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Drivers of hibernation in the brown bear

Evans, Alina; Singh, N.J.; Friebe, A.; Arnemo, Jon Martin; Laske, T.G.; Fröbert, O.; Swenson, Jon E.; Blanc, S.
Journal article, Peer reviewed
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URI
http://hdl.handle.net/11250/2381139
Date
2016
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  • Artikkel - fagfellevurdert vitenskapelig / Articles - peer-reviewed [1179]
Original version
Evans, A., Singh, N. J., Friebe, A., Arnemo, J. M., Laske, T., Fröbert, O., . . . Blanc, S. (2016). Drivers of hibernation in the brown bear. Frontiers in Zoology, 13(7). doi: 10.1186/s12983-016-0140-6   10.1186/s12983-016-0140-6
Abstract
Background: Hibernation has been a key area of research for several decades, essentially in small mammals in the

laboratory, yet we know very little about what triggers or ends it in the wild. Do climatic factors, an internal

biological clock, or physiological processes dominate? Using state-of-the-art tracking and monitoring technology on

fourteen free-ranging brown bears over three winters, we recorded movement, heart rate (HR), heart rate variability

(HRV), body temperature (Tb), physical activity, ambient temperature (TA), and snow depth to identify the drivers of

the start and end of hibernation. We used behavioral change point analyses to estimate the start and end of

hibernation and convergent cross mapping to identify the causal interactions between the ecological and

physiological variables over time.

Results: To our knowledge, we have built the first chronology of both ecological and physiological events from

before the start to the end of hibernation in the field. Activity, HR, and Tb started to drop slowly several weeks

before den entry. Bears entered the den when snow arrived and when ambient temperature reached 0 °C. HRV,

taken as a proxy of sympathetic nervous system activity, dropped dramatically once the bear entered the den. This

indirectly suggests that denning is tightly coupled to metabolic suppression. During arousal, the unexpected early

rise in Tb (two months before den exit) was driven by TA, but was independent of HRV. The difference between Tb

and TA decreased gradually suggesting that bears were not thermoconforming. HRV increased only three weeks

before exit, indicating that late activation of the sympathetic nervous system likely finalized restoration of euthermic

metabolism. Interestingly, it was not until TA reached the presumed lower critical temperature, likely indicating that

the bears were seeking thermoneutrality, that they exited the den.

Conclusions: We conclude that brown bear hibernation was initiated primarily by environmental cues, but

terminated by physiological cues.
Publisher
Frontiers in zoology
Journal
Frontiers in Zoology

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