Seasonal energetics and torpor use in North American flying squirrels

Seasonal cold temperatures require mammals to use morphological, behavioural, or physiological traits to survive periods of extreme cold and food shortage. Torpor is a physiological state that minimizes energy requirements by decreasing resting metabolic rate (MR) and body temperature (Tb). Many rodent species are capable of torpor, however, evidence in northern and southern flying squirrels (Glaucomys sabrinus and Glaucomys volans, respectively) has remained anecdotal. We experimentally attempted to induce torpor in wild-caught flying squirrels by lowering ambient temperature (Ta) and measuring MR using open-flow respirometry. We also studied seasonal differences in MR and Tb at various Ta. Both MR and Tb provided evidence for torpor in flying squirrels, but only infrequent, shallow torpor. MR decreased infrequently and any decreases were rarely sustained for longer than one hour. We found a significant positive relationship between Ta and Tb only in G. volans, which suggests that G. volans is more susceptible to low Ta compared with G. sabrinus, possibly due to their small body size. We observed no substantive seasonal or interspecific differences in the relation between MR and Ta, with the exception that northern flying squirrels expended more energy at cold Ta during warm season trials than other species-season combinations. The infrequency of torpor use in our experiments suggests that other energy-saving strategies, such as social thermoregulation, may limit the reliance on torpor in this lineage.