Flexibility is the key: metabolic and thermoregulatory behaviour in a small endotherm.

Hibernation is the most effective way to reduce thermoregulatory costs during periods of unfavourable environmental conditions. In preparation to hibernation, fat-storing hibernators accumulate large quantities of body fat, which increases their locomotor costs and also the risk of predation. As a consequence, there should be a strong selective pressure to restrict pre-hibernation fattening to a short-time period before the onset of hibernation. The edible dormouse (Glis glis) is characterized by having adapted its whole life history to the irregularly occurring mast-seeding pattern of the European beech (Fagus sylvaticus). Thus, the question arises how this small endotherm copes with huge differences in food availability between years. Therefore, we investigated body mass and thermal energetics of edible dormice during high and low food years. Our results demonstrate that during periods of low food availability, edible dormice enter an energy-saving mode with reduced body temperature (Tb ) and resting metabolic rate (RMR), and high torpor frequencies. During irregularly occurring short events of high food availability in mast years, however, Tb was higher, torpor did not occur, and RMR was drastically elevated possibly due to an enlarged digestive tract and the heat increment of feeding associated with a dietary switch to high-quality food and an increase in the amount of food ingested. This physiological flexibility allows edible dormice to efficiently accumulate body fat reserves under extremely different situations of food availability.