Genetic variation in sexual aggression and the factors that determine forced copulation success

Sexual conflict is common in nature and sometimes results in sexual aggression. An extreme case is forced copulation, where one individual forcibly mates with another individual who resists the mating. To understand what makes some males sexually aggressive, we established an experimental system that allowed us to quantify the characteristics that contribute to males' forced copulation success. In fruit flies (Drosophila melanogaster), sexually mature females can choose to accept or reject courting males; however, males can forcibly copulate with newly eclosed, sexually immature, teneral females. We tested males from 59 genotypes and found significant genetic variation in forced copulation success, with a broad-sense heritability of 0.16. We then chose three genotypes with the lowest and three genotypes with the highest forced copulation success rates and compared the behaviour of males from these two groups. Males from genotypes with high forced copulation success were more persistent in their pursuit of teneral females and mounted them more frequently than did males from the low-success genotypes. Males of the two categories, however, were similar in their attractiveness to both teneral and sexually mature females. Our results suggest that males vary in their pursuit strategies. Some males respond to female rejection signals by giving up and searching for receptive females, while other males persist in pursuit and coercion in spite of female objection. Our work highlights the practicality of using forced copulation in fruit flies as a model for further research on the mechanisms affecting variation in sexual coercion and forced copulation success and their evolutionary consequences.