Divorce and extra-pair paternity in the Lundy house sparrows

The question of why socially monogamous females engage in extra-pair behaviour is long-standing in evolutionary biology. Due to a lack of empirical support among passerine birds, recent work has moved away from the indirect-benefits hypothesis to explain extra-pair mating behaviour by females, instead favouring the hypothesis that this is the result of a pleiotropic effect. That is, a trait under strong positive selection in either or both sexes are genetically linked with another, potentially unrelated, trait. For example, genes beneficial to female fecundity (that promote within-pair solicitation of mating from a male partner) might also lead to extra-pair behaviour (by also promoting solicited copulations from extra-pair males). Here, we test two predictions from this hypothesis: We test the prediction that female divorce, measured as the number of social mates within a given year, is linked with (1) the number of extra-pair males engaged by the female and (2) the proportion of the female’s offspring that are extra-pair. Our results show that females who divorce their social partner are more likely to produce extra-pair offspring than those who maintain social monogamy, supporting the pleiotropy hypothesis. However, those females did not also have a higher proportion of extra-pair offspring. The number of broods initiated was also positively correlated with the number of extra-pair males that sired a female’s offspring, probably through increased opportunity for extra-pair males to sire offspring over a longer breeding season. Our results support the intrasexual pleiotropy hypothesis as a driver of female extra-pair behaviour.