Daily energy expenditure and the cost of activity in a free-living mammal.

Antelope ground squirrels (Ammospermophilus leucurus, 80-100 g) began surface activity 1.0-1.5 h after sunrise and ended it 0.5-1.25 h before sunset throughout the year near Barstow, California. Daily energy expenditure (DEE) of free-living animals measured with doubly labeled water (H3H18O) decreased from 1,340 kJ kg-1 d-1 in April to 970 in October. Resting metabolic rates (RMR) of freshly-captured, fed, ground squirrels varied through the year (22.1 J g-1 h-1 in August, 19.1 in January) but most of the change in DEE could be explained by differences in thermoregulatory costs between seasons. The ground squirrels had lower rates of resting metabolism at night (15.3J g-1 H-1) than during the day.The cost of activity (calculated by subtracting 24 h resting costs from total DEE during August and October, periods when thermoregulatory costs were negligible) was ∼ 550 kJ kg-1 d-1 in August and October. Thus, activity accounted for about 50% of the total DEE. The mean rate of energy expenditure during the activity period, calculated as activity cost (kJ d-1) divided by activity time (h d-1), then plus RMR, was about 3xRMR. This multiplication factor may be useful as an estimator of foraging costs or in estimating DEE from time budgets.