How Males Can Gain by Harming Their Mates: Sexual Conflict, Seminal Toxins, and the Cost of Mating

We suggest that damaging mating tactics, such as physical aggression, the evolution of genital barbs and spines, and the transfer of seminal toxins may serve as a general means by which males can induce females to avoid or to delay remating. Provided that cumulative damage has an accelerating impact on fitness, a female who has already been harmed by previous partner(s) may do best to refrain from remating to avoid suffering still further damage. Consequently, a male can gain through the imposition of mating costs, even though this may reduce female fitness because by doing so he minimizes the chances that his mate will copulate again. We develop a game theoretical model of this possibility, focusing on toxin transfer as an illustrative example. We show that toxicity as a means of inhibiting remating is phenotypically stable over a broad range of conditions (although, under some circumstances, it may be necessary to invoke other selective pressures to account for the initial evolution of toxicity). The model predicts that toxin transfer should be more common (and involve greater levels of toxicity) in species with greater last-male mating advantage; it is also most likely where the poison inflicts strongly accelerating, dose-dependent costs on females.