Lower but not upper thermal tolerance differs with opportunity for thermoregulation in a semi-fossorial lizard

Theory and data suggest that environmental pressures that can be alleviated behaviorally are less likely to elicit physiological responses than those that cannot. The warmest conditions experienced by most terrestrial organisms are caused by direct solar radiation and can be mitigated behaviorally by avoiding direct sunlight. However, the coldest conditions often occur when there is little thermal heterogeneity, reducing opportunities for behavioral mitigation. Therefore, we hypothesize that lower thermal tolerance, but not upper thermal tolerance, will differ in western skinks (Plestiodon skiltonianus) occupying habitats that differ in availability of warm and cold temperatures – southern v. eastern aspects. We placed operative models that recorded temperatures in the open, in shade, and under rocks on each aspect. We also captured lizards to assess thermal tolerance. Variance in environmental temperature increased with maximum temperature, which was higher on southern aspects, indicating greater opportunity for behavioral thermoregulation. Southern aspects also had more temperatures above the western skink's thermal set range and fewer temperatures below this range. The average environmental temperature was also below the thermal set range on both aspects, with temperatures being closer to this range on southern aspects. Despite differences in non-optimal warm and cold temperatures, only lower thermal tolerance differed with aspect: lizards from eastern aspects tolerated colder temperatures than those from southern aspects. These results suggest that behavioral thermoregulation can mitigate non-optimal warm temperatures but not cold temperatures. They also underscore the importance of maintaining habitat features responsible for thermal heterogeneity to help buffer species against increasing temperatures.