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.
Menstrual synchrony
Researchers are divided on whether menstrual synchrony would be adaptive. McClintock has suggested that menstrual synchrony may not be adaptive but rather epiphenomenonal, lacking any biological function. Among those who postulate an adaptive function, one argument is that menstrual synchrony is only a particular aspect of the much more general phenomenon of reproductive synchrony, an occurrence familiar to ecologists studying animal populations in the wild. Whether seasonal, tidal or lunar, reproductive synchrony is a relatively common mechanism through which co-cycling females can increase the number of males included in the local breeding system.