Effects of Body Mass, Feeding, and Circadian Cycles on Metabolism in the Lizard Sceloporus occidentalis

We examined aspects of pre- and postprandial metabolism in the diurnally active Western Fence Lizard, Sceloporus occidentalis, by measuring rates of oxygen consumption (Vo2) and carbon dioxide production (Vco2) at 30°C. Sceloporus occidentalis exhibited strong circadian variation in metabolism that continued throughout digestion, with diurnal peaks in metabolism up to four times as high as nocturnal minimum values (standard metabolic rate, SMR). Metabolism increased with increasing body size (mass range = 3.65–19.02 g), with mass exponents ranging from 0.61–0.82. Metabolism of lizards fed meals equivalent to 1.4, 2.9, and 3.9% of their body mass was elevated above fasting metabolism, although significant differences in metabolism were not detected among the three meal sizes. Maximum metabolism during digestion was from 1.2–1.3 times that of maximum fasting metabolism, a value similar to that of other small, frequently feeding lizards. Specific dynamic action (SDA) ranged from 2.38–22.02 mL O2 and 1.54–14.54 mL CO2, or 0.05 −0.44 kJ, which is equivalent to 9.4–17.0% of the ingested energy. Mean respiratory quotients (RQ) ranged from 0.65–0.68, indicating lipids were the primary energy substrate used during both fasting and digestion.