Torpor Duration in Relation to Temperature and Metabolism in Hibernating Ground Squirrels

To understand the basis of the thermally sensitive physiological processes that govern the duration of individual bouts of torpor, we determined torpor bout duration in relation to air temperature ($T_{a}$), body temperature ($T_{b}$), and oxygen consumption ($\dot{V}O_{2}$) in hibernating golden-mantled ground squirrels, Spermophilus saturatus, during the midst of the hibernation season when bout length was stable. Over the range of $T_{a}$ from 8 down to 2 C, length of torpor bouts increased as $T_{b}$ and $\dot{V}O_{2}$ decreased, and the difference between $T_{b}$ and $T_{a}$ (ΔT) remained about 1.5 C. However, when $T_{a}$ was lowered from 2 C to-2 C, the trend reversed and torpor duration and $T_{b}$ decreased, whereas $\dot{V}O_{2}$ and ₂T increased. This response is due to an increase in heat production that keeps $T_{b}$ from declining below about 1 C. We examined our data in terms of the hypothesis that arousal is determined by accumulation of metabolites during torpor; if this is true, then the arousal frequency should be directly proportional to energy metabolism ($\dot{V}O_{2}$). Using linear and multiple regression, we found that $T_{b}$ and $T_{a}$ are better predictors of torpor duration than $\dot{V}O_{2}$. The best relationship was, however, achieved by regression of $T_{b}$ and $\dot{V}O_{2}$. We conclude that $T_{b}$ per se also contributes, along with metabolism, to determining the length of torpor bouts.