THE EFFECT OF HE-O-2 EXPOSURE ON METABOLIC-RATE, THERMOREGULATION ANDTHERMAL CONDUCTANCE DURING NORMOTHERMIA AND DAILY TORPOR

Recently it was proposed that the low metabolic rate during torpor may be better explained by the reduction of thermal conductance than the drop of body temperature or metabolic inhibition. We tested this hypot hesis by simultaneously measuring body temperature and metabolic rate as a function of ambient temperature in both torpid and normothermic s tripe-faced dunnarts, Sminthopsis macroura (Marsupialia; approx. 25 g body mass), exposed to either air or He-O-2 (21% oxygen in helium) atm ospheres. He-O-2 exposure increases the thermal conductance of homeoth ermic mammals by about twofold in comparison to an air atmosphere with out apparent side-effects. Normothermic S. macroura exposed to He-O-2 increased resting metabolic rate by about twofold in comparison to tha t in air because of the twofold increase in apparent thermal conductan ce. Torpid S. macroura exposed to He-O-2 at ambient temperatures above the set-point for body temperature showed a completely different meta bolic response. In contrast to normothermic individuals, torpid indivi duals significantly decreased or maintained a similar metabolic rate a s those in air although the apparent thermal conductance in He-O-2 was slightly raised. Moreover, the metabolic rate during torpor was only a fraction of that of normothermic individuals although the apparent t hermal conductance differed only marginally between normothermia and t orpor. Our study shows that a low thermal conductance is not the reaso n for the low metabolic rates during torpor. It suggests that interrel ations between metabolic rate and body temperature of torpid endotherm s above the set-point for body temperature differ fundamentally from t hose of normothermic and homeothermic endotherms.