Thermoregulation and energetics in hibernating black bears: metabolic rate and the mystery of multi-day body temperature cycles.

Black bears overwintering in outdoor hibernacula in Alaska decrease metabolism to as low as 25 % basal rates, while core body temperature (T b) decreases from 37 to 38 °C to a mid-hibernation average of 33 °C. T b develops cycles of 1.6–7.3 days length within a 30–36 °C range, with no circadian component. We do not know the mechanism or function underlying behind the T b cycles, although bears avoid T b of <30 °C and shorter cycles are predicted from higher rates of heat loss in colder conditions. To test this we manipulated den temperatures (T den) of 12 hibernating bears with body mass (BM) from 35.5 to 116.5 kg while recording T b, metabolic rate (M), and shivering. T b cycle length (0.8–11.2 days) shortened as T den decreased (partial R 2 = 0.490, p < 0.001). Large bears with low thermal conductance (TC) showed more variation in T b cycle length with changes in T den than did smaller bears with high TC. Minimum T b across cycles was not consistent. At low T den bears shivered both during rising and decreasing phases of T b cycles, with minimum shivering during the fastest drop in T b. At higher T den the T b pattern was more irregular. Mean M through T b cycles was negatively correlated to T den below lower critical temperatures (1.4–10.4 °C). Minimum M (0.3509 W/kg ± 0.0121 SE) during mid-hibernation scaled to BM [M (W) = 1.217 × BM (kg)0.6979, R 2 = 0.855, p < 0.001]. Hibernating thermal conductance (TC) was negatively correlated to BM (R 2 = 0.721, p < 0.001); bears with high TC had the same T b cycle length as bears with low TC except at high T den, thus not supporting the hypothesis that cooling rate alone determines T b cycle length. We conclude that T b cycling is effected by control of thermoregulatory heat production, and T b cycling may not be present when hibernating bears use passive thermoregulation. More intense shivering in the rising phase of cycles may contribute to the prevention of muscle disuse atrophy. Bears hibernating in cold conditions use more energy during hibernation than in warmer conditions. At T den below lower critical temperature, no extra energy expenditure results from T b cycling compared to keeping a stable T b.