Camp. Biochem. Physiol. Vol. I zyxwvutsrqponmlkjihgfedc IZA, No. I, pp. 59-66, 1995 C o p yrig ht 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0300-9629195 $9.50 + 0.00 zyxwvutsrqpo Pergamon 0300-%29(95)00089-5 Influence of torpor on daily energy expenditure of the dasyurid marsupial S’wzinthopsis zyxwvutsrqponmlkjihgfedcbaZYXWV crassicaudata J. C. Holloway and F. Geiser Department of Zoology, University of New England, Armidale NSW 2351, Australia Daily torpor is an energy-conserving mechanism, used by many small marsupials to reduce energy expenditure during adverse environmental conditions. Since little is known about how much energy is actually saved by torpor in marsupials and how this is related to the duration of torpor bouts, we investigated the effect of ambient temperature zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFE (T.) on different metabolic states and the effect of torpor bout duration in Smintiropsis crassicaudata (16 g) on average daily metabolic rate (ADMR). T, had a significant effect on the active and resting metabolic rates (MR), both being higher at T, 12OC than T, 18OC (P < 0.001, t-test). In contrast, the ADMR at T, 12’C and T, 18OC did not differ significantly, and it appears that the increased cost for thermoregulation in normothermic active and resting animals at T,, 12OC was compensated by a decrease in MR during torpor. Torpor bout duration was negatively correlated with ADMR. Torpor bouts of 5 hr reduced ADMR by 12-16%. Torpor bouts of more than 10 hr reduced ADMRs by 3030%. Our study shows that torpor can significantly reduce daily energy expenditure in S. crassicaudatu, and this may be important for the survival of this species in the wild. Key words: Average daily metabolic rate; Daily torpor; Energy savings; Marsupial; Sminthopsis; Torpor bout. Comp. Biochem. Physiol. lIZA, 5946, 1995. Introduction Torpor is an energy-conserving mechanism used by small mammals to reduce energy expenditure during periods of cold exposure and food short- age. For hibernators which undergo deep, pro- longed torpor, bouts last for days or weeks, body temperature (T,,) may approach O”C, and the minimum metabolic rate (MR) during tor- por may be as low as 1% of that of normother- mic resting individuals at the same ambient temperature (Ta) (Geiser, 1988). In those species that use daily torpor, bouts last for several hours, T, is generally regulated between 10°C and 25°C and minimum MRs are higher than in hibernating animals, usually ranging between 18 and 3 1% of the value in normothermic resting individuals (Tannenbaum and Pivorun, 1984; Wang, 1989). zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Correspondence to: J. C. Holloway, Dept of Zoology, University of New England, Armidale NSW 2351, Aus- tralia. Tel. 067 73 2885/067 73 2868; Fax 067 711869. Received 2 1 January 1995; revised 24 March 1995; accepted 30 March 1995. These general patterns of torpor appear to be similar between marsupial and placental mam- mals. Within the marsupials, deep, prolonged torpor has been observed in the pygmy possums (Burramyidae) and feathertail glider (Acrobati- dae), and shallow daily torpor in the small possums (Petauridae) and insectivorous/ carnivorous marsupials (Dasyuridae) (Geiser, 1994). While the extent of Tb and MR reduction, and torpor bout duration in small marsupials are known, little is known about by how much the daily energy expenditure can be reduced by the use of torpor and how this is related to the duration of torpor bouts. Although torpor can reduce MR substantially, daily energy savings during torpor are decreased by the energy costs of endothermic arousals (Ellison and Skinner, 1992). Moreover, MRs during torpor do not always remain at a constant minimum, but may fluctuate (Geiser et al., 1986). To obtain a better understanding of the effect of torpor on daily energy expenditure, we inves- 59