Opportunistic hibernation by a free-ranging marsupial

Knowledge about the thermal biology of heterothermic marsupials in their native habitats is scarce. We aimed to examine torpor patterns in the free ranging western pygmy-possum ('Cercartetus concinnus'), a small marsupial found in cool temperate and semi-arid habitat in southern Australia and known to express aseasonal hibernation in captivity. Temperature telemetry revealed that during two consecutive winters four out of seven animals in a habitat with Mediterranean climate used both short (<24 h in duration) and prolonged (>24 h) torpor bouts (duration 6.4±5.4 h and 89.7±45.9 h, respectively). Torpor patterns were highly flexible among individuals, but low ambient temperatures facilitated torpor. Maximum torpor bout duration was 186.0 h and the minimum body temperature measured was 4.1°C. Individuals using short bouts entered torpor before sunrise at the end of the active phase, whereas those using prolonged torpor entered in the early evening after sunset. Rewarming from torpor usually occurred shortly after midday, when daily ambient temperature increased. We present the first quantitative data on a marsupial species expressing opportunistic hibernation during winter in the wild, and show that torpor use in 'C. concinnus' is strongly influenced by small-scale microclimatic conditions.