1. The extent to which free-ranging large north-temperate mammals seasonally adjust thermoregulation and their energy expenditure under fully natural conditions are unknown.
2. Therefore, using telemetry we measured the heart rate (as a proxy for metabolic rate), rumen temperature (Tr) and locomotor activity (LA) over 2 years for 20 free-ranging Alpine ibex (Capra ibex ibex) living at high altitudes in the Alps.
3. Ibex showed strong seasonal changes in mean daily heart rate with a winter nadir of about 60% below the summer peak. Only 40% of this variation could be attributed to the changes in daily mean Tr, LA, wind chill, body size and snowfall. The unexplained residual variation in heart rate still showed a strong seasonal pattern.
4. The amplitude of daily rhythms in Tr was twice as high during the winter when compared with summer. This was predominantly due to lower daily minimum Tr. Thus, the substantial down-regulation of endogenous heat production during winter – as indicated by heart rate – had surprisingly small effects on Tr, indicating decreased thermal conductance.
5. Rewarming from the daily Tr minimum during the morning hours was independent of heart rate throughout the year, and occurred phase-delayed to the increase in black bulb temperature (BBT). The effects of BBT and LA on the rate of rewarming were maximized within a small range of BBT around 0 °C. This suggests that the ibex moved at sunrise to the closest sunny spot to facilitate extensive basking.
6. The energetic benefits of basking can explain the strong residual seasonality of heart rate in Alpine ibex. This partially ectothermic strategy – together with metabolic depression – apparently enables a thrifty use of body fat reserves, the major metabolic fuel during winter, and thus survival of extremely harsh winter conditions despite the virtual absence of food. Therefore, hypometabolism and passive rewarming by basking may be of general importance as a strategy for non-hibernating mammals to survive winter in strongly seasonal habitats.