Sea otters spend most of their lives in the water—even in the winter. Although they have thick waterproof fur, they don’t have blubber to keep them warm like other marine mammals. Instead, their secret to staying warm is in their cells. Mitochondria are the energy-producing parts of a cell. The mitochondria in sea otters’ muscle cells leak out excess energy in the form of heat.

Their mitochondria could also explain sea otters’ extreme metabolism, which burns three times as much energy as other animals their size. “This could be a game changer in terms of how we think about the evolution of all marine mammals,” says Terrie Williams. She studies how an animal’s anatomy interacts with its environment at the University of California, Santa Cruz.