Extensive use of torpor in 13-lined ground squirrels in the fall prior to cold exposure |
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Authors: | Rae L Russell Philip H O’Neill L Elaine Epperson Sandra L Martin |
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Institution: | (1) Department of Cell and Developmental Biology, University of Colorado School of Medicine, P.O. Box 6511, MS 8108, Aurora, CO 80045, USA; |
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Abstract: | Mammalian hibernation is characterized by profound reductions in body temperature (T
b) and metabolic, heart and respiratory rates. These reductions are characteristic of torpor, which is temporally confined
to winter. Hibernators including ground squirrels are heterothermic in winter, cycling between multiday periods of torpor
with low T
b and brief periods of rewarming. In contrast, ground squirrels remain homeothermic during summer, like non-hibernating mammals.
The transition between the homeothermic and heterothermic phases of the circannual rhythm of hibernation is often overlooked
in hibernation studies. Here, we examined the use of torpor throughout the fall transition in laboratory-housed 13-lined ground
squirrels by recording core body temperature with an implanted data logger. As is typical of laboratory-based hibernation
studies, animals were kept in standard housing prior to being moved into a cold, dark room to simulate natural hibernation
conditions. Significantly, the vast majority of both male and female ground squirrels expressed torpor in the fall while still
housed conventionally and prior to cold exposure. The expression of torpor was not predicted by body weight or age, rather
it appears to be preprogrammed in a time-dependent manner that is independent of, yet enhanced by, environmental cues. The
timing and duration of these torpor bouts occurring prior to cold exposure were also remarkably sporadic. Thus, it is not
possible to know with certainty which animals are torpor-naive before cold exposure in the absence of continuous measurement
of body temperature. We conclude that fall animals encompass variable points in the transition between summer and winter phases
of the circannual cycle of hibernation, thereby confounding studies in which they are used as non-hibernating controls. Conversely,
these fall transition animals offer unique opportunities to define the molecular changes that accompany and enable hibernation. |
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