Seasonal and daily rhythms of body temperature in the European hamster (Cricetus cricetus) under semi-natural conditions.

Body temperature of five European hamsters exposed to semi-natural environmental conditions at 47 degrees N in Southern Germany was recorded over a 1.5-year period using intraperitoneal temperature-sensitive radio transmitters. The animals showed pronounced seasonal changes in body weight and reproductive status. Euthermic body temperature changed significantly throughout the year reaching its maximum of 37.9 +/- 0.2 degrees C in April and its minimum of 36.1 +/- 0.4 degrees C in December. Between November and March the hamsters showed regular bouts of hibernation and a few bouts of shallow torpor. During hibernation body temperature correlated with ambient temperature. Monthly means of body temperature during hibernation were highest in November (7.9 +/- 0.8 degrees C) and March (8.2 +/- 0.5 degrees C) and lowest in January (4.4 +/- 0.7 degrees C). Using periodogram analysis methods, a clear diurnal rhythm of euthermic body temperature could be detected between March and August, whereas no such rhythm could be found during fall and winter. During hibernation bouts, no circadian rhythmicity was evident for body temperature apart from body temperature following ambient temperature with a time lag of 3-5 h. On average, hibernation bouts lasted 104.2 +/- 23.8 h with body temperature falling to 6.0 +/- 1.7 degrees C. When entering hibernation the animals cooled at a rate of -0.8 +/- 0.2 degrees C.h-1; when arousing from hibernation they warmed at a rate of 9.9 +/- 2.4 degrees C.h-1. Warming rates were significantly lower in November and December than in January and February, and correlated with ambient temperature (r = -0.46, P < 0.01) and hibernating body temperature (r = -0.47, P < 0.01). Entry into hibernation occurred mostly in the middle of the night (mean time of day 0148 hours +/- 3.4 h), while spontaneous arousals were widely scattered across day and night. For all animals regression analysis revealed free-running circadian rhythms for the timing of arousal. These results suggest that entry into hibernation is either induced by environmental effects or by a circadian clock with a period of 24 h, whereas arousal from hibernation is controlled by an endogenous rhythm with a period different from 24 h.