Sex-role reversal and the absence of extra-pair fertilization in Wilson's phalaropes.

Extra-pair fertilizations are common in birds, which has led to great interest in how this phenomenon is regulated at a proximate level and how extra-pair fertilizations, and extra-pair fertilization avoidance, shape avian social systems. In Wilson's phalaropes, Phalaropus tricolor, incubation and brood-rearing is performed exclusively by males. Males are able to rear only a single brood of four during a breeding season. This suggests that males have a high level of paternity in clutches and broods under their care and, thus, that extra-pair fertilizations are infrequent. In contrast, female social dominance, lack of territoriality and frequent interactions among breeding adults suggest that both males and females have the opportunity to engage in extra-pair copulations. Using DNA fingerprint band-sharing between putative parents and offspring, we found no evidence of extra-pair fertilizations among 51 offspring from 17 families of phalaropes. Copulation disruption by non-copulatory adults, ability of females to reject copulation attempts and potential fitness benefits to females by avoiding extra-pair fertilizations were sufficient to explain the absence of extra-pair fertilizations in Wilson's phalaropes. We propose that sex-role reversal affects the relative costs and benefits to females of seeking extra-pair fertilizations. At the time of clutch completion, females have invested particularly heavily in their clutches due to intense competition among females to gain and keep a mate during the pre-laying and laying periods. After clutch completion, nest success requires significant male parental care. Benefits to females in gaining extra-pair fertilizations may be offset by the risk of losing male parental care. Copyright 1998 The Association for the Study of Animal Behaviour. Copyright 1998 The Association for the Study of Animal Behaviour.