Indirect genetic effects improve female extra-pair heritability estimates.

The question of why females engage in extra-pair behaviours is long-standing in evolutionary biology. One suggestion is that these behaviors are maintained through pleiotropic effects on male extra-pair behaviors and lifetime reproductive success (genes controlling extra-pair behaviours are shared between sexes, but only beneficial to one, in this case, males). However, for this to occur extra-pair behaviour must be heritable and positively genetically correlated between sexes. Although previous studies have suggested low heritability with no evidence for between-sex genetic correlations in extra-pair behaviours, indirect genetic effects (those derived from the behaviour of others, IGEs) from the social partner, the influence of the social partners genotype on the phenotype of an individual, have not been considered, despite the potential to uncover hidden heritability. Using data from a closed house sparrow population with a genetic pedigree spanning two decades, we tested the influence of IGEs on heritability and genetic correlation estimates of extra-pair behaviour. We found that the inclusion of IGEs improved model fit for both male and female extra-pair heritability. While IGEs did not change between-sex genetic correlations, we found a reduction in uncertainty in our estimates. Future studies should consider the effect of IGEs on the mechanisms of sex specific extra-pair behaviour.