Menstruation is the shedding of the uterine lining (endometrium). It occurs on a regular basis in reproductive-age females of certain mammal species. Overt menstruation (where there's bleeding from the uterus through the vagina) is found primarily in humans and close evolutionary relatives such as chimpanzees. The females of other placental mammal species have estrous cycles, in which the endometrium is reabsorbed by the animal (covert menstruation) at the end of its reproductive cycle. Many zo… (More on Menstruation)