Cooperation for copulation: a novel ecological mechanism underlying the evolution of coalition for sharing mating opportunities

Cooperation, or the act of benefiting others at the cost of the benefactor9s fitness, has been a central issue in evolutionary theory. Non-human animals sometimes show coalitions or male-male cooperation to confront a male rival and challenge the rank hierarchy. Here we observed novel types of coalitions in wild stump-tailed macaques; multiple males actively shared the mating opportunities, i.e., a male copulated with a female, while his ally waited his turn and guarded them. Our mathematical simulations revealed that lack of estrous signs, as well as large numbers of males in a group, possibly enhance facultative sharing of females. This is the first demonstration of the sharing of females in non-human primates, and shed light on the evolutionary theory of cooperation.