Menstruation (mammal)
Menstruation in mammals is the shedding of the uterine lining (endometrium). It occurs on a regular basis in uninseminated sexually reproductive-age females of certain mammal species.
Menstrual synchrony
In mammalian mating systems generally, and among primates in particular, female spatio-temporal distribution – how clumped females are in the environment and how much they overlap their fertile periods – affects the ability of any single male to monopolize matings. The basic principle is that the more females are fertile at any one time, the harder it is for any single male to monopolize access to them, impregnating all simultaneously at the expense of rival males. In the case of nonhuman primates, once the number of co-cycling females rises above a critical threshold, a harem-holder may be unable to prevent other males from invading and mating with his females. A dominant male can maintain his monopoly only if his females stagger their fertile periods, so that he can impregnate them one at a time (see figure a, right). Suppose a group of female baboons need between them just one dominant male, desirable in view of his high-quality genes. Then, logically, they should avoid synchronizing their cycles. By the same token, if males during the course of human evolution became valued by females for additional purposes – hunting and bringing home food, for example – then females should resist being controlled by dominant male harem-holders. If males are useful partners to have and keep around, then ideally each female should have at least one for herself. Under those circumstances, according to this argument, the logical strategy would be for females to synchronize as tightly as they can (see figure b, right).