Comparative studies of genital anatomy and sexual behaviour in male primates show that penile morphology and copulatory patterns tend to be more specialized in species which have a multimale or dispersed (non-gregarious) mating system. The penis may be longer and more complex morphologically in such species. Copulatory patterns involving a series of intromissions or prolonged single intromissions are more common than in species with a monogamous or polygynous mating system. Elongation of the baculum occurs in species which prolong intromission into the post-ejaculatory period. Sexual selection may have favoured the evolution of such features in species where females mate with a number of males rather than with a single partner. Relative testis size is also greatest under such conditions and larger testes occur in some non-gregarious or seasonally breeding prosimians, as well as in anthropoids with multimale mating systems.