Numerous reports have documented the occurrence of same-sex sociosexual behaviour (SSB) across the natural world. However, distributions of the behaviour within a species are needed to test popular theories describing its evolutionary underpinning, above all, whether the behaviour can be heritable and therefore evolve, and consequently if the behaviour carries fitness costs due to harsh trade-offs with reproductive effort. Chapter 1 provides this intraspecific distribution by using detailed observations collected across three years of the social and mounting behaviour of 236 male semi-wild male rhesus macaques. Results showed that male-male mounting was more common than male-female mounting, and that the likelihood of exclusive SSB orientations (and duly high reproductive costs) were low. Chapter 2 demonstrates that historical theories of social group sex-ratio and dominance (potentially mediating limited-female access) explain SSB only marginally, with increasing age instead weakly influencing both increased dominance rank and decreased SSB. Results therefore opened the possibility of individual identity, and consequently genetic background, influencing the expression of the behaviour. Using a comprehensive pedigree, this chapter provides the first evidence of vertebrate repeatability (19.3%) and heritability (6.4%) of SSB in the natural world. Furthermore, a positive genetic correlation between same-sex mounter and mountee activities indicated a common underpinning to different forms of SSB. In contrast, there was no genetic correlation between male-male and male-female mounting, providing further evidence of a decoupling between SSB and costs to missed mating opportunities. Chapter 3 studies pedigree offspring sired to directly show no evidence of a cost to SSB, but instead that the behaviour predicted coalitionary partnerships associated with likely fitness benefits. Together, the results presented here demonstrate that SSB can be common amongst individuals, can evolve, and is unlikely to be costly, with implications for both animal and human research.