Learned Sex Discrimination and the Evolution of Same-Sex Sexual Behavior

AbstractThe sociosexual environment shapes the expression of same-sex sexual behavior (SSB). Empirical studies on SSB in insects often find that sex discrimination, when plastic, is weaker under male-biased sex ratios, ostensibly contradicting theory and experimental evolution that show that stronger sex discrimination evolves under male-biased sex ratios. We develop theoretical models to assess the role of the sex ratio on learned sex discrimination. We find that males have the strongest sex discrimination when they have the greatest opportunity to experience conspecifics from which they learn. Learned sex discrimination is most likely to evolve when innate discrimination is costly, costs to learning are low, mating with the opposite sex is costlier than attempted matings with the same sex, and late matings produce more offspring. Learning from unsuccessful mating attempts with males typically outcompetes other learning strategies, although learning from successful matings with females or learning from all mating attempts can evolve when individuals have few opportunities to mate or costs to discrimination are low. We argue that the life history of insects may favor learning from successful matings or all mating attempts and thus drive the apparent disconnect between the effect of the sex ratio on learned versus innate sex discrimination; we also provide nonadaptive alternatives.