How Do Sea Otters Stay Warm? The Densest Fur of Any Animal

Why staying warm is hard for a sea otter

Cold water steals heat far faster than cold air; water conducts heat roughly 25 times better than air. The sea otter lives full time in the North Pacific, where the sea is often colder than 10 °C (50 °F) in Alaska and rarely much warmer than about 15 °C (59 °F) even off central California.

Size makes it harder still. Small animals have more skin surface for their body weight than large ones, so they lose heat faster. At 14–45 kg (31–99 lb), the sea otter is one of the smallest marine mammals, far smaller than the seals, sea lions and whales that share its waters. Those big animals carry a thick layer of fat called blubber, but sea otters have almost none. They solve the problem in a different way.

The densest fur of any animal

Sea otter fur contains roughly 100,000–150,000 hairs per square centimeter, or up to about 1 million per square inch. A fingertip-sized patch of its skin holds roughly as many hairs as a person has on the whole head, and it is the densest coat measured in any animal.

The fur has two layers. Long guard hairs form a protective, water-repelling outer layer, while the short, very fine underfur below is packed so tightly that water cannot easily push through it. Between the underfur and the skin sits a thin layer of air, warmed by the body. As long as that air layer stays in place, the otter’s skin stays dry, even though the animal is soaking in cold seawater.

The system has one weak point. When an otter dives deep to forage, water pressure squeezes the air layer, so the fur insulates less well during long, deep dives. Sea otters also never go through a sudden molt: they shed and replace hairs gradually throughout the year, so the coat never thins out all at once.

This remarkable fur nearly caused the species’ extinction. It was so prized that fur traders called sea otter pelts “soft gold”, and between 1741 and 1911 hunters killed perhaps a million otters.

Grooming: a full-time job

Fur only insulates when it is clean, untangled and fluffed with air. Dirt, food scraps or oils from prey make the hairs clump together, and cold water can then reach the skin. For this reason sea otters spend a large part of each day grooming.

A grooming otter rubs and kneads its fur with its paws, combs it with its claws, squeezes water out, rolls over and over in the water and blows air into the coat with its mouth. Its loose skin and very flexible skeleton allow it to reach almost every part of its body, including its back. After eating, otters often roll in the water to wash away bits of food.

Mothers spend long periods grooming their pups, whose fluffy baby fur traps so much air that they float like corks. A pup cannot keep its own coat in good condition until it is several months old.

A fast metabolism and a big appetite

Fur alone is not enough. A sea otter’s metabolic rate, the speed at which its body burns fuel, is about two to three times that of a land mammal of similar size. Research on sea otter muscle has found that it produces heat even while the animal is resting, because the muscle cells release part of their energy as heat instead of using it for movement.

All that heat comes from food. A sea otter eats about a quarter of its body weight every day, with estimates ranging from about 20% to more than 30%. For a 30 kg (66 lb) otter, that is roughly 7–9 kg (15–20 lb) of sea urchins, crabs, clams, mussels and other prey. Digestion is fast, and food can pass through the gut in as little as three hours.

To find enough food, sea otters spend many hours diving and foraging, and some crack hard shells open with rocks; see Do sea otters use tools? A mother with a growing pup has to catch food for two, which makes nursing very demanding and can leave females thin by the time the pup becomes independent.

Other tricks for saving heat

Sea otters also manage heat with posture. Their paws have much less fur than the body, so when resting they often fold their front paws on their chest or hold them over their face, and lift their hind flippers out of the water. On warm days they do the opposite and dip their hind feet in the sea to cool off.

Large lungs, about 2.5 times the size expected for a land mammal of the same weight, together with the air in the fur, keep sea otters very buoyant. They can float effortlessly on their backs with much of the body held at the surface. In parts of Alaska, sea otters also haul out on rocks and beaches to rest, especially in bad weather.

Resting in groups called rafts, often wrapped in kelp, helps them stay in sheltered places instead of drifting into open water; read more in Why do sea otters hold hands?

Why oil spills are so harmful

Because sea otters depend on clean fur, oil is one of the greatest threats they face. Oil mats the hairs together, the air layer collapses, and cold water reaches the skin. An oiled otter can die of hypothermia within a short time. Otters that try to lick and groom the oil off swallow it and breathe in fumes, which damages the lungs, liver and kidneys.

The Exxon Valdez spill in Prince William Sound, Alaska, in 1989 killed thousands of sea otters, and leftover oil affected the local population for many years. Because the populations in California, Washington and British Columbia live along fairly short stretches of coast, a single large spill could harm a large share of their otters. Preventing spills and preparing rescue plans are major parts of sea otter conservation.

Fur vs blubber: sea otters compared with seals

Blubber works whatever the condition of the skin surface, but it is heavy and takes time to build. Fur works only when it is perfectly clean. The sea otter’s strategy lets a small animal live year-round in cold seas, at the cost of constant grooming, endless eating and great vulnerability to oil.