Harmful mating tactics in hermaphrodites.

While empirical data suggest that sperm competition and multiple mating both contribute to the evolution of harmful mating tactics in hermaphrodites, a precise understanding of their interaction is lacking. We therefore formulate a game-theoretical model of mating behavior in hermaphrodites, where harmful mating tactics confer an advantage in sperm competition while simultaneously reducing the mating partner's survival. The model predicts evolutionarily stable values of resource allocation between sexual functions and the degree of harmful mating. Our analysis provides support for the empirical observation that harmful mating is associated with multiply mating species in which sperm precedence strongly favors the first mate. The model also shows that this criterion becomes less important as harmful mating tactics become more efficient. As harmful tactics make sperm displacement more effective, a consequence is a more female-biased resource allocation. Provided that fertilized egg production is not limited by availability of sperm, a more female-biased allocation should increase the number of offspring produced, but the model instead shows that harmful mating tactics more than countercompensate, leading to reduced fitness. Hermaphrodites that use harmful mating tactics may therefore be at a disadvantage when competing with other species for a limited resource.