Extra-pair paternity, age and paternal care: fitness consequences and genetic correlations

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Infidelity is common among many taxa, but we still do not know how selection pressures shape variation in, and drive evolution of extra-pair behavior. Males can reap additional fitness benefits from extra-pair offspring, because extra-pair fathers do not have to spend resources on costly parental care. This is however not the case for females, and today it is still unclear why females produce extra-pair offspring. The indirect benefits hypothesis has been suggested to explain female infidelity, but it is not well supported. Despite many studies on the topic, empirical evidence for indirect fitness benefits is ambiguous at best. Sexually antagonistic selection, where male and female extra-pair behavior are genetically linked, is another hypothesis to explain why females engage in extra-pair behavior, but empirical for this evidence is rare. Not accounting for social feedback effects between meting partners however may cloak potential genetic correlations. This project aims at a comprehensive assessment of the fitness consequences of female infidelity, focusing on two major sources of costs, and at assessing the genetic correlation between the sexes accounting for social feedback. One major source of costs to females may be the negative association between paternal care and polygamy. This effect is relatively well established among species, but the variation within-individuals, the resulting fitness consequences and predicted selection pressures, are largely unexplored. This may be because especially the costs are hard to assess, as they likely differ within- and between individuals, which needs specific analyses, in turn requiring large sample sizes. The uniquely suited long-term data on wild birds available for this project overcome these problems, and allow detailed analyses. A second source of costs is not often acknowledged, but may contribute greatly to our understanding of female infidelity: Older males of supposedly higher quality often have more extra-pair offspring than younger males. Empirical data suggests that older males sire offspring of poor quality, which could offset the direct fitness benefits old extra-pair sires supposedly gain. I propose to conduct detailed analyses in the long-term data and to complement this by focused experiments on captive individuals, to understand age-related variation in extra-pair behaviour. I then propose to estimate the genetic correlation, and to account for social feedback, the indirect effects, in extra-pair behavior. Finally, a comprehensive fitness analysis of costs and benefits of extra-pair behavior will synthesize the results. This project will allow us to better distinguish between the indirect genetic benefits hypothesis and the antagonistic selection hypothesis, and further our understanding of how variation in extra-pair behaviors within populations can be maintained.