Field Metabolic Rate and Food Requirement Scaling in Mammals and Birds

Field metabolic rates (FMRs or H F ), all measured using doubly labeled water, of 23 species of eutherian mammals, 13 species of marsupial mammals, and 25 species of birds were summarized and analyzed allometrically (log 1 0 —log 1 0 regressions). FMR is strongly correlated with body mass in each of these groups. FMR scales differently than does basal or standard metabolic rate in eutherians (FMR slope = 0.81) and marsupials (FMR slope = 0.58), but not in birds (FMR slope — 0.64 overall, but 0.75 in passerines and 0.75 in all other birds). Medium—sized (240—550 g) eutherians, marsupials, and birds have similar FMRs, and these are ≈17 times as high as FMRs of like—sized ectothermic vertebrates such as iguanid lizards. For endothermic vertebrates, the energy cost of surviving in nature is enormous compared with that for ectotherms. Within the eutherians, marsupials, or birds, FMR scales differently for the following subgroups: rodents, passerine birds, herbivorous eutherians, herbivorous marsupials, desert eutherians, desert birds, and seabirds. Equations are given for use in predicting daily and annual FMR and food requirement of a species of terrestrial vertebrate, given its body mass.