If the oestrus cycle is good enough for the rest of the mammals, why isn’t it good enough for the great apes? Menstruation seems like a waste of bodily fluid to me and anything that leaves a trail of strong-smelling blood can’t be good for survival. Is year-round fertility really worth it?
Nobody knows, and like most things nobody knows there are lots of wild theories around to explain it.
The simplest explanation that fits all the facts is simply that it exists because there is no major detriment in it existing. Before the invention of birth control and complex marriage rituals women would either be pregnant or nursing pretty much non-stop. As a result most women probably had only half a dozen periods in their entire lives. That’s simply not inconvenient anough to warrant any change.
As for why this apparently wasteful process evolved in the first place, the best answer yet again 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 estrus. If a female doesn’t get pregnant then she simply continues to move right along with the estrus 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 estrus. Even housecats tend to have major problems if a queen comes into season and doesn’t fall pregant 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 pregant very rapidly if she comes into season, so there doesn’t need to be any other mechanism to terminate estrus. 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 estrus 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 estrus 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 estrus 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 pregant on most cycles. A system ‘designed’ as an emergency override in rare circumstances was suddenly being used as a routine mechanism every month. Not surprisngly 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.
I just wanted to commend you, Blake on your eminently clear and understandable explanation.
Yes, me too. That was great. Thanks. 