The costs of extra-pair behaviours in birds.

Extra-pair behaviours - reproductive behaviours, including those related to copulation and paternity of offspring, amongst animals outside of a social pair bond - have long intrigued behavioural ecologists, particularly from the female animal's perspective. Female extra-pair paternity is not supported by classic models of sexual selection yet is common across birds. Researchers recognise that variation in extra-pair paternity, both within and between species, is in part driven by the costs to the individual; however, empirical studies tend to focus on benefits. This is because benefits are often measured through direct fitness measurements, whereas costs have more complex, indirect pathways to fitness. However, both the prevalence and magnitude of a cost are experienced in the context of 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 extra-pair paternity and extra-pair copulation. For example, we review both the costs of producing extra-pair offspring and the behaviour associated with extra-pair paternity. We conclude that the costs of extra-pair paternity and extra-pair copulation are understudied, but 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 extra-pair paternity.