Effect of torpor on the water economy of an arid-zone marsupial, the stripe-faced dunnart (Sminthopsis macroura)

Metabolic rate and evaporative water loss ( EWL) were measured for a small, arid- zone marsupial, the stripe- faced dunnart ( Sminthopsis macroura), when normothermic and torpid. Metabolic rate increased linearly with decreasing ambient temperature ( T-a) for normothermic dunnarts, and calculated metabolic water production ( MWP) ranged from 0.85 +/- 0.05 ( T-a = 30 degrees C) to 3.13 +/- 0.22 mg H2O g(-1) h(-1) ( T-a = 11 degrees C). Torpor at T-a = 11 and 16 degrees C reduced MWP to 24 - 36% of normothermic values. EWL increased with decreasing T-a, and ranged from 1.81 +/- 0.37 ( T-a = 30 degrees C) to 5.26 +/- 0.86 mg H2O g(-1) h(-1) ( T-a = 11 degrees C). Torpor significantly reduced absolute EWL to 23.5 - 42.3% of normothermic values, resulting in absolute water savings of 50 - 55 mg H2O h(-1). The relative water economy ( EWL/ MWP) of the dunnarts was unfavourable, remaining > 1 at all T-a investigated, and did not improve with torpor. Thus torpor in stripe- faced dunnarts results in absolute, but not relative, water savings.