Adaptive dynamics of sexual signaling and discrimination under different mating conditions

A wide variety of animals engage in same-sex sexual behavior (SSB). Some instances of SSB appear to arise from individuals mating indiscriminately without regard to the sex of their partners. Theory suggests that indiscriminate mating strategies can be selected if sexes are not completely distinguishable—that is, without strong signals of sexual identity. We model the coevolution of sexual signaling and discrimination using the adaptive dynamics approach to long-term trait evolution. Our model relaxes two major assumptions from past phenomenological theory. First, we consider a mechanistic mating process with tunable encounter rates and mating costs. Second, we vary reproductive investment roles to allow for better correspondence with empirical examples of SSB. We confirm that coevolution of sexual signaling and discrimination can lead to two distinct equilibria: one with no sexual signals and indiscriminate mating; the other with perfect signaling and exclusively different-sex sexual behavior. Selection for indiscriminate mating is strongest at very low or very high encounter rates. Mating costs lead to selection for reduced overall mating rate, but this can lead to either more or less SSB, depending on how this reduction is achieved. Overall, our model highlights the importance of ecologically relevant parameters for the evolution of SSB.