Effect of a helox atmosphere on the thermal, metabolic and hygric physiology of the scincid lizard Egernia kingii.

Helox is a mix of 21% oxygen in helium, used to manipulate the density, thermal and diffusive environment of plants and endothermic vertebrates, but its physiological effects for ectotherms is relatively unknown. We examine how helox affects the physiology of King’s skink (Egernia kingii) when thermoconforming and thermoregulating. Helox had no effect on the body temperature of skinks; basking skinks compensated for higher heat loss in helox by shuttling less to the cool end of the thermal gradient. Higher air temperature and basking had the expected effects on metabolic rate (Q10=2.5–2.7), and although helox did not influence oxygen consumption, the respiratory exchange ratio was lower in helox, presumably reflecting a change in metabolic substrate. Evaporative water loss of skinks was impacted by temperature, but not helox, reflecting the relatively small contribution of the diffusive boundary layer to total evaporative resistance. Agar models of skinks however, had a considerably higher evaporative water loss in helox (helox: air ratio = 1.83), reflecting the importance of the boundary layer for diffusion when the surface resistance is low. Cutaneous evaporative water loss measured directly with a Tewameter probe was significantly lower for dorsal than ventral skin, but there was no difference in cutaneous evaporative water loss for lizards that had been in helox compared to air. Ectothermic lizards are a useful model for investigating the effects of helox on the thermal, metabolic and hygric physiology without the complication of thermal and insulative effects observed for endothermic birds and mammals. However, exposing lizards to a helox atmosphere is not a useful methodology for assessing the potential for physiological regulation of evaporative water loss for lizards due to the insignificance of the diffusive boundary layer to resistance.