One of the most pervasive results of studies analyzing ecological variation of metabolic rates in endotherms is that ambient temperature is negatively correlated with mass-independent basal metabolic rate (residual BMR). However, to date, no study has evaluated if the strength of this correlation change with the size of the sampled species. Here we used published data on body mass (mb), BMR, and annual mean temperature (Tmean) for 465 mammal species (and/or subspecies) to analyse this point. Both conventional and phylogenetic analyses indicate that correlation coefficients between residual BMR and Tmean increased with body mass, from values close to -0.5 at body sizes <100 g to values close to 0 at body size >1,000 g. Consequently, the current vision about the relevance of ambient temperature shaping the evolution of metabolic rates in endotherms probably is affected by the large number of small species in both nature and physiological data sets. Although this result does not deny the relevance of ambient temperature for most of the extent endothermic species, it claim for a pluralistic approach to explain the evolution of residual BMR. For instance, it could be possible that ambient temperature is the best predictor of residual BMR at the left side of the body size distribution curve, but that factors related with food availability (e.g. habitat productivity) could be the best predictors at the right side of this curve. Further studies aimed to collect more data on metabolic rates are welcome if we wish to achieve a more comprehensive understanding on animal energetics.