SEXUAL SELECTION BY SPERM COMPETITION IN BIRDS - MALE-MALE COMPETITION OR FEMALE CHOICE

Sperm competition is a potential source of variance in male mating success. Therefore the evolutionary interpretation of sperm competition should be made within the framework of sexual selection theory and should include both male-male competition and female choice mechanisms. Previous sperm competition theory has suffered because it has generally focussed on just the conflict between males. Here we argue that sperm competition and its impact on sexual selection cannot be fully understood without considering female choice and the associated conflicts between females and their mates. For example, male mate guarding and other paternity guarding behaviours have generally been interpreted as an outcome of male-male competition because they prevent other males from fertilizing passive or resisting females. However, they could also arise out of a conflict between mates in which paternity guards prevent females from seeking fertilizations with other males. We show that a balanced approach of contrasting the two mechanisms of sexual selection yields new and testable predictions about traits associated with sperm competition.