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.
Sexual swelling
To successfully ensure paternal confusion, Hrdy predicted that ovulation must be randomly distributed across the term where the sexual swelling is of maximum tumescence. This would ensure that males were unable to use the swelling as a signal of female fertility. One example of this stems from research into sooty mangabey ("Cercocebus atys"); the females of this species have been found to produce sexual swellings both when they are fertile and when they are with child. Additionally, sexual swellings are only observed during the most fertile period of a female’s menstrual cycle in 26-35 species of anthropoid primates, in contrast to Hrdy's prediction.