Variation in Metabolic Rates of a Lizard: Use of SMR in Ecological Contexts

Abstract

Standard metabolic rates (SMR) are frequently used to estimate the maintenance costs of free-ranging ectotherms. Ecologically relevant sources of variation in the rate of metabolism not accounted for by SMR (e.g. nutritional status, time of day) can significantly affect estimates of the maintenance costs of free-ranging ectotherms. This study reports the effects of temperature, sex, population and time of day on the rate of oxygen consumption of resting Sceloporus merriami. The rate of oxygen consumption of fasted lizards (standard conditions) was strongly dependent on temperature, sex and population, and it was not constant over the inactive period (scotophase). Average rates of oxygen consumption were as much as four times higher than the minimal rate (SMR) for all individuals. Our results indicate that the cost of maintenance of S. merriami calculated using SMR is from two to four times lower than the cost estimated from the average rates of oxygen consumption of fasted lizards. These results have important implications for ecologists interested in comparing lizard productivity with similarly sized endotherms. Rates of oxygen consumption at 32 and 370C increased by as much as 37% after lizards were fed 10-15% of their live mass in crickets. This effect was not observed at 250C. Because oxygen consumption is not constant over the scotophase and because free-ranging lizards usually have food in their guts, we conclude that when estimating the energy budgets of free-ranging lizards ecologists should use the rate of oxygen consumption averaged over the scotophase of fed animals, not SMR.