The Evolution of Same-sex Sexual Behavior: Using Old Theory to Answer New Questions

The evolution of mating preferences and the consequences of the resulting sexual selection are intensively studied topics in evolutionary biology. Nevertheless, until relatively recently, the evolution of perhaps the most fundamental of all mating preferences, the preference for mating with opposite-sex individuals, has received little attention. In the past two decades, however, there has been an explosion of literature on the evolution and expression of same-sex sexual behavior (SSB) in nonhuman animals. Here, I show that literature on SSB has strong connections to many insights from classic theory on mate choice. I argue that explicitly integrating this "old theory" on mate choice proves tremendously informative for understanding the evolution of SSB. To do so, I review what is known about the expression and evolution of SSB. I begin by explaining why selection may favor imperfect sex discrimination (i.e., the failure to accurately assess the sex of potential mating partners), how imperfect sex discrimination manifests in SSB empirically, and the origin of sex discrimination. I then review experimental studies on how varying social conditions, such as density and sex ratio, influence the plastic expression of SSB. Next, I turn to evidence for adaptive SSB, wherein the act of engaging in sexual behavior with same-sex conspecifics carries fitness benefits. I conclude by arguing that connecting results on SSB to "classic" work on mating preferences leads to a deeper understanding of how SSB evolves.