Effects of helium/oxygen and temperature on aerobic metabolism in the marsupial sugar glider, Petaurus breviceps

Helox (79% helium and 21% oxygen) has often been used for thermobiological studies, primarily because helium is thought to be metabolically inert and to produce no adverse effects other than increasing heat loss. However, the se assumptions have been questioned. As basal metabolic rate (BMR) represen ts maintenance energy requirements for vital body functions, potential phys iological effects of helox should be reflected in changes of BMR. In this s tudy, sugar gliders were subjected to both air and helox atmospheres over a wide range of T-a 's, including the thermoneutral zone (TNZ), to determine (1) whether helox has any influence other than on heat loss and (2) the ma ximum heat production (HPmax) and thermal limits of this species. Although thermal conductance in the TNZ increased in helox, BMR was similar in air a nd helox (0.55 +/- 0.07 and 0.57 +/- 0.06 mL g(-1) h(-1), respectively). Th e TNZ in helox, however, was shifted upwards by about 3 degreesC. Below the TNZ, sugar gliders were able to withstand an effective temperature of -24. 7 +/- 7.3 degreesC with an HPmax of 3.14 +/- 0.36 mL g(-1) h(-1). The low e ffective temperature tolerated by sugar gliders shows that they are compete nt thermoregulators despite their apparent lack of functional brown fat. Si milarities of BMRs in air and helox suggest that the effect of helox is res tricted to an increase of heat loss, and, consequently, helox represents a useful tool for thermal physiologists. Moreover, the lack of increase of BM R in helox despite an increase in thermal conductance of sugar gliders sugg ests that BMR is not a function of body surface.