Dormice voluntarily enter torpor at ambient temperatures ranging between 0-28 degrees C. This study describes heart rate, ventilation frequency, O2-consumption (defined as metabolic rate), CO2-production and body temperature during entrance into torpor. Their temporal relationship was analysed during the time course of metabolic depression at different ambient temperatures. Body temperature and heart rate were measured in unrestrained dormice with implanted transmitter. Ventilation frequency was monitored by total body plethysmography or infrared video monitoring. To compare entries into torpor at different T(a) these periods were distinguished into four different phases: the resting phase prior to torpor, the phase of pre-torpor adjustments, the reduction phase and the phase of steady state torpor. In the pre-torpor phase, dormice increased their ventilation, metabolic rate and heart rate, indicating that the torpid state is initiated by an enhanced metabolic activity for about an hour. This was followed by a rapid reduction of ventilation, metabolism and heart rate, which reached their minimum values long before body temperature completed its decline. The results of the present study show that the entrance into torpor is caused by an active respiratory, cardiac and metabolic depression.
