Group hibernation does not reduce energetic costs of young yellow-bellied marmots

Abstract

We investigated mechanisms of energy conservation during hibernation. The amount of time torpid was significantly less for groups of three young marmots than for marmots hibernating singly. Mean daily mass loss (DML; as mg d -1 g-1 immergence mass) averaged 1.33 for single marmots and 1.46 for grouped young. Animals were active 17.3% of the time, which used 82.4% of the energy, and were torpid 82.7% of the time, which used 17.6% of the energy expenditure. During longer torpor bouts, more time was spent in deep torpor, which decreased the hourly cost of a complete bout. Bout oxygen consumption V̇o2, percent time in deep torpor, and body temperature (TB) during deep torpor changed seasonally and were curvilinearly related to when in the hibernation period the measurements were made and probably represent a stage in the circannual metabolic cycle. The decrease of environmental temperature (TE) to 2°C significantly increased metabolism. Potential costs of low TE were reduced by allowing TB to decrease, thereby reducing the TB to T E gradient. Average monthly metabolic rate was high early and late in the hibernation period when time spent euthermic was greater and when V̇o2 was higher. Over the hibernation period, energy saved averaged 77.1% and 88.0% of the costs for winter and summer euthermic metabolism, respectively. Hibernation costs were reduced by the seasonal changes, the high percentage of time in torpor, the rapid decline in V̇o2 following arousal, and allowing TB to decline at lower TE. Asynchrony in the torpor cycles increased energy expenditures in group hibernators, which negated possible beneficial effects of group hibernation.