Field metabolic rates (FMRs) of 61 species of mammals, as measured with doubly labelled water, range from 29 kJ day-1 (0.34 W) in pipistrel bats to 49 MJ day-1 (570 W) in northern elephant seals, which is a range of 1678 times. Most of this variation is due to differences in body mass; the least-squares, log-log regression of mammalian FMR on body mass (kJ day-1 = 5.27 g0.723) has a coefficient of determination (r2) of 0.961, indicating that variation in log(body mass) accounts for 96% of variation in log(FMR). The scaling of FMR in marsupials (kJ day-1 = 10.83 g0.582, 17 species) differs significantly from that in eutherian mammals (kJ day-1 = 4.63 g0.762, 44 species), and the respective r2-values (0.978 and 0.972) indicate that these taxonomic infraclasses explain another 1% of variation in log(FMR). After adjusting for mass and infraclass effects, residual variation is still substantial (2.5-fold among marsupials and 6-fold among eutherians). What accounts for this variation? Neither taxonomic order (or family within the Marsupialia), diet category (e.g. herbivore, camivore), nor habitat (e.g. marine, tundra) explained much residual variation, except that desert-dwelling eutherians had significantly lower FMRs than expected for eutherians of their mass. The failure of taxonomic and ecological categories to account for residual variation may be due, in part, to small sample sizes and skewed distributions of these categories along the mass axis, but it seems likely that other sources of variation, such as season, sex, age, ambient temperature, daily behaviour pattern and food availability may have large effects on FMR that are not accounted for in this analysis.
