The role of energy expenditure in resource acquisition and energy allocation in free-ranging North American red squirrels (Tamiasciurus hudsonicus)

Energy is a currency of life because all organisms must expend energy to survive and reproduce. I examined the central role that energy expenditure plays in the lives of free-ranging female North American red squirrels ( Tamiasciurus hudsonicus) in Yukon, Canada. I found that energy acquisition while hoarding white spruce (Picea glauca) cones in autumn was characterized by a type III functional response; when cones were scarce, squirrels switched to hoarding mushrooms, whereas when cones were abundant, squirrels were unable to hoard more than a small proportion of the cones present in their territory. The strong satiation of cone hoarding supports the primary prediction of the predator satiation hypothesis for the evolution of masting in white spruce trees. I also determined the daily energy expenditure (DEE, kJ/day) of female squirrels using the doubly-labeled water technique during the activity stages of nonbreeding (winter and summer), lactation, and hoarding. The primary determinant of energy expenditure throughout the year was the activity stage that the individual was engaged in, with females having the highest energy expenditure during hoarding and lactation. Ambient temperature (Ta) during the sampling interval was the second most important determinant of energy expenditure, but counter to expectations based on thermoregulatory requirements, Ta was positively correlated with energy expenditure. Levels of DEE were positively correlated with an estimate of physiological somatic damage (plasma protein carbonyls). The somatic damage experienced by females during lactation was greater than that experienced by nonbreeding females during summer and winter. Levels of somatic damage were reduced during lactation if individuals had access to supplemental food, which was associated with squirrels having higher levels of total antioxidant capacity. High energy expenditure during lactation was correlated with increased reproductive success. Lactating females with higher DEE also had increased physiological somatic damage, but there was no relationship between lactation DEE and survival. My thesis establishes for the first time the intuitive importance of energy expenditure for determining energy acquisition and mediating life-history trade-offs between reproduction and survival.