruation is a rare evolutionary phenomenon with most species simply reabsorbing the uterus lining. Even among species that do menstruate humans are considerably the worst off as bleeding in other species is minimal and they do not show signs of intensive pain, the exception is this poor shrew that does demonstrate altered behavior.
The question becomes why would this occur, especially multiple times, and why are humans so much worse off? Long story short, the amount an embryo attaches to the uterine lining differs between species, this is referred to as how invasive it is and there’s a bunch of categories. decidualization only occurs in species with invasive embryos that tear into the maternal tissue. Invasive embryos need a very thick lining to avoid damaging the uterus. Basically the lining is both to protect the mother from the embryo, and nourish a very high maintenance embryo.
Another factor is that often times the embryos of menstruating species are not viable and the complexity of the lining allows the body to determine if the embryo is viable before putting in a bunch of resources into growing it and this is very important as menstruating mammals have a low number of offspring, usually 1 per pregnancy and a lower amount of pregnancies throughout lifespan.
This lining would take a lot of energy to maintain, also it’s a lot of material, so if a pregnancy does not occur it’s more efficient to expel it and remake it later rather than maintaining it all the time. Due to the amount of material that needs to be expelled this can cause pain or discomfort. The reason it is beneficial to prepare the lining in advance of a pregnancy rather than when it occurs is that by then the damage from the invasive embryo would have been done.
As for why humans are so much worse off it’s unclear. Human embryos are very aggressive and likely because our big human brains take A LOT of resources to develop and thus need a very complex lining to avoid damaging the uterus while still supporting the embryo. There is also the idea that humans do not continuously face the pressure that other menstruating mammals do that if you are impaired by pain you are much more likely to be eaten by a predator so statistically most of the individuals who survive have less impairing menstruation cycles.
TLDR mammals that menstruate have invasive embryos and a lower amount of average offspring so a protective, quality, uterine lining is required in advance of a pregnancy and if no pregnancy occurs it is more efficient to expel the lining rather than maintaining it all the time. Humans are special in that ours are extra wasteful and painful than the others yay us!
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