Int J Med Sci 2013; 10(5):508-514. doi:10.7150/ijms.4476 This issue

Research Paper

Body Temperature during Hibernation Is Highly Correlated with a Decrease in Circulating Innate Immune Cells in the Brown Bear (Ursus arctos): A Common Feature among Hibernators?

Berolla Sahdo1, Alina L. Evans2,3, Jon M. Arnemo2,4, Ole Fröbert5, Eva Särndahl1,5, ✉, Stéphane Blanc6 ✉

1. Department of Clinical Medicine, School of Health and Medical Sciences, Örebro University, SE-701 82 Örebro, Sweden.
2. Department of Forestry and Wildlife Management, Hedmark University College, Campus Evenstad NO-2418 Elverum, Norway.
3. Section of Arctic Veterinary Medicine, Norwegian School of Veterinary Science, NO-9292 Tromsø, Norway.
4. Department of Wildlife, Fish and Environmental Studies, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden.
5. Department of Cardiology, Örebro University Hospital, SE-701 85 Örebro, Sweden.
6. Université de Strasbourg, IPHC, 23 rue Becquerel 67087 Strasbourg, France, CNRS, UMR7178, 67037 Strasbourg, France.

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Sahdo B, Evans AL, Arnemo JM, Fröbert O, Särndahl E, Blanc S. Body Temperature during Hibernation Is Highly Correlated with a Decrease in Circulating Innate Immune Cells in the Brown Bear (Ursus arctos): A Common Feature among Hibernators?. Int J Med Sci 2013; 10(5):508-514. doi:10.7150/ijms.4476. Available from

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Background: Hibernation involves periods of severely depressed metabolism (torpor) and decreases in body temperature (Tb). Small arctic mammals (<5kg), in which Tb generally drop drastically, display leukopenia during hibernation. This raised the question of whether the decreased leukocyte counts in mammalian hibernators is due to torpor per se or is secondary to low Tb. The present study examined immune cell counts in brown bears (Ursus arctos), where torpor is only associated with shallow decreases in Tb. The results were compared across hibernator species for which immune and Tb data were available.

Methods and Results: The white blood cell counts were determined by flow cytometry in 13 bears captured in the field both during summer and winter over 2 years time. Tb dropped from 39.6±0.8 to 33.5±1.1°C during hibernation. Blood neutrophils and monocytes were lower during hibernation than during the active period (47%, p= 0.001; 43%, p=0.039, respectively), whereas no change in lymphocyte counts was detected (p=0.599). Further, combining our data and those from 10 studies on 9 hibernating species suggested that the decline in Tb explained the decrease in innate immune cells (R2=0.83, p<0.0001).

Conclusions: Bears have fewer innate immune cells in circulation during hibernation, which may represent a suppressed innate immune system. Across species comparison suggests that, both in small and large hibernators, Tb is the main driver of immune function regulation during winter dormancy. The lack of a difference in lymphocyte counts in this context requires further investigations.

Keywords: Brown bear, Ursus arctos, Hibernation, Innate immunity, Leukocytes, Torpor.