Adaptive dynamics of discriminate and indiscriminate mating

Same-sex sexual behavior (SSB) occurs across many animal taxa. Some instances of SSB likely arise from animals' mating indiscriminately and/or with imperfect assessment of their partner's sex. Selection on indiscriminate mating strategies depends on the costs and benefits of mating and the distinguishability of sexes-which in turn depend on evolvable signals of sex. Other factors like population density (which determines the rate of encountering potential mates) and investment roles (i.e., which sex invests more in reproduction) are likely relevant to the evolution of same-sex and different-sex mating, but there is no consensus as to the direction of these effects. We develop a new model of adaptive coevolution of sexual signaling and discrimination, explicitly incorporating encounter rate, mating costs, and investment roles. Coevolutionary dynamics typically lead to two alternative equilibria: one with no sexual signals and indiscriminate mating; the other with high-quality signaling and exclusively different-sex mating. Low-density conditions favor indiscriminate strategies, due to the steep opportunity costs of missing a possible mate. High density can favor either discriminate or indiscriminate strategies, depending on investment roles. By providing theoretical predictions for the effects of encounter rate and investment roles, our model bridges gaps between theoretical and empirical work on SSB.