Jørgen S. Søraker, Jamie Dunning

Extra-pair behaviours, where individuals copulate outside of an established pair bond, resulting in extra-pair paternity (EPP) of offspring, have long intrigued behavioural ecologists. Of particular interest is why females of otherwise socially monogamous species engage in extra-pair behaviours. Although researchers recognise that the drivers of variation in EPP, both within-species and between-species, also depend on the negative consequences (costs) of the behaviour to the individual, empirical studies mostly focus on benefits rather than costs. This is partly because benefits are often measured in currency close to fitness, whereas costs often have more indirect and complex pathways to fitness. Both the prevalence and magnitude of a cost are experienced in the context an individual’s environment and may affect fitness, either directly (by affecting reproduction or survival) or indirectly (through the fitness of offspring). Here, we review our current understanding of costs associated with EPP and extra-pair copulations (EPC), e.g. both the costs of producing extra-pair offspring and the behaviour associated with EPP. We conclude that the costs of EPP and EPC are likely a key factor shaping this behaviour. More research, particularly empirical and experimental studies in taxa other than birds, is needed to understand the intricate cost-benefit equation underlying EPP.