Sexual conflict over sperm ejection in monogamous pairs of kittiwakes Rissa tridactyla

Socially monogamous partners suffer conflicting interests concerning various aspects of reproduction such as parental care, copulation and fertilization. Female black-legged kittiwakes commonly eject their mates' sperm immediately following copulations. Because sperm ejection reduces male sperm competitiveness and paternity assurance, males and females have conflicting interests as regards sperm ejection. Males whose mates ejected their sperm at least once remained longer on their mates' backs after the last insemination which apparently prevented the females from ejecting sperm. These results suggest that compelling females to retain their sperm may be a previously unidentified tactic employed by males to assure their paternity. Females tried to prevent their mates from witnessing sperm ejection by ejecting sperm after their mates departed from the nest. Females were more likely to eject sperm when they terminated the copulations by unbalancing the male. The conflict over sperm ejection was related to the ability of the females to end the copulations which covaried with the body mass of their mates. These findings suggest that conflicts in monogamous pairs also exist over the disposition of sperm.