Menstruation and Menopause in Monkeys

Menstruation is the cyclic shedding of the endometrium in the absence of pregnancy. Although many people believe their cats or wild Peruvian poodles also have menstrual bleeds, only great apes and humans do, and what is seen in pets is actually bleeding associated with ovulation. Most other mammals will simply resorb the lining. It's actually not clear what the evolutionary advantage of the shedding is. Some people have speculated that it cleans the womb from potential pathogens, based on the high macrophage content of the menstrual blood. Others have proposed it was a primitive visual cue, to reassure men that if they mate, the child will be theirs and also, that if they leave their menstruating females for a long hunting trip, they will not come back to an impregnated female.
Great apes are also subject to menopause, although again it is not clear why menopause seems to be a triggered, regulated cessation of reproductive capacity and menstruation. Some monkeys, like capuchins (Johnny the monkey!), rhesus, as well as whales and elephants are also thought to undergo menopause. The grandmother theory proposes that the loss of fertility and sexual drive frees up a lot of time to nurture existing offspring and 2nd generation offsprings. A recent study of gorillas in captivity at the Brookfield zoo showed changes in behaviour of females undergoing menopause, including what seemed to be reactions to hot flashes and sudden and irregular increases in libido, particularly directed at younger males.