I was reading Cecil’s colomn on Menstruation and the moon and in it he mentions the cycle of other animals including rats. I used to have pet rats and they did get horny according to that cycle but they never got a period like we do. Does anyone know if there are any animals that bleed periodicaly other than humans. It can’t be an evolutionary plus, can it?
Link to article referenced:
What’s the link between the moon and menstruation?
AFAIK only primates menstruate, that is lose the uterine lining as part of the normal reproductive cycle. Other mammals undergo a process of estrus where the uterine lining is absorbed directly back into the bloodstream. These mammals may lose blood during estrus, but this is breakthrough bleeding associated with thickening of the uterine liing and occcurs before the animal becomes fertile. In contrast primate menstruation is the shedding of the uterine itself, and occurs after the animal has ceased to be fertile.
Obviously it is. The only question is why. There are several theories for this. The one I personally favour is that it prevents infection.
Many, possibly even most, small mammals have to fall preganat once the oestrus cycle is entered. If they do not fall pregnant there is no mechanism for shedding the uterine lining. It stays withing the urterus and eventually becomes infected, leading to death. This is most notable in cats and ferrets, where unmated females will usually die one they come into season. However all carnivores suffer form this problem to some degree including dogs. If primates are descended from similarly engineered animals the reason for menstruation seems obvious. The uterine lining can be shed harmlessly if no males are available and the females can go on to breed during the next season.
It might seem odd, but it’s quite possible that the ability to resorb the uterine lining evolved only on a specific line of the mammal family tree, afer primates and carnivores diverged. As a result we have developed novel techniques for dealing with the problem independantly.