Figure 2.

Hysteresis plots of heart rate (HR) versus body temperature $\left(T_{\text{b}}\right)$. Data are from six Arctic ground squirrels with complete recordings of all four phases of a hibernation bout. Arrows indicate the directions of entrance into and arousal from torpor. HR significantly decreased before the onset of the reduction in $T_{\text{b}}$, followed by a more gradual decrease in HR as $T_{\text{b}}$ started falling. HR increased drastically over about 4 h during the arousal phase. Differences in the amount of data points during each phase are due to the length of the different phases. IBA = interbout arousal.