A cold-adapted reptile becomes a more effective thermoregulator in a thermally challenging environment

Thermoregulation is of great importance for the survival and fitness of ectotherms as physiological functions are optimized within a narrow range of body temperature (Tb). The precision with which reptiles thermoregulate has been proposed to be related to the thermal quality of their environments. Although a number of studies have looked at the effect of thermal constraints imposed by diel, seasonal and altitudinal variation on thermoregulatory strategies, few have addressed this question in a laboratory setting. We conducted a laboratory experiment to test whether tuatara, Sphenodon punctatus (order Rhynchocephalia), a cold-adapted reptile endemic to New Zealand, modify their thermoregulatory behaviour in response to different thermal environments. We provided tuatara with three thermal treatments: high-quality habitat [preferred Tb (Tsel) could be reached for 8 h/day], medium-quality habitat (Tsel available for 5 h/day) and low-quality habitat (Tsel available for 3 h/day). All groups maintained body mass, but tuatara in the low-quality habitat thermoregulated more accurately and tended to maintain higher Tbs than tuatara in the high-quality habitat. This study thus provides experimental evidence that reptiles are capable of adjusting their thermoregulatory behaviour in response to different thermal constraints. This result also has implications for the conservation of tuatara. A proposed translocation from their current habitat to a higher latitudinal range within New Zealand (similar to the shift from our 8 h/day to our 5 h/day regime) is unlikely to induce thermoconformity; rather, tuatara will probably engage in more effective thermoregulatory behaviour.