Energetics of arousal episodes in hibernating arctic ground squirrels

Arctic ground squirrels overwintering in northern Alaska experience average soil temperature of -10°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°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°C took longer, consumed more oxygen, and attained higher peak rates of oxygen consumption when compared to 2°C. T_a had no significant effect on cost or duration of the euthermic phase. Animals recooled faster at -12°C than at 2°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°C and only 23% at -12°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.