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.
Spiny mouse
One species of spiny mouse was recently discovered to have a menstrual cycle which would make them important as a model organism to study menstruation and reproductive disorders, as Mus musculus, the common laboratory mouse, does not menstruate.