Regulation of body temperature and energy requirements of hibernating Alpine marmots (Marmota marmota)

Body temperature and metabolic rate were recorded continuously in two groups of marmots either exposed to seasonally decreasing ambient temperature (15 to 0 degrees C) over the entire hibernation season or to short-duration temperature changes during midwinter. Hibernation bouts were characterized by an initial 95% reduction of metabolic rate facilitating the drop in body temperature and by rhythmic fluctuations during continued hibernation. During midwinter, we observed a constant minimal metabolic rate of 13.6 ml O-2.kg(-1).h(-1) between 5 and 15 degrees C ambient temperature, although body temperature increased from 7.8 to 17.6 degrees C, and a proportional increase of metabolic rate below 5 degrees C ambient temperature. This apparent lack of a Q(10) effect shows that energy expenditure is actively downregulated and controlled at a minimum level despite changes in body temperature. However, thermal conductance stayed minimal (7.65 +/- 1.95 ml O-2.kg(-1).h(-1).degrees C-1) at all temperatures, thus slowing down cooling velocity when entering hibernation. Basal metabolic rate of summer-active marmots was double that of winter-fasting marmots (370 vs. 190 ml O-2.kg(-1).h(-1)). In summary, we provide strong evidence that hibernation is not only a voluntary but a well-regulated strategy to counter food shortage and increased energy demands during winter.