Mammalian and avian torpor is highly effective in reducing energy expenditure. However, the extent of energy savings achieved and thus long-term survival appear to differ between species capable of multiday hibernation and species restricted to daily heterothermy, which could, however, be due to thermal effects. We tested how long-term survival on stored body fat (i.e. time to lean body mass), crucial for overcoming adverse periods, is related to the pattern of torpor expressed under different ambient temperatures (Ta: 7 °C typical of hibernation, 15 and 22 °C typical of daily torpor) in the small marsupial hibernator the pygmy-possum (Cercartetus nanus). Possums expressed torpor at all Tas and survived without food for 310 days on average at Ta 7 °C, 195 days at Ta 15 °C, and 127 days at Ta 22 °C. At Ta 7 and 15 °C, torpor bout duration (TBD) increased from < 1-3 to ~ 5-16 days over 2 months, whereas at Ta 22 °C, TBD remained at < 1 to ~ 2 days. At all Tas daily energy use was substantially lower and TBD and survival times of possums much longer (3-12 months) than in daily heterotherms (~ 10 days). Such pronounced differences in torpor patterns and survival times even under similar thermal conditions provide strong support for the concept that torpor in hibernators and daily heterotherms are physiologically distinct and have evolved for different ecological purposes.
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