Eurasian Red Squirrels Show Little Seasonal Variation in Metabolism in Food-Enriched Habitat.

Energy expenditure and ambient temperature (T) are intrinsically linked through changes in an animal's metabolic rate. While the nature of this relationship is stable, the breadth of change in thermoregulatory cost varies with body size and physiological acclimatization to season. To explore seasonal metabolic changes of small mammals, we studied a population of Eurasian red squirrels (Sciurus vulgaris) in a seminatural environment with a year-round supply of natural and supplemented food. In each season we measured the metabolic rate of wild-caught red squirrels, using open-flow respirometry, and hypothesized that individuals would make adjustments to contend with seasonal weather conditions. In comparison to summer animals, we predicted that winter squirrels would show (1) an increase in metabolic rate within the thermoneutral zone, (2) a decrease in the lower critical temperature of the thermoneutral zone, (3) a shallower slope of resting metabolic rate with decreasing T, and (4) lower thermal conductance. Surprisingly, we observed only minor changes in resting metabolic rate, and energetic modeling suggested that the scope of change was unlikely to be of ecological consequence. Hair area density was higher in winter than in summer, corresponding to a slightly elevated thermal conductance in summer, while body mass was reasonably constant year-round. We conclude that the scope of physiological seasonal adaptation is minimal when food is abundant and that squirrels instead rely on adjustments in activity to reduce exposure to low T. We suggest that this may explain the squirrel's success in a wide range of habitats, including urban areas, which require a rapid and flexible response to environmental changes and may indicate the capacity of other small mammal species to cope with environmental disturbance.