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.
Barbary macaque
Although Barbary macaques are sexually active at all points during a female’s reproductive cycle, male Barbary macaques determine a female’s most fertile period by sexual swellings on the female. Mating is most common during a female’s most fertile period. The swelling size of the female reaches a maximum around the time of ovulation, suggesting that size helps a male predict when he should mate. This is further supported by the fact that male ejaculation peaks at the same time that female sexual swelling peaks. There is not a sufficient change in female sexual behaviour around the time of ovulation in order to demonstrate to the male that the female is fertile. The swellings, therefore, appear necessary for predicting fertility.