Julia Schroeder, Hannah L Dugdale , Shinichi Nakagawa , Alexandra M Sparks, Terry Burke

The social environment can influence phenotypes through indirect genetic effects (IGEs), whereby genetic variance among interacting individuals explains some of the phenotypic variance. Empirical studies of wild populations often ignore IGEs especially among unrelated individuals, probably due to data limitations. This is problematic because IGEs can crucially affect estimates of heritable variation and subsequently influence the predicted evolutionary change. We here present a quantitative genetic analysis of biparental care in a natural bird population using a genetic pedigree. For both sexes, the conventionally calculated repeatability (15% in the female trait and 19% in the male trait) was lower than the total heritable variation including IGEs (24% in the female trait, and 25% in the male trait). These estimates of total heritable variation was also larger compared to conventionally calculated heritability (13% in both sexes), suggesting that parental care can evolve through social selection. Furthermore, we detected statistically significant genetic covariance between direct genetic effects, and between IGEs and direct genetic effects. Our work showcases how IGEs can represent substantial and important hidden heritable variance and highlights the importance of considering IGEs for theoretical models of parental care for ecology and evolution.