Thermal Dependence of Locomotion and Aggression in a Xantusiid Lizard

The thermal dependence of sprint speed, exertion distance and time, and aggressive response was examined in the island night lizard, Xantusia riversiana, over a body temperature range of 10-35 C. Critical thermal limits were 6.6 and 39.0 C. Thermal optima for sprint speed and distance approximated preferred body temperature (28.3 C) and declined at higher or lower body temperatures. Exertion time was largely independent of body temperature, and lizards ran for about 70-90 s. Speed during exertion sprinting declined exponentially with increasing time and distance covered. Maximum oxygen consumption at 30 C$\dot{{\rm V}}_{{\rm o}_{2^{{\rm max}}}$occurred immediately following burst sprinting and was greater than values recorded for other species of lizards of similar body mass; factorial aerobic scope relative to standard$\dot{{\rm V}}_{{\rm o}_{2}}$was 29. Maximal aggressive responses occurred at low body temperatures which are suboptimal for sprinting, but aggressive behavior was always preceded by a sprint to exhaustion. Temperature and exercise state appear to be interacting factors influencing aggressive responses. Xantusia riversiana is a diurnal but reclusive lizard with relatively low preferred body temperature and low temperature optimum for sprinting. In the dense vegetative scrub and rocks that the lizards inhabit, sprint escape from predators is the initial response at all activity body temperatures. Sprint performance also represents capacity for struggle against restraint, and the increased aggressive response of exhausted lizards at low body temperatures may be a defensive response compensating diminished capacity for struggle at low body temperatures.