Temperature tolerances of Southeast Australian reptiles examined in relation to reptile thermoregulatory behaviour and distribution

Abstract

The Critical temperatures (defined as the upper and lower limits at which the animal can still right itself) were determined for 29 lizard species (13 genera) and four snake species (four genera) of Southeast Australia. In addition to these Critical temperatures, acclimation of the Critical temperatures and also some lethal temperatures were recorded for several of these species. The mean summer Critical Minimum values ranged between 2.2 and 9.8°C. Thigmotherms (Gekkonidae, Lygosominae, Elapidae) and posturing heliotherms (Agamidae, Scincinae, Varanidae) generally had high Critical Minimum values and the shuttling heliotherms (Lygosominae, Elapidae) had the lowest values. The rate of acclimation of the Critical Minimum of ten lizard species was similar and complete acclimation took place within ten days. Following acclimation the final or ultimate Critical Minimum of some species fell below the body freezing point of-0.52°C and locomotion could occur while the lizard was supercooled. Mean summer Critical Maximum values ranged between 37.0 and 44.8°C. Determination of the Lethal Minimum temperatures by continuously cooling the specimens was complicated by the fact that supercooling occurred followed by nucleation or sudden freezing of the body tissues. The Lethal Minimum was best determined by holding specimens at different constant temperature levels then calculating the body temperature at which the species could survive for an indefinite time period. It is concluded, from the data presented in this paper, that if environmental temperatures were to limit the distribution of reptiles then the Critical Minimum level would have more ecological significance than the Critical Maximum.