Sea Otter (Enhydra lutris): Facts, Diet and Habitat

Appearance: a body built around fur

A sea otter has a stocky, rounded body, a broad head with a blunt muzzle, small ears and long, pale whiskers. The front paws are short and mitten-like, with retractable claws and tough pads that grip slippery prey. The hind feet are long, flat and fully webbed, more like flippers than paws, and the outer toe is the longest, which helps the otter paddle while lying on its back. The tail is short, thick and slightly flattened.

The coat is usually deep brown with silvery flecks, but it ranges from yellowish or grayish brown to almost black. The head, throat and chest are paler than the body and often become grizzled or nearly white in older animals. Unlike seals, whales and sea lions, the sea otter has no insulating layer of blubber. It relies instead on fur with roughly 100,000–150,000 hairs per square centimeter (up to about 1 million per square inch), the densest coat known in any animal.

Long guard hairs protect a short, fine underfur that traps a layer of air against the skin. Loose skin and an unusually flexible skeleton let the otter reach and groom every part of its body. Read more in How do sea otters stay warm?

The 32 teeth include broad, flat, rounded molars that crush shells instead of slicing meat. Sea otters that eat a lot of sea urchins can even have purple-stained teeth and bones.

How big is a sea otter?

Males are clearly larger than females, and exceptional males of up to 54 kg (119 lb) have been recorded. The Asian subspecies is the largest, while southern sea otters in California are smaller, rarely over about 32 kg (70 lb). Otters living at low density, with more food to go around, are heavier on average than those in crowded areas.

The sea otter outweighs every other member of the weasel family, including the giant otter of South America, which is longer but lighter. Among marine mammals, though, it is tiny: an adult harbor seal, at about 50–170 kg (110–375 lb), is usually several times heavier.

Habitat and range

Sea otters live in shallow coastal waters, usually within about 1 km (0.6 mi) of shore and in water around 15–23 m (50–75 ft) deep. Rocky coasts with kelp forests are the classic home, but they also live over sand and mud in sheltered bays and estuaries such as Elkhorn Slough in California. They can survive among drifting sea ice but not where ice is frozen to the shore, which sets the northern limit of the range.

Three subspecies are recognized:

• Asian sea otter (E. l. lutris): the Kuril Islands, Kamchatka and the Commander Islands of Russia, with a small group off eastern Hokkaido, Japan.
• Northern sea otter (E. l. kenyoni): the Aleutian Islands and Alaska south to British Columbia and Washington.
• Southern or California sea otter (E. l. nereis): the central California coast, roughly from San Mateo County to Santa Barbara County.

Before the fur trade, sea otters lived in an arc from northern Japan to central Baja California in Mexico. They have never lived in the Atlantic or in Europe. Most individuals stay in a home range a few kilometers long all year, although young males may travel much farther.

What do sea otters eat?

Sea otters are carnivores that eat more than 100 kinds of prey, mostly animals living on the seabed. Sea urchins, crabs, clams, mussels, abalone, snails, chitons, sea stars, octopuses and fat innkeeper worms are typical meals. In parts of Alaska and the Aleutian Islands, slow-moving bottom fish can make up a large share of the diet, while further south fish are rarely eaten.

Because a sea otter burns energy so fast to stay warm, it eats about a quarter of its body weight every day, sometimes more. It hunts in short dives, usually lasting about a minute and rarely more than four minutes, and it finds prey by touch with its sensitive paws and whiskers. Sea otters can dive to around 100 m (330 ft), but most feed in much shallower water.

A sea otter carries its catch to the surface in a pouch of loose skin under each forearm, then eats while floating on its back, using its chest as a table. It bites through the underside of sea urchins, where the spines are shortest, and often cracks hard shells by pounding them on a stone. Find out more in Do sea otters use tools?

Individual otters often specialize in certain prey and feeding techniques, and pups usually copy their mothers.

Behavior: rafts, grooming and daily life

Sea otters are mostly active by day. They typically feed in the early morning and again in the afternoon, rest around midday and sometimes forage at night, especially mothers with pups. Depending on how much food is available, they spend roughly a quarter to more than half of the day foraging.

Each otter hunts alone, but they rest together in groups called rafts, usually of 10 to 100 animals and sometimes many more; the largest recorded raft held over 2,000 otters. Rafts are normally single-sex, with males and females using different areas. To avoid drifting while they sleep, otters wrap themselves in kelp, and they occasionally hold paws.

