1. Patterns of torpor, arousal and activity in free-living Greater Horseshoe Bats, Rhinolophus ferrumequinum (Schreber), were investigated during the hibernation period by using temperature-sensitive radio-transmitters.
2. Torpor bouts varied between 0·1 and 11·8 days, with individual means ranging from 1·3 to 7·4 days. Torpor bout duration decreased with increasing ambient temperature.
3. Activity duration varied from 37 min to 54 h 24 min, with individual means ranging from 2 : 29 to 8 : 58 h. Activity duration increased with ambient temperatures above approximately 10 °C.
4. Ten of 11 bats synchronized their arousals with dusk. The circadian rhythm of one bat showed a free-running pattern over a period of about 5 weeks. Arousals were more highly synchronized, and closer to dusk, in individuals with lower body condition.
5. That bats forage in mild weather is supported by the strong synchronization of arousals with dusk, especially in bats with low body condition.
6. Patterns of torpor and subsequent activity are consistent with predictions that torpor lasts until a critical metabolic or water imbalance is achieved. Because metabolism and water loss are temperature-dependent, torpor bout duration decreases with increasing temperature. The imbalance is corrected during subsequent activity, which is relatively constant in duration until a temperature threshold of 10 °C, above which increasing levels of foraging lead to longer activity bouts.