Sexual selection and sperm competition in primates: What are male genitalia good for?

One hundred twenty‐five years ago, in The Descent of Man and Selection in Relation to Sex , 1 Charles Darwin proposed the theory of sexual selection, as distinct from natural selection, to explain why, in some species, males have such magnificent ornaments and, in other species, such impressive weapons. He suggested two processes, which we now term female choice and male‐male competition: either females choose particularly ornate males or, alternatively, relatively passive females accept the winner of fights among males. By now, knowledge of species in which the females are more brightly colored or aggressive than males has led to a more general formulation of the principle of sexual selection, in which, instead of “females”, we write “the sex with the lower potential reproductive rate”, and, instead of “male”, “the sex with the higher potential reproductive rate”. 2