Why do humans and greater apes menstruate, and other animals don't?

Answer 1

It isn't just great apes, menstruation is very widespread amongst all primates including monkeys and lemurs.

The best answer for why appears to be "because it never needed to change". It's not uncommon in carnivores, especially small carnivores, for there to be no specific mechanism to end oestrus. If a female doesn't get pregnant then she simply continues to move right along with the oestrus cycle until she does, or until she dies. That's probably most advanced in the weasels and polecats, where if a female is prevented from getting pregnant she will rapidly die from the hormonal imbalances brought about by oestrus. Even housecats tend to have major problems if a queen comes into season and doesn't fall pregnant because there simply isn't any effective mechanism to end the cycle. IOW it's not so much a cycle as a switch that activates a positive feedback loop.

The primates are relatively closely related to the carnivores, and the first primates probably inherited the same system. This system isn't a problem amongst small animals with communal social structure or overlapping ranges. Any female can become pregnant very rapidly if she comes into season, so there doesn't need to be any other mechanism to terminate oestrus. But at some point our ancestors presumably adandonded that living arrangement and became more dispersed. Under those circumstances a female may not always be able to find a mate. As a result a solution evolved to terminate oestrus and get rid of the uterine lining before it became infected. In truly estral animals that's achieved by resorbing the lining, but our ancestors either never evolved that baility or had lost it. So we went down the other path and simply shed the lining to get rid of it.

That's not an ideal solution, but evolution doesn't produce ideal solutions, it produces a workable solution with what's available. Menstruation worked. It terminated the oestrus event, it got rid of the uterine lining and prevented infection. It wasted a few resources, but that's better than death. And for our primarily carnivorous ancestors the resource loss was fairly minor. And it left a small amount of blood and marginally increased the amount of scent produced, but that's still better than a large amount of certain death. And the system wasn't intended to be employed every time oestrus was triggered. It was only ever meant to be an emergency brake.

Then modern humans evolved. We invented ridiculous marriage rituals and birth control that meant that most women weren't falling pregnant on most cycles. A system 'designed' as an emergency override in rare circumstances was suddenly being used as a routine mechanism every month. Not surprisingly this system wasn't very good and became a major inconvenience. Emergency systems tend to be like that: they work perfectly for what they are meant to do, but you wouldn't want to work with them all the time.

Answer 2

Other animals do... they go into heat, that is the same thing. They lose blood and tissue as well... when female dogs go through with this you have to put a little diaper on them so they don't leak all over your house.

Answer 3

You've never had a female dog, have you? And you weren't paying attention in biology class when they talked about mammals, were you?

Answer 4

Other mammals do... it's called estrus (aka being in heat, like female dogs).

Answer 5

Nearly all mammals do. Yep... had a female dog once... once... gross... little diaper... lots of howling... lots of pain... yep... only once did I have a female dog... get a male dog... yuck....