COST OF LOCOMOTION AND DAILY ENERGY-EXPENDITURE BY FREE-LIVING SWIFT FOXES (VULPES VELOX) - A SEASONAL COMPARISON

We studied the daily energy expenditure of free-living nonreproductive carnivorous swift foxes (Vulpes velox, average mass 2.1 kg) on shortg rass prairie in southeastern Colorado in summer and winter in relation to air temperature, daily activity pattern, movement rate, and daily movement distance. The field metabolic rate (FMR) was measured with do ubly labeled water, and activity and movements were monitored by radio telemetry. During their nighttime activity period in winter, swift fox es traveled large distances (18.5 +/- 0.6 km/d). Locomotion costs (est imated from daily movement distance in winter, using an allometric equ ation) accounted for at least 21% of total daily expenditure, the high est proportion reported for a mammal. During their nocturnal activity periods (winter vs. summer), swift foxes apparently traveled farther ( ca. 13.0 vs. 5.7 km/night, using equal sampling intervals) and were ac tive longer (ca. 12.9 vs. 11.3 h/night) in colder air. Nevertheless, F MR in winter (1488 kJ/d) was significantly lower than during summer (2 079 kJ/d). We review available data for other free-living eutherians a nd show that low temperatures in winter are not necessarily associated with increases in FMR.