Grooming takes up a large part of the day. Otters rub, comb and squeeze their fur, roll in the water and blow air into the coat to keep it clean and waterproof. Sea otters make a range of sounds: pups give high cries often compared to a gull’s, content females coo, and alarmed adults may whistle, hiss or scream.

Reproduction and life cycle

Sea otters are polygynous, meaning a male mates with several females, often those living in the territory he defends. Mating and births happen all year, with seasonal peaks. Thanks to delayed implantation, in which the fertilized egg waits before it starts developing, the time from mating to birth varies from about four to twelve months.

The single pup is born in the water, with open eyes and a thick, fluffy coat that traps so much air that it floats like a cork and cannot dive. Twins occur in about 2% of births, but a mother can raise only one. She carries the pup on her belly, grooms it for hours and leaves it floating, sometimes wrapped in kelp, while she dives for food. The baby fur is replaced by adult fur at about 13 weeks.

Pups nurse for about four months to a year and usually become independent at six to eight months. Survival is low in the first winter; one estimate suggests that only about 25% of pups live through their first year. Females can breed at three to four years of age; males mature at about five but often do not father pups until later.

In the wild, males usually live 10–15 years and females 15–20 years, with a maximum of about 23 years. Worn-down teeth may limit how long wild otters survive. In captivity the record is 28 years, reached by a female at the Seattle Aquarium.

Predators and threats

Healthy adults have few natural predators. Orcas are the main one in Alaska and are suspected of causing much of the steep Aleutian decline that began in the 1990s. In California, great white sharks bite many sea otters, apparently mistaking them for seals, and the bites are often fatal even though the sharks do not eat them. Bald eagles can take pups, and on land, bears, coyotes and wolves occasionally kill otters.

Oil spills are the most serious threat. Oil mats the fur so it can no longer hold air, and an oiled otter can quickly die of cold; the Exxon Valdez spill in Alaska in 1989 killed thousands. Disease is another major cause of death in California, including the parasite Toxoplasma gondii, which reaches the sea in runoff carrying cat droppings, as well as toxins from harmful algal blooms. Otters also drown in fishing gear and are sometimes killed illegally, and their low genetic diversity, a legacy of near extinction, may slow recovery.

Sea otters and kelp forests: a keystone species

The sea otter is a classic keystone species, an animal whose effect on its ecosystem is far larger than its numbers suggest. By eating sea urchins such as the purple sea urchin, otters stop urchins from grazing kelp forests down to bare rock. Where otters vanished, many reefs became urchin barrens; where otters returned, kelp forests grew back. Read the full story in Why are sea otters a keystone species?

Conservation status: from fur trade to recovery

Before commercial hunting, there were an estimated 150,000–300,000 sea otters. Their fur, the finest of any animal, was among the most valuable in the world, and between 1741 and 1911 hunters killed perhaps a million. By 1911 only about 1,000–2,000 survived in scattered colonies. That year, the United States, Russia, Japan and Great Britain (for Canada) signed a treaty that ended commercial hunting.

Protection and reintroductions brought the species back to about two-thirds of its former range. Otters moved from Alaska re-established populations in Southeast Alaska, British Columbia and Washington, and the California population grew from a remnant group of about 50 animals off Big Sur to around 3,000 today. A worldwide total of roughly 107,000 was estimated in the mid-2000s, but steep declines have since been reported in the Aleutian Islands and Russia.

The IUCN Red List classifies the sea otter as Endangered. In the United States, the southern sea otter and the southwest Alaska population are listed as threatened under the Endangered Species Act, and all sea otters are protected by the Marine Mammal Protection Act.

Myths and facts

Myth: the sea otter is the smallest marine mammal. It is one of the smallest, but the marine otter of Peru and Chile is smaller, weighing only a few kilograms.

Myth: sea otters live in Britain or Norway. Otters hunting along European sea coasts are Eurasian otters, the same species found on rivers. See sea otter vs river otter for how to tell the two apart.

Myth: sea otters eat a lot of starfish. Cartoons often show them with sea stars, but sea stars are usually a minor part of the diet; urchins, crabs and clams matter far more.

Fact: sea otters can live their whole lives at sea. Unlike seals and sea lions, which must haul out on land or ice to give birth, sea otters can mate, give birth and raise pups entirely in the water.