Thermal sensitivity of sprint-running in the lizard Sceloporus undulatus: support for a conservative view of thermal physiology.

The thermal sensitivity of sprint-running ability was investigated in two populations of Sceloporus undulatus that occupy thermally distinct habitats. Integration of field and laboratory data indicates that lizards inhabiting a cool, high-elevation habitat are frequently active at body temperatures that retard sprint-running velocity, which could affect adversely their ability to evade predators and to capture prey. These negative effects might be expected to select for local adaptation of thermal physiology. No differences in thermal physiology (optimal temperature for sprinting, critical thermal limits) were found, however, between lizards from the two habitats.Preferred body temperature of Sceloporus undulatus is lower than the body temperature that maximizes sprint velocity but is still well within an 'optimal performance range' where lizards can run at better than 95% of maximum velocity. Analysis of data from other studies shows a similar concordance of preferred body temperature and temperatures that maximize sprint velocity for some, but not all lizard species studied.Low diversity of predators and high levels of food may compensate in part for the reduced sprinting ability of highelevation lizards active at low body temperatures. The lack of population differentiation supports the view that lizard thermal physiology is evolutionarily conservative.