Sexual selection favors harmful mating in hermaphrodites more than in gonochorists.

Hermaphroditic animals often exhibit mating mechanisms that seem more damaging than those in species with separate sexes. Our analyses indicate that this difference is real. While females only remate when the benefit is positive, hermaphrodites remate even when this implies losing female fitness. This occurs because hermaphrodites can outweigh losses in the female function by gaining paternity. In an extended model we ask whether this favors the evolution of more male harm in hermaphrodites. When male harm only suppresses remating in the receiver it neither evolves in hermaphrodites nor in gonochorists. However, when male harm is coupled to a fertilization advantage, it evolves in both forms of gender expression with the highest levels in hermaphrodites. Hence, hermaphrodites are more prone to be caught in costly escalations than gonochorists. We discuss the implications for the evolution of gender expression in animals and plants.