Environmental variability affects individual variation in activity onset at large geographic scales

Abstract Timing is an essential component of the phenotype, influencing individual fitness and shaping ecological interactions. Although variation in the timing of activity can be substantial, the degree to which this variation stems from within-individual variation vs. among-individual variation is poorly understood and represents a key knowledge gap with distinct ecological and evolutionary implications. Within-individual variation in timing may allow individuals to adjust to unfamiliar or shifting cues, while consistent among-individual variation in timing may lead to maladaptive behavior in novel conditions (eg, light pollution or novel predators). We estimated activity onset from 821 captures of 254 individual white-footed mice, Peromyscus leucopus, across the upper Midwest, Northeast, Mid-Atlantic, and southern Appalachia regions of the United States. In each locale, we quantified the total phenotypic variation in activity onset and partitioned this into its within- and among-individual components. We then examined whether environmental factors, such as variable climate or habitat structure, influenced these components of variation. We found that within-individual variation in timing consistently exceeded among-individual variation, and individuals from populations at cooler, drier sites with seasonal fluctuations in precipitation and temperature exhibited more within-individual behavioral variation than mice at warmer, wetter sites with less seasonal change. Neither within- nor among- individual variation was significantly associated with habitat structure heterogeneity. Our results reveal that most variation in timing across this species’ range derives from within-individual variation and suggests that individuals may have the capacity to adjust the timing of activity onset in response to local conditions.