Plasticity and regional heterothermy of upper thermal tolerance in the ring-necked snake

Phenotypic plasticity of upper and lower thermal tolerance during parts of the annual cycle with high variation and unpredictability should theoretically be important for temperate ectotherms. However, whether thermal tolerance is plastic is unclear for most ectotherms. In addition, many ectotherms have regional heterothermy (variation in body temperature among body regions), and yet how regional heterothermy might impact plasticity of thermal tolerance is unknown. We studied the plasticity of upper thermal tolerance in response to warm temperatures in a population of the temperately distributed ring-necked snake (Diadophis punctatus) that exhibits regional heterothermy whereby head temperatures are substantially warmer than cloaca temperatures. We measured voluntary thermal maximum (VT_max, a measure of upper thermal tolerance) of ring-necked snakes, incubated ring-necked snakes at two temperatures (30 °C or 34 °C) at two durations (1 h or 3 h), and remeasured VT_max. We found that ring-necked snakes exhibited plasticity of VT_max even for short durations at relatively mild but warmer temperatures. We also found that the VT_max measured in the mouth was substantially higher than that measured in the cloaca, and that mouth VT_max was less plastic than cloacal VT_max. Together, these results suggest that at least some temperate ectotherms can exhibit plasticity of upper thermal tolerance in response to warm temperatures, and that interpretations of plasticity of VT_max depend on where temperature is measured in species with regional heterothermy. These findings have implications for understanding how variation in plasticity of thermal tolerance among body regions might impact the biological response to rapid environmental change.