AbstractWhile basal metabolic rate (BMR) scales proportionally with body mass (Mb), it remains unclear whether the relationship differs between mammals from aquatic and terrestrial habitats. We hypothesized that differences in BMR allometry would be reflected in similar differences in scaling of O2delivery pathways through the cardiorespiratory system. We performed a comparative analysis of BMR across 63 mammalian species (20 aquatic, 43 terrestrial) with aMbrange from 10 kg to 5318 kg. Our results revealed elevated BMRs in small (>10 kg and <100 kg) aquatic mammals compared to small terrestrial mammals. The results demonstrated that minute ventilation, that is, tidal volume (VT)·breathing frequency (fR), as well as cardiac output, that is, stroke volume·heart rate, do not differ between the two habitats. We found that the “aquatic breathing strategy”, characterized by higherVTand lowerfRresulting in a more effective gas exchange, and by elevated blood hemoglobin concentrations resulting in a higher volume of O2for the same volume of blood, supported elevated metabolic requirements in aquatic mammals. The results from this study provide a possible explanation of how differences in gas exchange may serve energy demands in aquatic versus terrestrial mammals.