Prolonged torpor use during winter by a free-ranging bat in subtropical Australia

Prolonged torpor or hibernation is commonly employed by bats found in temperate regions. In contrast, data on the use of prolonged torpor in bats from tropical or subtropical regions is entirely lacking. We used radio-telemetry to measure skin temperatures (Tskin) of free-raning Nyctophilus bifax (~11g) during winter in a coastal subtropical habitat in north-eastern New South Wales. All bats employed torpor bouts that lasted for >48 hours on at least one occasion during the stud period. The minimum Tskin we recorked was 10.8°C. Torpor patterns were affected by ambient temperature (Ta); during cooler periods bats remained torpid throughout the whole day, whereas on warmer days they aroused near sunset, likely to forage for several hours before becoming torpid again. The Tskin of torpid bats was influenced by their choice of tree roost, such that their daily Tskin fluctuations were greater in exposed roosts compared to more insulated ones. We provide the first evidence for the use of prolonged torpor in a free-ranging bat that is restricted to tropical and subtropical habitats. This illustrates the energetic constraints facing small endotherms even in subtropical climates and their flexibility to vary the duration of torpor and activity to suit prevailing conditions.