1. The regulation of energy expenditure in relation to food availability and its consequences for individual fitness in free-ranging animals are poorly understood. Increased daily energy expenditure (DEE) may be viewed as the result of two different processes: expenditure may be forced upwards by low food availability (forcing hypothesis) or enabled to increase by high levels of food resources (enabling hypothesis). Several studies have suggested long-term fitness costs due to increased mortality as a trade-off to increased DEE.
2. We examined the relationship between energy expenditure and an indirect measure of food availability, and the short-term fitness consequences associated with changes in DEE in a small, Arctic seabird, the little auk (Alle alle). We measured DEE of 43 parent little auks by the doubly labelled water method during two consecutive breeding seasons and inferred food availability from plasma concentrations of the stress hormone corticosterone (CORT).
3. We found that DEE was elevated by 26·7% in the year with reduced levels of CORT, indicating that little auks responded to increased food availability by increasing their DEE. These results support the enabling hypothesis. Elevated DEE was presumably caused by increased parental effort as reflected by higher chick provisioning rates and larger chick meals, and was associated with fitness benefits in terms of enhanced current reproductive success.
4. Contrary to earlier studies, our data did not indicate adverse effects associated with elevated DEE; there was no negative relationship between DEE and the probability of adults returning to the colony the following year. Instead, adult return rate was positively related to body mass, with lower return rates when food was limited.
5. These results suggest that ecological consequences associated with limited resource availability may outweigh possible direct negative physiological effects of elevated DEE.