Seasonal patterns in behavior and glucocorticoid secretion of a specialist Holarctic tree squirrel (Sciurus aberti)

Seasonally breeding mammals must make constant adjustments in behavior and physiology to manage energetic trade-offs between survival and reproduction. Despite encountering high levels of climate and resource variability across the year, specialist Abert’s squirrels (Sciurus aberti), lack the capacity to express hibernation or pronounced morphological adaptations to seasonality. Using accelerometer and GPS devices, we assessed how abiotic environmental factors, reproduction, and resource abundance influenced levels of activity and daily range size in a rural and food-supplemented suburban population of squirrels. We also quantified fecal cortisol metabolites (FCM) in squirrels to assess patterns of glucocorticoid secretion. While changes in weather predicted activity levels in both populations, seasonal variation in activity levels were reduced in food-supplemented compared to rural squirrels. In contrast to activity, daily range size was not affected by weather but was a better predictor of sex-specific reproductive investment. Comparisons between populations suggest that food-supplemented squirrels forage more efficiently within smaller areas. Across both sexes and populations, squirrels showed no sexual dimorphism in body size, no major patterns of seasonal weight change, and no associations between body mass and FCM concentrations; however, FCMs were lower in the food-supplemented compared to rural population during late-spring. Taken together, activity levels and FCM concentrations appear primarily influenced by weather and seasonal fluctuations in food availability, whereas daily range size reflects sexual asymmetries in seasonal reproductive investment. Overall, squirrels appear to rely largely on behavioral adjustments to cope with novel environmental heterogeneity, rather than changes in morphology or GC secretion.