Anim. Behav., 1998, 55, 995–1002 Sex-role reversal and the absence of extra-pair fertilization in Wilson’s phalaropes DAVID J. DELEHANTY*, ROBERT C. FLEISCHER², MARK A. COLWELL* & LEWIS W. ORING* *Department of Biology, University of North Dakota at Grand Forks ² National Z oological Park, Smithsonian Institution ( Received 4 June 1996; initial acceptance 17 September 1996; final acceptance 7 July 1997; M S. number: 7619) Abstract. 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 a ffects 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.  1998 The Association for the Study of Animal Behaviour Extra-pair fertilizations are widespread among birds (reviewed in Westneat et al. 1990; Birkhead & Møller 1992; Westneat & Sargent 1996) and appear to be the outcome of intersexual conflict as individuals act to maximize their fitness (Westneat & Sargent 1996). Many factors have been hypoth- esized to regulate the frequency of extra-pair fertilizations within birds. Mate guarding of females by males (Beecher & Beecher 1979; Møller & Birkhead 1993) reduces extra-pair fertilization frequency by preventing extra-pair copulations. Territoriality may function in extra-pair copula- tion avoidance by limiting interactions among breeding adults, even though many territorial species sometimes have high rates of extra-pair fertilizations (e.g. Bray et al. 1975; Gibbs et al. 1990; Morton et al. 1990; Stutchbury et al. 1994). High encounter rates among breeding adults can promote extra-pair fertilizations (Westneat 1994; Stutchbury & Morton 1995; but see Dunn et al. 1994). Thus, breeding synchrony and coloniality have been associated with extra-pair fertilization frequency (e.g. Decker et al. 1993; Haig et al. 1993; Stutchbury & Morton 1995). Variance in mate quality promotes extra-pair fertilizations (e.g. Morton et al. 1990; Smith et al. 1991; Burley et al. 1994; Forslund & Pa ¨rt 1995), especially Correspondence and present address: D. J. Delehanty, Department of Biology-BRRL/314, University of Nevada, Reno, NV 89557, U.S.A. (email: djdele@scs.unr.edu). R. C. Fleischer is at the Molecular Genetics Laboratory, National Zoological Park, Smithsonian Institution, Washington D.C., 20008, U.S.A. M. A. Colwell is at the Department of Wildlife, Humboldt State University, Arcata, CA 95521, U.S.A. L. W. Oring is at the EECB Program, 1000 Valley Road, Reno, NV 89512, U.S.A., and the Department of Environmental and Resource Sciences, 1000 Valley Road, University of Nevada, Reno, NV 89512, U.S.A. 0003–3472/98/040995+ 08 $25.00/0/ar970670  1998 The Association for the Study of Animal Behaviour 995