Abstract
Metabolism underpins all life‐sustaining processes and varies profoundly with body size, temperature and locomotor activity. A current theory explains some of the size‐dependence of metabolic rate (its mass exponent, b ) through changes in metabolic level ( L ). We propose two predictive advances that: (a) combine the above theory with the evolved avoidance of oxygen limitation in water‐breathers experiencing warming, and (b) quantify the overall magnitude of combined temperatures and degrees of locomotion on metabolic scaling across air‐ and water‐breathers. We use intraspecific metabolic scaling responses to temperature (523 regressions) and activity (281 regressions) in diverse ectothermic vertebrates (fish, reptiles and amphibians) to show that b decreases with temperature‐increased L in water‐breathers, supporting surface area‐related avoidance of oxygen limitation, whereas b increases with activity‐increased L in air‐breathers, following volume‐related influences. This new theoretical integration quantitatively incorporates different influences (warming, locomotion) and respiration modes (aquatic, terrestrial) on animal energetics.