The costs of extra-pair mating

Abstract

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.

Disentangling the age-dependent causal pathways affecting multiple paternity in house sparrows

“…Since then, suggested hypotheses describing the potential benefits of this behaviour for females include: (a) 'good genes' where offspring fitness is improved by higher quality or attractive male genes (including the 'sexy son' hypothesis [9]), that females identify from male phenotypic cues [4,10,11]; (b) 'compatible genes' where offspring fitness increases due to a better genetic match between their mother and the extra-pair male [12][13][14]; (c) 'fertility assurance' by increasing the chance of females producing fertilized eggs [15,16]; and (d) MP as a form of genetic bet-hedging under unpredictable environments [17]. However, there might also be costs associated with multiple mating, such as reduced paternal care to the brood, search costs for additional partners and parasite transmission [18]. Individuals involved in MP are not expected to be a random subset of the adult population [19], and traits correlated with MP in both males and females have attracted considerable attention [19][20][21].…”