Indiscriminate Mating and the Coevolution of Sex Discrimination and Sexual Signals

The presence of same-sex sexual behavior across the animal kingdom is often viewed as unexpected. One explanation for its prevalence in some taxa is indiscriminate mating-a strategy wherein an individual does not attempt to determine the sex of its potential partner before attempting copulation. Indiscriminate mating has been argued to be the ancestral mode of sexual reproduction and can also be an optimal strategy given search costs of choosiness. Less attention has been paid to the fact that sex discrimination requires not just the attempt to differentiate between the sexes but also some discernible difference (a signal or cue) that can be detected. To address this, we extend models of mating behavior to consider the coevolution of sex discrimination and sexual signals. We find that under a wide range of parameters, including some with relatively minor costs, indiscriminate mating and the absence of sexual signals will be an evolutionary end point. Furthermore, the absence of both sex discrimination and sexual signals is always evolutionarily stable. These results suggest that an observable difference between the sexes likely arose as a by-product of the evolution of different sexes, allowing discrimination to evolve.

Automated summary

The presence of same-sex sexual behavior across the animal kingdom is often seen as surprising. One possible explanation is indiscriminate mating, where individuals do not try to determine the sex of potential partners before copulation. This strategy is believed to be an ancestral mode of reproduction and can be a beneficial strategy considering the costs of selective mating. It is important to note that sex discrimination requires not only the attempt to differentiate between sexes, but also some detectable difference (a signal or cue). Based on modeling of mating behavior, it is found that under various parameters, including some with minor costs, indiscriminate mating and the absence of sexual signals can be an evolutionary endpoint. Additionally, the absence of both sex discrimination and sexual signals is always evolutionarily stable. These findings suggest that the observable differences between sexes likely arose as a by-product of the evolution of different sexes, allowing for the evolution of sex discrimination.