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.
Female
Most mammalian females have two copies of the X chromosome as opposed to the male which carries only one X and one smaller Y chromosome (but some mammals, such as the platypus, have different combinations). To compensate for the difference in size, one of the female's X chromosomes is randomly inactivated in each cell of placental mammals while the paternally derived X is inactivated in marsupials. In birds and some reptiles, by contrast, it is the female which is heterozygous and carries a Z and a W chromosome whilst the male carries two Z chromosomes. Intersex conditions can also give rise to other combinations, such as XO or XXX in mammals, which are still considered as female so long as they do not contain a Y chromosome, except for specific cases of testosterone deficiency/insensitivity in XY individuals while in the womb. However, these conditions frequently result in sterility.