Comparative evidence for a positive association between divorce and extra-pair paternity in birds

Abstract

Although most bird species are socially monogamous, they show a large variation in both divorce rate and the proportion of extra-pair paternity (EPP). Recently, adaptive explanations of avian monogamy have considered divorce and EPP as related behavioural strategies by which individuals paired with low quality mates can improve their breeding status within ecological and time constraints. It has been suggested that, at both the intra- and inter-specific levels, divorce rate should be associated with the frequency of EPP. Divorce and EPP could coexist as alternative strategies whose relative frequencies would depend on ecological conditions, resulting in a negative association between the two phenomena. Or, they can be seen as synergistic tactics co-varying with variation in mate quality between populations or species and are thus positively associated. Here, applying two different comparative methods: the comparison of independent linear contrast and the pairwise comparison of closely related taxa, we show that high rates of divorce are positively associated with high rates of extra-pair paternity in socially monogamous species of birds, even when controlling for survival rate as a potentially confounding variable. This constitutes the first comparative evidence for a relation between divorce and adultery in animals. We discuss this result in relation to recent hypotheses on the adaptiveness of divorce and extra-pair copulations in birds and natural variation in mate quality among populations or species. The pattern found is most consistent with the hypothesis that birds may divorce to gain a better breeding position, but alternatives cannot be fully ruled out.