Sexual behaviours are likely to have evolvedunder strong selective pressures since they aredirectly related to the breeding success of indi-viduals.It is striking that the sexual behaviour ofhumans diVers so markedly from that of thetwo chimpanzee species with which we sharemore than 99% of our genome. The behaviourof the less closely related gorilla is more akin tothat of many mammals—for example,ungulates/sea elephants, in which sexuallydimorphic dominant males herd smaller fe-males into groups over which they maintaincopulatory monopoly. Still more distantlyrelated apes—for example, gibbons, are notsexually dimorphic and are monogamous.Both humans and chimpanzees displaysexual dimorphism which inclines sociobiol-ogists to the uncomfortable view that thehuman’s “natural” sexual behaviour may benot dissimilar from that of the chimpanzee.However, there are major diVerences in chimpand in human sexuality. The relative size of thepenis and the relative time spent during copu-lation are described by Diamond