Figure 4. Body-temperature homeostasis is actively controlled during daily torpor.

(a) Protocol of the fasting-induced daily torpor experiment. Animals were placed in the T_A-constant chamber on Day 0; data were recorded for 72 hours from the beginning of Day 1. Food was removed and returned at the beginning of Day 2 and Day 3. Water was freely accessible throughout the experiment. (b) The minimum T_B at various T_As. Including the following panels, red and blue denote normal and torpid status, respectively. For normal status, the minimum T_B of the dark phase of Day 1 was used for analysis. For torpid status, the minimum T_B during torpor was used for analysis. As in the following panel d, the dots with the vertical error bars denote the observed mean and SEM of the minimum variables (T_B in b, VO₂ in d) at each T_A, and the line and the shaded area denote the mean and the 89% HPDI intervals of the estimated minimum variables. (c) The posterior distribution of the slope (a₁) of T_A−T_B relationship. Including the following distribution panels in this figure, the bold and thin lines denote the mean and the 89% HPDI intervals of the estimated values. The bin size is 0.005. (d) The minimum VO₂ at various T_As. Minimum VO₂ was defined as the VO₂ recorded when the T_B was minimum. (e) The posterior distribution of the slope (a₂) of T_A-VO₂ relationship. The bin size is 0.001 ml/g/hr/°C. (f) The rate of successful daily torpor induction at various T_As. When T_A was above 12 °C, all animals entered daily torpor. (g) The averaged torpor duration for each episode. One torpor episode is tended to be shorter when the T_A gets higher. (h, i) The posterior distribution of the estimated mean (n = 6) of T_B (h) and VO₂ (i) at a T_A of 8 °C. Because the observed mean (dashed lines) is larger than the 89% HPDI in both T_B and VO₂, when T_A = 8 °C, the animal exhibited higher metabolism than expected. The bin size is 0.1 °C and 0.01 ml/g/hr for T_B and VO₂, respectively.