Morphological and Physiological Adaptations Support a 2.9‐Fold Higher Mass‐Specific Resting Metabolic Rate in Sea Otters Compared to Terrestrial Mammals

ABSTRACT Sea otters ( Enhydra lutris ) exhibit morphological and physiological adaptations supporting a mass‐specific resting metabolic rate (RMR) 2.9‐fold higher than predicted for terrestrial mammals of similar size. Their elevated metabolism results primarily from thermogenic mitochondrial proton leak to compensate for heat loss in the marine environment because of their high surface‐area‐to‐volume ratio, combined with their reliance on fur insulation rather than blubber. Key respiratory adaptations include an enlarged lung volume, 3.5‐fold greater than predicted allometrically, and a proportionately elevated tidal volume, enabling efficient oxygen uptake and carbon dioxide elimination. Complementary cardiovascular adaptations include a heart mass 1.3‐fold greater than the allometric prediction, a correspondingly elevated stroke volume, normal heart rate, increased hemoglobin concentration, and enhanced arterial oxygen content. Together, these adaptations result in an elevated cardiac output and enhanced convective oxygen transport to support the elevated RMR. These multi‐level adaptations, present even in young animals, likely evolved under selective pressures favoring thermogenesis over increased insulation by blubber, representing a convergent evolutionary strategy consistent with the principle of symmorphosis, similar to terrestrial athletic mammals adapted for sustained aerobic performance rather than energy conservation. Additional benefits of enlarged lungs, such as enhanced buoyancy and pulmonary oxygen stores, may also have contributed to their evolution.