Northern sea otters (Enhydra lutris kenyoni) are common in the waters surrounding Kodiak Island. Our summer guests love watching and photographing these cute, curious animals as we pass by them in our boat. Some otters lie placidly on their backs and watch us motor past, and others dive from the perceived danger we create. A few otters repeatedly twist and turn in the water or turn summersaults, and we laugh at their comical antics. The truth is, though, sea otters don’t turn summersaults because they are having fun; these movements have the much more practical application of keeping the animals warm.

A marine mammal must maintain a body temperature near 100° F (37.8° C), and in Alaska, where the water temperature drops as low as 35° F (1.67° C), keeping warm can be a challenge. Other marine mammals have a thick layer of blubber to insulate themselves from the cold, but sea otters have very little fat and depend mainly on their fur for insulation. Sea otters have the thickest fur of any animal, with 850,000 to one million hairs per square inch (up to 150,000 per square centimeter). It is their dense, beautiful fur that made them so valuable to fur traders in the 1700’s and 1800’s.

A sea otter’s fur consists of two layers. Long guard hairs form the outer layer, providing a protective coat to keep the underfur dry. The extremely dense underfur provides warmth, but for the fur to insulate efficiently, it must be clean, so sea otters spend a large portion of each day grooming and cleaning their fur.

In addition to cleaning his fur, an otter will somersault in the water and rub his body to trap air bubbles in his fur. These bubbles not only provide insulation but also help to keep his skin dry. An otter’s underfur ranges from brown to black, and the guard hairs vary from light brown to silver or black. Alaskan sea otters often have lighter fur on their heads, and the fur usually lightens as an otter gets older.

In addition to their warm fur, sea otters maintain their body heat by burning calories at a rapid rate. A sea otter’s metabolism is two to three times higher than the metabolism of a similar-sized land mammal. Because its metabolic rate is so high, a sea otter must eat 23 to 33 percent of its body weight each day. This means a fifty-pound otter will eat 11 to 16 lbs. (5 to 7.3 kg) of food every day.

Because very little fur covers an otter’s paws, they lose heat rapidly when submerged in cold water, otters conserve heat by keeping their forepaws out of the water and their hind flippers folded over their abdomens when resting and floating.

Since sea otters are dependent on their fur to keep them warm and insulated from the cold ocean water, and because they must continually groom their fur to maintain its insulating properties, they are extremely vulnerable to the effects of pollution. When oil or another pollutant soils an otter’s fur, the fur becomes matted, and it can no longer keep the animal warm. Matted fur can lead to hypothermia and death from exposure. When the otter tries to clean his fur to remove the pollutant, he ingests the toxin, which is also often fatal.

When the Exxon Valdez struck a reef in Prince William Sound in 1989 and spilled eleven million gallons of crude oil, nearly 1000 sea otter carcasses were recovered, and biologists estimated the actual number of sea otters killed by the oil spill was anywhere from 41% to 80% higher than the number of dead otters observed.

Sea otters have many interesting adaptations to a life spent in the ocean. In this post, I told you how much sea otters need to eat each day just to stay warm, and next week, I’ll write about what sea otters eat and how they catch and eat their prey.

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Robin Barefield is the author of three Alaska wilderness mystery novels, Big Game, Murder Over Kodiak, and The Fisherman’s Daughter. To download a free copy of one of her novels, watch her webinar about how she became an author and why she writes Alaska wilderness mysteries. If you like audiobooks, check out her audiobook version of Murder Over Kodiak. Also, sign up below to subscribe to her free, monthly newsletter on true murder and mystery in Alaska.