Thermal relations of metabolic rate reduction in a hibernating marsupial

We tested whether the reduction of metabolic rate (MR) in hibernating Cercartetus nanus (Marsupialia, 36 g) is better explained by the reduction of body temperature (T-b), the differential (Delta T) between T-b and air temperature (T-a), or thermal conductance (C). Above the critical T-a during torpor (T-tc) of 4.8 +/- 0.7 degrees C, where the T-b was not regulated, the steady-state MR was an exponential function of T-b (r(2) = 0.92), and the overall Q(10) was 3.3. However, larger Q(10) values were observed at high T-b values during torpor, particularly within the thermoneutral zone (Q(10) = 9.5), whereas low Q(10) values were observed below T-b 20 degrees C (Q(10) = 1.9). The Delta T did not change over T-a 5-20 degrees C, although MR fell, and therefore the two variables were not correlated. Below the T-tc, T-b was regulated at 6.1 +/- 1.0 degrees C and MR increased proportionally to Delta T. Our study suggests that MR in torpid C. nanus is largely determined by temperature effects and metabolic inhibition. In contrast, Delta T explains MR only below the T-tc and C appears to affect MR only indirectly via changes of T-b, suggesting that Delta T and C play only a secondary role in MR reduction during hibernation.