WHY DO FEMALE BIRDS REJECT COPULATIONS FROM THEIR MATES

Female birds frequently reject copulations from their mates, suggestin g a conflict between the sexes. This study analyses behavioural data o f socially monogamous razorbills, Aka torda, to examine whether female s rejected their males because of conflicts over fertilization or the pair bond. Among pairs, females rejected 9-70% of their mates' copulat ion attempts and prevented their mates from completing 42-100% of succ essful copulations. Copulations terminated by females were half the du ration of those terminated by males, and females terminated fewer firs t copulations than subsequent ones on the same day. These findings ind icate that females were motivated to copulate less frequently and for shorter durations than their mates. The sperm competition hypothesis p redicts that females reject their mates to increase the probability of being fertilized by extra-pair males. This hypothesis was not support ed because females rejected extra-pair males similarly to their mates. The female-mate-guarding hypothesis predicts that females guard their pair bond by copulating frequently with their mates, thereby deprivin g the males of time and energy to copulate with and form bonds with ot her females. This prediction was consistent with a significant negativ e correlation between the percentage of copulation attempts that femal es accepted from their mates, and the number of extra-pair copulations that their mates attempted. However, this correlation was not caused by a trade-off of males copulating with their mates instead of attempt ing extra-pair copulation because males attempted most extra-pair copu lations on days when their mates were absent. A new hypothesis is prop osed, namely, that females reject their mates to test the male's commi tment to provide essential parental contributions after egg-laying. Th e 'testing-of-the-bond' hypothesis is consistent with the findings but requires testing.