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The bear circadian clock doesn't 'sleep' during winter dormancy.

Heiko T Jansen1, Tanya Leise2, Gordon Stenhouse3

  • 1Department of Integrative Physiology and Neuroscience, College of Veterinary Medicine, Washington State University, Mailstop 7620, Veterinary and Biomedical Research Bldg., Room 205, Pullman, WA 99164-7620 USA.

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Torpid bears maintain functional circadian rhythms, demonstrating that their internal body clocks remain active and responsive to light cues even during winter dormancy. This internal timekeeping is a normal aspect of their physiology.

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Area of Science:

  • Chronobiology
  • Animal Physiology
  • Mammalian Hibernation

Background:

  • Circadian rhythms synchronize biological functions to light:dark cycles.
  • Bears undergo shallow torpor with metabolic suppression during winter.
  • The presence and function of circadian rhythms in torpid bears remain largely uncharacterized.

Purpose of the Study:

  • To confirm free-running circadian rhythms of body temperature and activity in torpid grizzly bears.
  • To assess the functional responsiveness of these rhythms to environmental light.
  • To investigate molecular clock operation in peripheral tissues of torpid bears.

Main Methods:

  • Monitoring activity and body temperature in captive and wild denning bears.
  • Measuring ambient light exposure in natural dens.
  • Analyzing cultured skin fibroblasts for molecular clock gene expression.
  • Estimating circadian parameters using wavelet transforms and spectral analyses.

Main Results:

  • Torpid captive bears expressed circadian rhythms of activity and body temperature in constant darkness.
  • Rhythm periods differed between juvenile and adult bears but were consistent between captive and wild adults.
  • Light exposure phase-shifted activity rhythms, consistent with known mammalian responses.
  • In vitro studies confirmed molecular circadian rhythms in peripheral tissues.

Conclusions:

  • The circadian system is functional in torpid bears, including peripheral tissues.
  • These rhythms are responsive to light, indicating normal physiological regulation.
  • Circadian timekeeping is an integral part of torpid bear physiology.