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 (HHO) decreased from 1,340 kJ kg d in April to 970 in October. Resting metabolic rates (RMR) of freshly-captured, fed, ground squirrels varied through the year (22.1 J g h 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 H) 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 d 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) divided by activity time (h d), 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.