Thermal dependence of endurance and locomotory energetics in a lizard.

The thermal dependencies of endurance and the rates of oxygen cnsumption (VO2) and carbon dioxide production (VCO2) were determined for the lizard Dipsosaurus dorsalis walking on a treadmill. The thermal dependencies of endurance and maximal VO2 (VO2 max) are nerly identical. The maximal sustainable speed and the speed at which VO2 max is attained at each experimental body temperature are the same. The net cost of locomotion (the slope of VO2 vs. speed) is termperature independent. Resting VO2 and the incremental change in VO2 required to support locomotion at a sustainable speed are higher at 40 degrees C than at 25 degrees C. Therefore, the total cost of locomotion (VO2 divided by speed) is greater at 40 degrees C. However, the minimum cost of transport, attained at the maximal sustainable speed, is temperature independent. The results clearly indicate that maintenance of a high body temperature is energetically expensive for Dipsosaurus, but that the associated expanded activity capacity enables this lizard to avoid an obligate increase in the energetic requirements of locomotion.