Three major forms of dormancy in mammals have been classified: hibernation in endotherms is characterised by reduced metabolic rate (MR) and body temperature (Tb) near ambient temperature (Ta) over prolonged times in the winter. Estivation is a similar form of dormancy in a dry and hot environment during summertime. Daily torpor is defined as reduced MR and Tb lower than 32 degrees C, limited to a duration of less than 24 h. The edible dormouse (Glis glis) is capable for all three distinct forms of dormancy. During periods of food restriction and/or low Ta, daily torpor is displayed throughout the year, alternating with hibernation and estivation in winter and summer respectively. We recorded Tb, O2-consumption and CO2-production in unrestrained dormice at different Ta's for periods of up to several months. Cooling rate and rate of metabolic depression during entrance into the torpid state was identical in all three forms of dormancy. The same was true for thermal conductance, maximum heat production, duration of arousal and cost of an arousal. The only difference between hibernation and daily torpor was found in the bout duration. A daily torpor bout lasted 3-21 h, a hibernation bout 39-768 h. As a consequence of prolonged duration, MR, Tb and also the Tb - Ta gradient decreased to lower values during hibernation bouts when compared to daily torpor bouts. Our findings suggest that all three forms of dormancy are based on the same physiological mechanism of thermal and metabolic regulation.
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