Sexual conflict in waterfowl: why do females resist extrapair copulations?

Forced copulation is a male reproductive strategy in a variety of animals but rare among avian species, with the notable exceptions of waterfowl (family Anatidae) and at least 1 passerine species, the New Zealand stitchbird or hihi Notiomystis cincta. The presence of forced extrapair copulation in these species challenges the perception that females control extrapair copulations (EPC) across avian species. A noteworthy behavioral discrepancy is believed to exist between waterfowl and passerines in that female waterfowl are widely assumed to always resist EPC, whereas female passerines often pursue EPC. This difference in female behavior between avian groups is perplexing in light of the fact that unconditional resistance to EPC exposes female waterfowl to risk of serious injury. I consider 5 hypotheses to explain the female unconditional resistance strategy in waterfowl and focus on the controversial idea that resistance could represent a female mate choice strategy in a system dominated by male force. This resistance as mate choice hypothesis relies on indirect benefits to females through biasing paternity in favor of manipulative or genetically high-quality males and predicts that unconditional resistance versus conditional acceptance of EPC reflects the presence or absence of forced copulation in the mating system. Although indirect selection is widely regarded as unimportant in the evolution of female defensive traits when direct costs are large, I argue that indirect selection could nonetheless play an important role in the evolution of female strategies under sexual conflict.