Animals at the extremes: Hibernation and torpor

## 4.5 Summary

### 5 Physiological adaptations – respiration and energy provision

# 5.1 Introduction

The change in BMR observed in all hibernators has traditionally been viewed as a passive response that is a consequence of hypothermia. However, many studies have provided evidence for temperature-independent regulation of BMR. In the alpine marmot (Marmota marmota), a BMR that is less than 5% of summer levels is maintained despite the frequent fluctuations in body temperature between 8 and 18° C. The mechanism of body temperature regulation in marmots, during long periods of hibernation, has become clearer following investigations of T b and BMR throughout this phase. Entry into hibernation is facilitated by a precipitate drop in BMR that precedes slower temperature changes, then throughout the winter, bursts of thermogenesis occur quite independently of T a (Figure 27) (Ortmann and Heldmaier, 2000).

Figure 27 (a) Time course of body temperature (T b) over an entire hibernation season in the alpine marmot (Marmota marmota). Ambient temperature (T a) decreased step wise from 15° C in autumn to 0° C in spring. (b) Time course of weight-specific metabolic rate (top) and T b and T a (bottom) over a time period of 10 days and then 3 days. T a was decreased from around 7° C at the end of December through the beginning of January to around 2.5° C in mid-February. Note that bursts of higher metabolic rate occur during deep hibernation which do not synchronize with changes in T b
(a) Time course of body temperature over an entire hibernation season in the alpine marmot (Marmota marmota). Ambient temperature decreased step wise from 15C in autumn to 0C in spring. (b) Time course of weight-specific metabolic rate (top) and body temperature and ambient temperature (bottom) over a time period of 10 days and then 3 days. Ambient temperature was decreased from around 7C at the end of December through the beginning of January to around 2.5C in mid-February. Note that bursts of higher metabolic rate occur during deep hibernation which do not synchronize with changes in temperature.
Ortmann, S. and Heldmaier, G. (2000) Regulation of body temperature and energy requirements…, American Journal of Physiology, 278. Copyright © American Physiological Society

It is widely acknowledged that mammals switch to the use of lipid from WAT during hibernation. A period of ‘fattening-up’ precedes the onset of hibernation, under the control of hormones which stimulate lipid storage. In mice induced to enter a near-torpid state, levels of leptin are low, reducing lipolysis and promoting lipid storage. However, BMR is lowered in the little brown bat (Myotis lucifugus; Figure 9), despite an increase in plasma leptin during the pre-hibernation period which suggests that weight gain is controlled by other hormones together with a resistance to leptin-induced satiety in this species.

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