Sea Otter

A sea otter wraps itself in kelp in Morro Bay, California.
Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: Mammalia
Order: Carnivora
Family: Mustelidae
Subfamily: Lutrinae
Genus: Enhydra
Fleming, 1828
Species: E. lutris
Binomial name
Enhydra lutris
(Linnaeus, 1758)

Modern and historical range

The sea otter (Enhydra lutris) is a marine mammal native to the coasts of the northern and eastern North Pacific Ocean. Adult sea otters typically weigh between 14 and 45 kg (30 to 100 lb), making them the heaviest members of the weasel family, but among the smallest marine mammals. Unlike most marine mammals, the sea otter's primary form of insulation is an exceptionally thick coat of fur, the densest in the animal kingdom. Although it can walk on land, the sea otter lives mostly in the ocean.

The sea otter inhabits nearshore environments where it dives to the sea floor to forage. It preys mostly upon marine invertebrates such as sea urchins, various molluscs and crustaceans, and some species of fish. Its foraging and eating habits are noteworthy in several respects. First, its use of rocks to dislodge prey and to open shells makes it one of the few mammal species to use tools. In most of its range, it is a keystone species, controlling sea urchin populations which would otherwise inflict extensive damage to kelp forest ecosystems. Its diet includes prey species that are also valued by humans as food, leading to conflicts between sea otters and fisheries.

Sea otters, whose numbers were once estimated at 150,000–300,000, were hunted extensively for their fur between 1741 and 1911, and the world population fell to 1,000–2,000 individuals in a fraction of their historic range. A subsequent international ban on hunting, conservation efforts, and reintroduction programs into previously populated areas have contributed to numbers rebounding, and the species now occupies about two-thirds of its former range. The recovery of the sea otter is considered an important success in marine conservation, although populations in the Aleutian Islands and California have recently declined or have plateaued at depressed levels. For these reasons (as well as its particular vulnerability to oil spills) the sea otter remains classified as an endangered species.


The first scientific description of the sea otter is contained in the field notes of Georg Steller from 1751, and the species was described by Linnaeus in his Systema Naturae of 1758.[2] Originally named Lutra marina, it underwent numerous name changes before being accepted as Enhydra lutris in 1922.[3] The generic name Enhydra, derives from the Ancient Greek en/εν "in" and hydra/ύδρα "water",[4] meaning "in the water", and the Latin word lutris, meaning "otter".[5] It was formerly sometimes referred to as the "sea beaver",[6] although it is only distantly related to beavers. It is not to be confused with the marine otter, a rare otter species native to the southern west coast of South America. A number of other otter species, while predominantly living in fresh water, are commonly found in marine coastal habitats. The extinct sea mink of northeast North America is another mustelid that adapted to a marine environment.


Although it is a relatively new marine mammal lineage, the sea otter can live in the ocean at all stages of life.

The sea otter is the heaviest member of the family Mustelidae,[7][8] a diverse group that includes the thirteen otter species and terrestrial animals such as weasels, badgers, and minks. It is unique among the mustelids in not making dens or burrows, in having no functional anal scent glands,[9] and in being able to live its entire life without leaving the water.[10] The only member of the genus Enhydra, the sea otter is so different from other mustelid species that as recently as 1982, some scientists believed it was more closely related to the earless seals.[11] Genetic analysis indicates that the sea otter and its closest extant relatives, which include the African speckle-throated otter, Eurasian otter, African clawless otter and oriental small-clawed otter, shared an ancestor approximately 5 million years ago (mya).[12]

Fossil evidence indicates that the Enhydra lineage became isolated in the North Pacific approximately 2 mya, giving rise to the now-extinct Enhydra macrodonta and the modern sea otter, Enhydra lutris.[3] The sea otter evolved initially in northern Hokkaidō and Russia, then spread east to the Aleutian Islands, mainland Alaska, and down the North American coast.[13] In comparison to cetaceans, sirenians, and pinnipeds, which entered the water approximately 50 mya, 40 mya, and 20 mya, respectively, the sea otter is a relative newcomer to a marine existence.[14] In some respects, however, the sea otter is more fully aquatically adapted than pinnipeds, which must haul out on land or ice to give birth.[15]


There are three recognized subspecies, which vary in body size and in some skull and dental characteristics:[7][16]

