The energetic cost of arousal from torpor in the marsupial Sminthopsis macroura: benefits of summer ambient temperature cycles

The costs of arousal from induced torpor were measured in the striped-faced dunnart (Sminthopsis macroura; ca. 25 g) under two experimental ambient te mperature cycles. The sinusoidal-type temperature cycles were designed to e valuate the effects of passive, ambient temperature heating during arousal from torpor in these insectivorous marsupials. It was hypothesised that die l ambient temperature cycles may offer significant energy savings during ar ousal in animals that employ daily torpor in summer as a response to unpred ictable food availability. The cost of arousal in animals in which passive, exogenous heating occurred was significantly lower than that in animals no t exposed to an ambient temperature cycle. The total cost of all three phas es of torpor (entry, maintenance and arousal) was almost halved when animal s were exposed to an ambient heating cycle from 15 degrees C to 25 degrees C over a 24-h period. In all animals, irrespective of the experimental ambi ent temperature cycle employed, the minimum torpor body temperature was 17- 18 degrees C. The body temperature (T-b) of animals exposed to exogenous he ating increased from the torpor T-b minimum to a mean value of 22.59 degree s C before endogenous heat production commenced. This relatively small incr ease in T-b of Ca. 5 degrees C through 'free' passive heating was sufficien t to account for the significant ca. three-fold decrease in the cost of aro usal and may represent an important energetic aid to free-ranging animals.