Energy Expenditure in Free‐Ranging Lizards

Metabolic rates of the western fence lizard Sceloporus occidentalis (Sauria: Iguanidae) were measured in the laboratory by gas exchange, and in the field with doubly labeled H 2 O. Field metabolic rates of adults during the spring reproductive season (0.22 ml CO 2 g — 1 h — 1 or 141.4j g — 1 day — 1 ) were not different from those during the fall nonreproductive season (0.20 ml CO 2 g — 1 h — 1 or 127.2j g — 1 day — 1 ). Field and laboratory metabolic rates of @V @V were not different from those of ♀ ♀ in spring or fall. Average daily field metabolic rates were 2.0 to 2.5 x resting laboratory rates measured on a simulated normal thermal cycle; estimated rates during the field activity period were 2.5 to 3.1 x resting levels measured at 35°C. Only 11 to 17% of total daily energy expenditure was used while lizards were inactive in their burrows. Daily field metabolism of this lizard is only 3 to 4% that of a bird or mammal of equal size.