  • The common sea otter, E. l. lutris (Linnaeus, 1758), ranges from the Kuril Islands to the Commander Islands in the western Pacific Ocean.[7] Also known as the Asian sea otter, it is the largest subspecies with a wide skull and short nasal bones.[17]
  • The southern sea otter, E. l. nereis (Merriam, 1904), is found off the coast of central California.[7] Also known as the Californian sea otter, it has a narrower skull with a long rostrum and small teeth.[17]
  • The northern sea otter, E. l. kenyoni[18] (Wilson, 1991), is native to Alaska and the Pacific west coast from the Aleutian islands to British Columbia, Washington, and northern Oregon [17]. After being extirpated from southern British Columbia south due to overhunting, it has since been re-introduced off Vancouver Island and the Olympic Peninsula. [7]

The reintroduction effort off the Oregon coast was not successful. However, reintroductions in 1969 and 1970 off the Washington coast were very successful and sea otters have been expanding their range since. They have now entered the Strait of Juan de Fuca and can be found almost as far east as Pillar Point. Individuals have even been seen in the San Juan Islands and northern Puget Sound.

Physical characteristics

A sea otter's thick fur makes its body appear much plumper on land than in the water.

The sea otter is one of the smallest marine mammal species.[10] Male sea otters weigh 22 to 45 kg (49 to 99 lb) and are 1.2 to 1.5 m (4 to 5 ft) in length. Females are smaller, weighing 14 to 33 kg (30 to 73 lb) and measuring 1.0 to 1.4 m (3 ft 3 in to 4 ft 7 in) in length.[19]

Unlike other marine mammals, the sea otter has no blubber and relies on its exceptionally thick fur to keep warm.[20] With up to 150 thousand strands of hair per square centimeter (nearly one million per sq in), its fur is the most dense of any animal.[21] The fur consists of long waterproof guard hairs and short underfur; the guard hairs keep the dense underfur layer dry. Cold water is thus kept completely away from the skin and heat loss is limited.[19] The fur is thick year-round, as it is shed and replaced gradually rather than in a distinct molting season.[22] As the ability of the guard hairs to repel water depends on utmost cleanliness, the sea otter has the ability to reach and groom the fur on any part of its body, taking advantage of its loose skin and an unusually supple skeleton.[23] The coloration of the pelage is usually deep brown with silver-gray speckles, however it can range from yellowish or grayish brown to almost black.[24] In adults, the head, throat, and chest are lighter in color than the rest of the body.[24]

The sea otter displays numerous adaptations to its marine environment. The nostrils and small ears can close.[25] The hind feet, which provide most of its propulsion in swimming, are long, broadly flattened, and fully webbed.[26] The fifth digit on each hind foot is longest, facilitating swimming while on its back, but making walking difficult.[27] The tail is fairly short, thick, slightly flattened, and muscular. The front paws are short with retractable claws, with tough pads on the palms that enable gripping slippery prey.[28]

Skeleton of a sea otter. The hind flippers are larger than the mitten-like front paws.

The sea otter propels itself underwater by moving the rear end of its body, including its tail and hind feet, up and down,[26] and is capable of speeds of up to 9 km/h (5.6 mph).[7] When underwater, its body is long and streamlined, with the short forelimbs pressed closely against the chest.[29] When at the surface, it usually floats on its back and moves by sculling its feet and tail from side to side.[30] At rest, all four limbs can be folded onto the torso to conserve heat, whereas on particularly hot days the hind feet may be held underwater for cooling.[31] The sea otter's body is highly buoyant because of its large lung capacity – about 2.5 times greater than that of similar-sized land mammals[32] – and the air trapped in its fur. The sea otter walks with a clumsy rolling gait on land, and can run in a bounding motion.[27]

Long, highly sensitive whiskers and front paws help the sea otter find prey by touch when waters are dark or murky.[10] Researchers have noted that when they approach in plain view, sea otters react more rapidly when the wind is blowing towards the animals, indicating that the sense of smell is more important than sight as a warning sense.[33] Other observations indicate that the sea otter's sense of sight is useful above and below the water, although not as good as that of seals.[34] Its hearing is neither particularly acute nor poor.[35]

An adult sea otter swimming on its back.

An adult's 32 teeth, particularly the molars, are flattened and rounded, designed to crush rather than cut food.[36] Seals and sea otters are the only carnivores with two pairs of lower incisor teeth rather than three;[37] the adult dental formula is:[38]


The sea otter has a metabolic rate two or three times that of comparatively sized terrestrial mammals. It must eat an estimated 25 to 38% of its own body weight in food each day in order to burn the calories necessary to counteract the loss of heat due to the cold water environment.[39][40] Its digestive efficiency is estimated at 80 to 85%,[41] and food is digested and passed in as little as three hours.[20] Most of its need for water is met through food, although, in contrast to most other marine mammals, it also drinks seawater. Its relatively large kidneys enable it to derive fresh water from sea water and excrete concentrated urine.[42]


Sensitive whiskers and forepaws enable sea otters to find prey using their sense of touch.

