Temperature sensors were placed in the abdominal cavity, on the neck, and outside the dens of 5 hibernating black bears (Ursus americanus) during early winter and removed at the end of winter before emergence of bears from their dens. Bears did not arouse from torpor throughout the winter test period. Abdominal temperature remained within a 1.5°C temperature range and did not appear to exhibit circadian rhythmicity. However, neck surface temperature of bears demonstrated elevated spikes from 2 to 30°C about 4 times each day. Adult students wearing the same neck sensors as bears exhibited similar spikes in body surface temperature when vigorously exercising in the cold. We suggest that bears engage in bouts of muscle activity during the winter denning period that may result in the retention of muscle strength without elevating their core body temperature and without arousing from torpor.