AREGU March 47/3

Ortmann, Sylvia, and Gerhard Heldmaier. Regulation of body temperature and energy requirements of hibernating Alpine marmots (Marmota marmota). Am. J. Physiol. Regulatory Integrative Comp. Physiol. 278: R698–R704, 2000.— Body temperature and metabolic rate were recorded continuously in two groups of marmots either exposed to seasonally decreasing ambient temperature (15 to 0°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 O2·kg ·h21 between 5 and 15°C ambient temperature, although body temperature increased from 7.8 to 17.6°C, and a proportional increase of metabolic rate below 5°C ambient temperature. This apparent lack of a Q10 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 6 1.95 ml O2·kg ·h21 · °C21) 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 O2·kg ·h21). In summary, we provide strong evidence that hibernation is not only a voluntary but a wellregulated strategy to counter food shortage and increased energy demands during winter.