Core body temperature (CBT) changes over a year measured intraperitoneally in a laboratory-housed 13-lined ground squirrel. Top: black line plots CBT, gray line plots ambient temperature. Hibernation is the heterothermic period; arrowheads bracket euthermic CBT in both the homeothermic (active) and heterothermic (hibernation) seasons. Note that the fall transition between homeothermy and heterothermy can begin to occur before ambient temperature is lowered to the winter temperature of 4°C (). The boxed region of the CBT trace is expanded in the bottom panel to emphasize the torpor arousal cycle. CBT nadirs at ∼4°C for multiple days during the torpor phase of hibernation during which time heart and respiratory rates decline from a summertime level of 200–300 to 3–5 beats/min and respiratory rate from 100–200 to 4–6 breaths/min (). During periodic arousal, heart, respiratory, and metabolic rates rapidly increase, and CBT returns to euthermia. After ∼12 h, metabolic, heart, and respiratory rates decline, CBT falls again, and the hibernator reenters torpor ().

A 13-lined ground squirrel emerges healthy and alert (A) after months of cycling between torpor (B) with low perfusion and arousal with rapid reperfusion during hibernation.