Timing of torpor bouts during hibernation in European hamsters (Cricetus cricetus L.).

Body temperature (Tb) of seven European hamsters maintained at constant ambient temperature (Ta = 8 degrees C) and constant photoperiod (LD 8:16) was recorded throughout the hibernating season using intraperitoneal temperature-sensitive HF transmitters. The animals spent about 30% of the hibernation season in hypothermia and 70% in inter-bout normothermy. Three types of hypothermia, namely deep hibernation bouts (DHBs), short hibernation bouts (SHBs), and short and shallow hibernation bouts (SSHBs), were distinguished by differences in bout duration and minimal body temperature (Tm). A gradual development of SSHBs from the diel minimum of Tb during normothermy could be seen in individual hamsters, suggesting a stepwise decrease of the homeostatic setpoint of Tb regulation during the early hibernation season. Entry into hibernation followed a 24-h rhythm occurring at preferred times of the day in all three types of hypothermia. DHBs and SHBs were initiated approximately 4 h before SSHBs, indicating a general difference in the physiological initiation of SSHBs on the one hand and DHBs and SHBs on the other. Arousals from SHBs and SSHBs also followed a 24-h rhythm, whereas spontaneous arousals from DHBs were widely scattered across day and night. Statistical analyses of bout length and the interval between arousals revealed evidence for a free-running circadian rhythm underlying the timing of arousals. The results clearly demonstrate that entries into hypothermia are linked to the light/dark-cycle. However, the role of the circadian system in the timing of arousals from DHBs remains unclear.