Adult males are about 5 feet long and weigh between 60 and 85 pounds. Their small size is a dubious distinction in an environment where sheer bulk is an adaptation against the heat-sapping properties of water. In addition, sea otters do not have the thick, insulating layer of blubber found in cetaceans and pinnipeds. They rely, instead, on their fur and a rapid metabolism to survive in the near-freezing waters of Alaska's seas. The sea otter's dark brown to black fur is the most dense of any marine mammal.
Air is better insulation than fat except for deep-diving animals.
The heat loss, however, is negligible in the relatively shallow 30-foot waters in which otters typically forage. In order for the fur to keep heat in and cold out, it must be clean and meticulously groomed. If the fur becomes matted or fouled with oil or other pollutants, cold seawater penetrates to the otter's skin, and the animal quickly dies of hypothermia. A sea otter spends many hours a day on its fur, squeezing water out, combing and fluffing it with its front claws and blowing air into it. Otters shed gradually and throughout the year so that they are never without this vital protection.