The sea otter is diurnal. It has a period of foraging and eating in the morning, starting about an hour before sunrise, then rests or sleeps in mid-day.[43] Foraging resumes for a few hours in the afternoon and subsides before sunset, and there may be a third foraging period around midnight.[43] Females with pups appear to be more inclined to feed at night.[43] Observations of the amount of time a sea otter must spend each day foraging range from 24 to 60%, apparently depending on the availability of food in the area.[44]

The sea otter spends much of its time grooming, which consists of cleaning the fur, untangling knots, removing loose fur, rubbing the fur to squeeze out water and introduce air, and blowing air into the fur. To an observer it appears as if the animal is scratching, however sea otters are not known to have lice or other parasites in the fur.[45] When eating, the sea otter rolls in the water frequently, apparently to wash food scraps from its fur.[46]


The sea otter hunts in short dives, often to the sea floor. Although it can hold its breath for up to five minutes,[25] dives typically last about one minute and no more than four.[19] It is the only marine animal capable of lifting and turning over boulders, which it often does with its front paws when searching for prey.[46] The sea otter may also pluck snails and other organisms from kelp and dig deep into underwater mud for clams.[46] It is the only marine mammal that catches fish with its forepaws rather than with its teeth.[20]

Under each foreleg, the sea otter has a loose pouch of skin that extends across the chest. In this pouch (preferentially the left one), the animal stores collected food to bring to the surface.[47] There, the sea otter eats while floating on its back, using its forepaws to tear food apart and bring it to its mouth. It can chew and swallow small mussels with their shells, whereas large mussel shells may be twisted apart.[48] It uses its lower incisor teeth to access the meat in shellfish.[49] To eat large sea urchins, which are mostly covered with spines, the sea otter bites through the underside where the spines are shortest, and licks the soft contents out of the urchin's shell.[48]

The sea otter's use of rocks when hunting and feeding makes it one of the few mammal species to use tools.[50] To open hard shells, it may pound its prey with both paws against a rock on its chest. To pry an abalone off its rock, it hammers the abalone shell using a large stone, with observed rates of 45 blows in 15 seconds.[19] Releasing an abalone, which can cling to rock with a force equal to 4,000 times its own body weight, requires multiple dives.[19]

To keep from drifting apart, sea otters may sleep holding paws.[51] Note the high buoyancy of the animals' bodies.

Social structure

Although each adult and independent juvenile forages alone, sea otters tend to rest together in single-sex groups called rafts. A raft typically contains 10 to 100 animals, with male rafts being larger than female ones.[52] The largest raft ever seen contained over 2000 sea otters. To keep from drifting out to sea when resting and eating, sea otters may wrap themselves in kelp.[53]

A male sea otter is most likely to mate if he maintains a breeding territory in an area that is also favored by females.[54] As autumn is the peak breeding season in most areas, males typically defend their territory only from spring to autumn.[54] During this time, males patrol the boundaries of their territories to exclude other males,[54] although actual fighting is rare.[52] Adult females move freely between male territories, where they outnumber adult males by an average of five to one.[54] Males who do not have territories tend to congregate in large male-only groups,[54] and swim through female areas when searching for a mate.[55]

The species exhibits a variety of vocal behaviors. The cry of a pup is often compared to that of a seagull.[56] Females coo when they are apparently content; males may grunt instead.[57] Distressed or frightened adults may whistle, hiss, or in extreme circumstances, scream.[56]

Although sea otters can be playful and sociable, they are not considered to be truly social animals.[58] They spend much time alone, and each adult can meet its own needs in terms of hunting, grooming, and defense.[58]

Reproduction and lifecycle

During mating, the male bites the nose of the female, often bloodying and scarring it.

