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 (Tb) decreases from 37 to 38 °C to a mid-hibernation average of 33 °C. Tb 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 Tb cycles, although bears avoid Tb of <30 °C and shorter cycles are predicted from higher rates of heat loss in colder conditions. To test this we manipulated den temperatures (Tden) of 12 hibernating bears with body mass (BM) from 35.5 to 116.5 kg while recording Tb, metabolic rate (M), and shivering. Tb cycle length (0.8–11.2 days) shortened as Tden decreased (partial R2 = 0.490, p < 0.001). Large bears with low thermal conductance (TC) showed more variation in Tb cycle length with changes in Tden than did smaller bears with high TC. Minimum Tb across cycles was not consistent. At low Tden bears shivered both during rising and decreasing phases of Tb cycles, with minimum shivering during the fastest drop in Tb. At higher Tden the Tb pattern was more irregular. Mean M through Tb cycles was negatively correlated to Tden 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, R2 = 0.855, p < 0.001]. Hibernating thermal conductance (TC) was negatively correlated to BM (R2 = 0.721, p < 0.001); bears with high TC had the same Tb cycle length as bears with low TC except at high Tden, thus not supporting the hypothesis that cooling rate alone determines Tb cycle length. We conclude that Tb cycling is effected by control of thermoregulatory heat production, and Tb 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 Tden below lower critical temperature, no extra energy expenditure results from Tb cycling compared to keeping a stable Tb.