Partial arousals from hibernation in hedgehogs in outdoor hibernacula.

Thermal properties of hibernacula and sequences of arousals have been studied in four adult hedgehogs for seven months starting in October. Departures and entries to the nesting chamber were continuously monitored together with ambient temperature and the temperature in the hibernacula.During the two first months of the experimental period nest departures were intermittently recorded, predominantly in the two females which also occasionally foraged. The longest periods spent continuously in the hibernaculum ranged from 129 to 178 days. The natural hibernation season for Danish hedgehogs was found to comprise the six months from October onwards when there is little shelter where hedgehogs normally roam.Ambient temperatures recorded were -11 to +13° C being subzero for half the total time measured. The nest temperatures generally were higher, and above 0° C during 78-99% of total time, most commonly ranging from 0° to 4° C and thus reflecting deep hibernation.Between December and May spontaneous increases in nest temperatures amounting to 7-26° C (average 18° C) and bringing these temperatures to 10-29.5° C were recorded in 58 cases. Fiftyfour arousals did not involve departure from the hibernaculum (partial arousals). In the remaining cases (full arousals) the preceding rewarming lasted 4 1/2-6 1/2 h and nest departures amounted to 10,2 and 5 min in one female hedgehog and 90 min in another.The hedgehogs showed 12-18 arousals, the mean duration of which was 34-44 h. The high energy expenditure associated with arousals however, was found to last on average 21 h during each arousal. It is hypothesized that the body temperature during arousals chiefly was below 35-37° C.The time between arousals was 3-15 days. Periods in hibernation averaged 7-8 days in the females and 9-10 days in the heavier males, being generally longest in January-February. Neither arousals nor re-entries into deep hibernation occurred at any particular time of the day. It is suggested that for undisturbed hedgehogs arousals are induced and controlled by endogenous factors. In conclusion it is stressed that future studies on hibernation should recognize the importance of individual variability in the response pattern and focus interest on the endogenous factors which govern this important process.