Daily and seasonal cycles of body temperature and aspects of heterothermy in the hedgehog Erinaceus europaeus

Intra-abdominal temperature-sensitive radio transmitters were used to collect more than 350 sets of body temperature (T_b) data from 23 captive adult hedgehogs over a 3-year period. Each data set comprised measurements made every 1/2 h for 24-h periods. Between 20 and 60 such data sets were recorded every calendar month, and a total of 17400 measurements of T_bwere collected. The hedgehogs were exposed to natural environmental conditions at 57°N in NE Scotland. Hedgehogs showed seasonal changes in mean daily euthermic T_b,with a July maximum of 35.9±0.2°C, a September minimum of 34.7±0.9°C, and a marked circadian T_bcycle that correlates closely with photoperiod. Maximal T_boccurred within 2 h of midnight and this pattern of nocturnal maximum and diurnal minimum T_bwas most marked between April and September. The circadian T_bcycle was least correlated with photoperiod during winter. Hibernal T_bduring winter correlated with ambient temperature (T_a),it was maximal in September (17.7±1.0°C) and minimal in December (5.2±0.9°C). Apart from the tracking of T_aand T_bduring hibernal bouts, with a time-lag of 4-6 h, circadian rhythmicity of hibernal T_bwas not evident. However, the T_bof hibernating hedgehogs rose significantly when T_afell below - 5°C, although the animals did not neccessarily arouse. Although hibernal bouts occurred between September and April, 89.5% of such bouts were recorded between November and February. The mean time of entry into hibernation was 01:45±5.1 h GMT while the mean time of the start of spontaneous arousal from hibernation was 11:53±4.8 h GMT. Therefore, during hibernation hedgehogs were either fully aroused at night, when euthermic hedgehogs have maximal T_b,or in deep hibernation around midday, when euthermic hedgehogs have minimal T_b.Since wild hedgehogs will feed during spontaneous arousal from hibernation, these timings are probably adaptive, and suggest that entry into, and arousal from, hibernation may be extensions of circadian cyclicity. Spontaneous bouts of transient shallow torpor (TST) were recorded throughout the year, with nearly 80% of observations occurring during August and September, at the start of the hibernal period. TST bouts lasted for 4.9±2.9 h, with T_bfalling to 25.8±3.1 °C. Only 20% of TST bouts immediately preceded hibernation and their duration did not correlate with T_aor body mass. TST bouts started at 06:51±4.7 h GMT, significantly later than entry into hibernation, and ended at 13:04±5.4 h GMT. The function of TST bouts is unclear, but they may be preparation for the hibernation season or a further energy conservation strategy. When arousing from hibernation hedgehogs warmed at a rate of 1.9±0.4°C·h^-1, and when entering hibernation cooled at 7.9±1.9°C·h^-1. Warming rates were slightly higher during mid-winter when T_band body mass were minimal, but cooling rates were 44% higher at the end of the hibernal period compared to the start. Cooling and warming rates were strikingly similar to those measured in hedgehogs at 31°N. These results demonstrate that thermoregulation in the hedgehog is closely regulated and changes on a seasonal basis, in meeting with requirements of surviving food shortages and low temperature during winter.