Menstruation
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.
Human placentophagy
Maternal placentophagy is defined as "a mother’s ingestion of her own placenta postpartum, in any form, at any time". Of the more than 4000 species of placental mammals, most, including herbivores, regularly engage in maternal placentophagy, thought to be an instinct to hide any trace of childbirth from predators in the wild. The exceptions to placentophagy include mainly humans, Pinnipedia, Cetacea, and camels.