How do sea otters produce extra body heat without shivering?

Sea otters produce extra body heat without shivering through a process known as "thermogenic leak".1

Unlike shivering, which generates warmth through rapid muscle contractions, thermogenic leak occurs within the mitochondria of muscle cells. In this process, the mitochondria release energy as heat rather than producing ATP for muscle movement. This internal heat production is exceptionally high in sea otters—reaching rates about three times higher than those of comparable land mammals—and is present even in infants.1

Because this constant metabolic heat production is so energy-intensive, sea otters must consume approximately 25 percent of their body weight in food daily to maintain their energy balance.1