While (apparently) the idea that human females living close together synchronize their menstrual cycles is still controversial, it's now been proven a species of monkey synchronizes the sexual receptiveness of a troupe's females. It's thought this provides a reproductive advantage to the females, since it helps to prevent the dominant male from monopolizing them. Think "embarrassment of riches," that sort of thing.

I'm just glad I wasn't the one forced to watch monkeys go at it for hours on end and then pick at their poo after they're done.