Internal warmth through skeletal muscle thermogenesis enables sea otters - the smallest of marine mammals - to survive in their cold, marine habitats, researchers report. The study provides insight into how the tiny creatures maintain a normal mammalian body temperature despite living their lives submerged in the chilly waters of the North Pacific Ocean. Many marine animals that live in cold water environments are large and are often wrapped in a thick insulating layer of subcutaneous blubber to maintain a normal core body temperature. On the other hand, sea otters lack both size and blubber and while their dense fur coat offers some insulation, it alone cannot offset heat loss into the surrounding cold waters. To account for this, sea otters have a basal metabolic rate (BMR) approximately three times greater than is predicted for similar mammals their size. The tissue-level source of this hypermetabolism, however, is unknown. Because skeletal muscle is a major determinant of whole-body metabolism, Traver Wright and colleagues characterized respiratory capacity and thermogenic leak in sea otter muscle from captive-raised, stranded, and wild populations of northern and southern sea otters. Wright et al. found that these otters are effectively internally warmed by way of thermogenic mitochondrial leak from skeletal muscle, and that this process could account for their observed hypermetabolism. What's more, the authors show that this muscle thermogenic capacity develops in neonates and reaches adult levels before mechanical function of the muscles matures, providing internal warmth from birth.