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.
Extended female sexuality
Research is fairly consistent in the finding that species with concealed estrus mate at all stages of their ovarian cycle. For instance, mating activity in assamese macaques ("Macaca assamensis") has been investigated. The researchers analysed the levels of progesterone in the monkeys, in order to establish the ovarian stage of the female, as progesterone peaks following the fertile window. They found that the females concealed estrus and were sexually receptive during the entire mating season. Concealed estrus and sexual receptivity (at all times of the ovarian cycle) aids paternity confusion. This is because the males are unsure of who mated with the female during her fertile period, and so do not know the identity of the father.