Estimating Indirect Effects in a Wild and Anti-Social Mammal

Abstract. Nearly all organisms interact with conspecifics at some point in their lives, and as such can affect one another’s phenotypes. Indirect genetic effects occur when an individual’s phenotype is affected by genes being expressed in another individual. This additional source of heritable variation within a population can enhance, or reduce adaptive potential, thereby accelerating or even reversing evolutionary change. Hence quantifying these indirect effects is crucial for our understanding of evolution, yet estimates of indirect genetic effects in wild animals are limited. We estimate indirect phenotypic, and indirect genetic effects, among both spatial and temporal neighbors in wild North American red squirrels (Tamiasciurus hudsonicus). Following our predictions, indirect effects of spatial neighbors were weak when populations densities were low, but stronger when population densities were high. Evidence that these effects had a heritable basis however was not conclusive, as indirect genetic effects were not significantly different from zero, although indirect phenotypic effects were. Nevertheless, the estimated effect size was such that, if present, indirect genetic effects are predicted to reduce trait evolution by 59% in high density years. Temporal neighbours, i.e. the previous squirrel to hold the territory, influenced the current squirrel's food hoard size, and breeding date, but neither of these effects had a heritable basis. We therefore found good evidence for indirect phenotypic effects among spatial and temporal neighbours, but evidence these effects were heritable was less strong. Potential effects are strong however, suggesting indirect effects should be considered in more systems, especially if they are heritable.