Sea otters are polygynous: males have multiple female partners. However, temporary pair-bonding occurs for a few days between a female in estrus and her mate.[46] Mating takes place in the water and can be rough, the male biting the female on the muzzle – which often leaves scars on the nose – and sometimes holding her head under water.[7][59]

Births occur year-round, with peaks between May and June in northern populations and between January and March in southern populations.[60] Gestation appears to vary from four to twelve months, as the species is capable of delayed implantation followed by four months of pregnancy.[60] In California, sea otters usually breed every year, about twice as often as sea otters in Alaska.[61]

Birth usually takes place in the water and typically produces a single pup weighing 1.4 to 2.3 kg (3 to 5 lb).[62] Twins occur in 2% of births; however, usually only one pup survives.[7] At birth, the eyes are open, ten teeth are visible, and the pup has a thick coat of baby fur.[63] Mothers have been observed to lick and fluff a newborn for hours; after grooming, the pup's fur retains so much air that the pup floats like a cork and cannot dive.[64] The fluffy baby fur is replaced by adult fur after about thirteen weeks.[2]

A mother floats with her pup on her chest. Georg Steller wrote, "They embrace their young with an affection that is scarcely credible."[65]

Nursing lasts six to eight months in California populations and four to twelve months in Alaska, with the mother beginning to offer bits of prey at one to two months.[66] The milk from a sea otter's two abdominal nipples is rich in fat and more similar to the milk of other marine mammals than to that of other mustelids.[67] A pup, with guidance from its mother, practices swimming and diving for several weeks before it is able to reach the sea floor. Initially the objects it retrieves are of little food value, such as brightly colored starfish and pebbles.[47] Juveniles are typically independent at six to eight months, however a mother may be forced to abandon a pup if she cannot find enough food for it[68] and at the other extreme, a pup may nurse until it is almost adult size.[62] Pup mortality is high, particularly during an individual's first winter – by one estimate, only 25% of pups survive their first year.[68] Pups born to experienced mothers have the highest survival rates.[69]

Females perform all tasks of feeding and raising offspring, and have occasionally been observed caring for orphaned pups.[70] Much has been written about the level of devotion of sea otter mothers for their pups – a mother gives her infant almost constant attention, cradling it on her chest away from the cold water and attentively grooming its fur.[71] When foraging, she leaves her pup floating on the water, sometimes wrapped in kelp to keep it from floating away;[72] if the pup is not sleeping, it cries loudly until she returns.[73] Mothers have been known to carry their pup for days after the pup's death.[65]

Females become sexually mature at around three or four years of age and males at around five; however, males often do not successfully breed until a few years later.[74] A captive male sired offspring at age 19.[62] In the wild, sea otters live to a maximum age of 23 years,[19] with average lifespans of 10–15 years for males and 15–20 years for females.[75] Several captive individuals have lived past 20 years, and a female at the Seattle Aquarium died at the age of 28 years.[76] Sea otters in the wild often develop worn teeth, which may account for their apparently shorter lifespans.[77]

Population and distribution

Sea otter floating in Morro Bay, California

Sea otters live in coastal waters 15 to 23 meters (50 to 75 ft) deep,[78] and usually stay within a kilometer (⅔ mi) of the shore.[79] They are found most often in areas with protection from the most severe ocean winds, such as rocky coastlines, thick kelp forests, and barrier reefs.[80] Although they are most strongly associated with rocky substrates, sea otters can also live in areas where the sea floor consists primarily of mud, sand, or silt.[81] Their northern range is limited by ice, as sea otters can survive amidst drift ice but not land-fast ice.[82] Individuals generally occupy a home range a few kilometers long, and remain there year-round.[83]

The sea otter population is thought to have once been 150,000 to 300,000,[6] stretching in an arc across the North Pacific from northern Japan to the central Baja California Peninsula in Mexico. The fur trade that began in the 1740s reduced the sea otter's numbers to an estimated 1,000 to 2,000 members in thirteen colonies. In about two-thirds of its former range, the species is at varying levels of recovery, with high population densities in some areas and threatened populations in others. Sea otters currently have stable populations in parts of the Russian east coast, Alaska, British Columbia, Washington, and California, and there have been reports of recolonizations in Mexico and Japan.[84] Population estimates made between 2004 and 2007 give a worldwide total of approximately 107,000 sea otters.[2][85][86][87][88]


Currently, the most stable and secure part of the sea otter's range is Russia.[89] Before the 19th century there were around 20,000 to 25,000 sea otters in the Kuril Islands, with more on Kamchatka and the Commander Islands. After the years of the Great Hunt, the population in these areas, currently part of Russia, was only 750.[85] As of 2004, sea otters have repopulated all of their former habitat in these areas, with an estimated total population of about 27,000. Of these, about 19,000 are in the Kurils, 2000 to 3500 on Kamchatka and another 5000 to 5500 on the Commander Islands.[85] Growth has slowed slightly, suggesting that the numbers are reaching carrying capacity.[85]