EXTRA-PAIR PATERNITY AND THE OPPORTUNITY FOR SEXUAL SELECTION IN A SOCIALLY MONOGAMOUS BIRD (DENDROICA-PETECHIA)

We used multi-locus DNA fingerprinting to characterise the genetic mat ing system of the socially monogamous yellow warbler (Dendroica petech ia). Over 2 years there were no instances of brood parasitism, but 59% of families (n = 90) contained extrapair sired young and 37% of offsp ring (n = 355) were of extra-pair paternity. Most hypotheses for extra -pair mating in monogamous species assume a paternity benefit to extra -pair sires, and focus on the benefit(s) to females. However, the assu mption of male benefit has been little tested. Among yellow warblers, known extra-pair sires were just as likely to be cuckolded as any male in the population, and there was at least one reciprocal exchange of extra-pair paternity. Nevertheless, among known extra-pair sires, the paternity gains from extra-pair paternity were, on average, greater th an the losses in their own families. These results show there is a pat ernity benefit to certain males. However, the benefit is not absolute but relative and therefore more difficult to measure. The results also suggest that patterns of extra-pair fertilisation are not determined by female choice alone. Most confirmed extra-pair mates were territori al neighbours, but some resided as far as three territories apart, and greater spatial separation was implied in other cases. Thus, the oppo rtunity for extra-pair mating is great. We estimate that as a result o f extra-pair fertilisations, variance in male mating success is increa sed somewhere between 3-fold and 15-fold over that which would result from within-pair reproduction alone. These findings affirm the potenti al importance of extra-pair reproduction for sexual selection in monog amous species and they support earlier suggestions that extra-territor ial forays by male yellow warblers are for the purpose of extra-pair m ating.