Bleeding, I mean
In: Biology
There are several species of animals that bleeds when they are in heat, for exampl dogs. But this is not menstration as in primates and a few other unrelated species. We do not know exactly why primates menstrate. We know how this developed. Most animals do not develop the placenta fully until they are pregnent and is therefore able to absorb the placenta back into their body if they are not pregnent. However in primates the cycle goes on as if we are already pregnant and then the placenta is discarded as if there was an abortion while in reality there was not any pregnancy at all.
It is speculated that this might be due to a higher number of genetic defects in primates then other animals. One example of this is that primates will suffer from Downs syndrom at a higher then normal rate. Most genetic disorders will result in babies being stillborn. However in primates a lot of genetic disorders will instead result in abortions at this early stage. It is therefore theorized that menstration is a system which helps primates abort fetuses at an early stage if there are genetic defects rather then carrying the baby full term as most animals would do. However this is just speculation and we can not know for sure. For example it could also be an undesired mutation which just happened to cling on.
There are a few other mammal species that menstruate. certain shrews, and some bats. Many other species have similar reproductive cycles, with their endometrium growing with each ovulation cycle, but most just reabsorb the endometrium, rather than shedding it and bleeding like we do.
As for why? Because we evolved that way. so why did we evolve that way? is it a beneficial adaptation? Or is it just a trait who’s cost is low enough that natural selection never eliminated it?
There have been lots of hypotheses over the years, most of which have not been well supported by evidence. They used to think it was to remove toxins, but that was disproven. then they thought it was to protect against STD’s, but that doesn’t seem to be true. The current theory is that it has to do with the fact that in menstruating species, the endometrium thickens before pregnancy occurs. in most other species it doesn’t fully thicken until an embryo has already implanted. so most species just don’t have as much endometrium as us.
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