Menstruation (mammal)
Most female mammals have an estrous cycle, yet only ten primates species, four bats species, the elephant shrew, and the spiny mouse have a menstrual cycle. The relatedness of these groups suggests that four distinct evolutionary events have caused menstruation to arise.
Cervix
Female marsupials have paired uteri and cervices. Most eutherian (placental) mammal species have a single cervix and single, bipartite or bicornuate uterus. Lagomorphs, rodents, aardvarks and hyraxes have a duplex uterus and two cervices. Lagomorphs and rodents share many morphological characteristics and are grouped together in the clade Glires. Anteaters of the family myrmecophagidae are unusual in that they lack a defined cervix; they are thought to have lost the characteristic rather than other mammals developing a cervix on more than one lineage. In domestic pigs, the cervix contains a series of five interdigitating pads that hold the boar's corkscrew-shaped penis during copulation.