Male house mice that have evolved with sperm competition have increased mating duration and paternity success

Sperm competition imposes strong selection on males to gain fertilizations and maximize paternity. Males have been shown to adapt to sperm competition by modifying their behaviour and/or reproductive physiology. We investigated the fitness effects of male responses to sperm competition in house mice, Mus domesticus. Males that had been evolving with (polygamy) and without (monogamy) sperm competition for 18 generations were subject to different frequencies of social encounters with conspecific males to generate a sperm competition ‘risk’ treatment and a ‘no risk’ treatment. After manipulation of their social environment for 15–22 days, males were forced to compete for fertilizations against a male with the same selection history that experienced no manipulation. We genotyped embryos at 16 days gestation to quantify paternity success, and thus the competitive ability, of the experimental males. While there was no treatment effect on mating behaviour and paternity success, males from polygamous lines had significantly increased mating duration and paternity success when positioned in the disfavoured role of the second male to mate. Thus, males that had evolved with sperm competition showed greater mating effort and fitness compared to males that had evolved without sperm competition. Whether this is due to a genetic divergence between the lines or a differential capacity of males with different selection histories to respond plastically to current levels of sperm competition remains to be tested.