Chimpanzees (Pan troglodytes) are an anthropoid ape species living across equatorial Africa. They are genetically the closest living relatives to humans, yet display marked differences in their sexual behavior. Chimpanzees engage in brief copulatory encounters and are highly promiscuous, mating with most or all available partners. Female chimpanzee sexual receptivity is defined by an enlarged swelling of the anogenital skin which incites sexual solicitations and male − male competition. Depending on male rank and opportunity, mating may occur promiscuously and uncontested within a large group. Alternatively, males may attempt to protect exclusive access to females by fending off competing males or by escorting the female away from competitors for an exclusive consortship. Salient features of the sexual system of chimpanzees include the importance of energetic condition for the fecundity of females, intense sperm competition among males, and coercive aggression between the sexes.