Menstruation (mammal)

Menstruation is the shedding of the uterine lining (endometrium). It occurs on a regular basis in uninseminated^[1] sexually reproductive-age females of certain mammal species.

Although there is some disagreement in definitions between sources, menstruation is generally considered to be limited to primates. It is common in simians (Old World monkeys, New World monkeys, and apes), but completely lacking in strepsirrhine primates and possibly weakly present in tarsiers. Beyond primates, it is known only in bats, the elephant shrew, and the spiny mouse species Acomys cahirinus.^{[2][3][4][5][6]} Overt menstruation (where there is bleeding from the uterus through the vagina) is found primarily in humans and close relatives such as chimpanzees.^[7]

Females of other species of placental mammal undergo estrous cycles, in which the endometrium is completely reabsorbed by the animal (covert menstruation) at the end of its reproductive cycle.^[8] Many zoologists regard this as different from a "true" menstrual cycle. Female domestic animals used for breeding—for example dogs, pigs, cattle, or horses—are monitored for physical signs of an estrous cycle period, which indicates that the animal is ready for insemination.