SCALING OF FIELD METABOLIC-RATE TO BASAL METABOLIC-RATE RATIO IN HOMEOTHERMS

Linear regression analyses were used to describe the allometric relationships of basal metabolic rate (BMR), field metabolic rate (FMR) and FMR:BMR ratio on body mass in non-reproducing individuals of bird, marsupial and eutherian mammal species. Measurements of FMR were made by the double labelled water method and of BMR by indirect calorimetry using O-2 consumption. All regressions were significant, including FMR:BMR ratio on body mass. In addition, the slope of BMR on body mass was significantly higher than that of FMR on body mass for each group of animals. This shows that the FMR:BMR ratio is not constant within each group and is greater for smaller than for larger animals, We propose that this difference is due mainly to thermoregulatory costs. The use of a constant FMR:BMR ratio to estimate FMR from BMR, as is commonly done, could result in erronoeus results. However, the ratio found in the allometric relationships between FMR:BMR ratio and body mass for the three groups of homeotherms could be indicative of trends in estimating FMR from BMR.