Independent influence of thermoregulatory cost on the lower and upper set-points of a heliothermic lizard.

Studies on ectothermic vertebrates generally lead to average indicators of thermal preferences measured in the laboratory, which do not say about responses to natural environmental change and may not inform about individual variation and its triggering mechanisms. We studied whether and how changes in costs of thermoregulation influence the preferred temperature (Tp ) of individual lizards and their energetic investment in thermoregulation by exposing specimens to three treatments of increasing costs, recording body temperature (Tb ) and distance walked (energetic investment). Moderate costs induced an investment trade-off between energy-investment in thermoregulation and Tb , and highlighted individual variation that decreased with higher costs. Lower average Tp 's were observed in the high costs trials as a result of the decreased values of the lower and upper voluntary Tb 's. As costs increased, lizards walked a shorter overall distance and accepted lower Tb 's, but lizards still engaged in costly thermoregulation. Individual variation resulted in two main thermoregulatory patterns that in certain ecological contexts may have an important role in decision-making and adjusting to temperatures that are far from optimal temperatures for performance and physiological processes.