Energetics of arousal episodes in hibernating arctic ground squirrels

Arctic ground squirrels overwintering in northern Alaska experience average soil temperature of -10 degrees C. To examine energetic costs of arousing from hibernation under arctic compared to temperate conditions, captive ground squirrels were maintained in ambient temperatures (T(a)) of 2, -5 and -12 degrees C. Rates of oxygen consumption and carbon dioxide production were used to estimate metabolic rate and fuel use during the three phases of arousal episodes: rewarming, euthermia, and recooling. Respiratory quotient comparisons suggest exclusive use of lipid during rewarming and mixed fuel use during euthermia. Animals rewarming from torpor at T(a) -12 degrees C took longer, consumed more oxygen, and attained higher peak rates of oxygen consumption when compared to 2 degrees C. T(a) had no significant effect on cost or duration of the euthermic phase. Animals recooled faster at -12 degrees C than at 2 degrees C, but total oxygen consumption was not different. T(a) had no significant effect on the total cost of arousal episodes when all three phases are included. Arousal episodes account for 86% of estimated costs of a complete hibernation cycle including torpor when at 2 degrees C and only 23% at -12 degrees C. Thus, due to the higher costs of steady-state metabolism during torpor, proportional metabolic costs of arousal episodes at T(a) characteristic of the Arctic are diminished compared to relative costs of arousals in more temperate conditions.