Sea Otters: Characteristics, Behavior and Reproduction | Sea Life, Islands and Oceania

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SEA OTTERS

northern sea otters in Washington state

Sea otters (Enhydra lutris) are marine mammals that live in the Pacific Ocean and in the minds of many people are very cute. Sascha Bos wrote in HowStuffWorks: Their thick fur, fluffy paws and round, mischievous faces make sea otters some of the most adorable animals in the ocean. These cuddly animals hold each other's paws when they sleep, and babies often hitch a ride on their mothers. They're also incredibly playful. [Source:Sascha Bos, HowStuffWorks, March 29, 2024]

Sea otters are 1.2 to 1.5 meters (3.9 to 4.9 feet_ long and weigh 16 to 40 kilograms (35 to 90 pounds) and that includes a lot of hair. Otter have the densest hair of any animal. Air trapped in the fibers keeps them warm and allows them to float on the surface. Daily grooming is necessary for upkeep. Because their fur is so durable, warm and valuable sea otters were nearly hunted to extinction. The maximum estimated lifespan of sea otters in the wild is 23 years. [Source: Joe Allegra; Rhiannon Rath; Aren Gunderson, Animal Diversity Web (ADW) |=|]

Sea otters live in shallow coastal waters, often in and around kelp beds. They like to float on the surface of the sea on their backs with their arms folded across their chest. They usually eat and and mothers nurse their young in this position. When foraging, sea otters dive to the sea bottom and gather shellfish, sea urchins and mollusks. Abalones are a favorite food of sea otters, Sea otters cradle the shells on their chests and crack the shells open with rocks, a skill that is learned when they are young.

There are 13 recognized species of otters. Otters are part of the Mustelidae family of carnivorous mammals along with minks, weasels, wolverines and badgers. The sea otter is the largest member of the weasel family, yet the smallest marine mammal in North America. Otters are a keystone species, meaning their presence or absence strongly affects populations of other species in the area where they live. Along the Pacific coast, sea otters help control the sea urchin population. Fewer sea urchins in turn help prevent kelp forests from being overgrazed. In California, research has found that sea otters also enhance seagrass beds, and in Alaska’s Glacier Bay National Park, sea otters’ expansion into the area marked a gradual return of a more diverse ecosystem. According to National Geographic: In British Columbia sea otters often dig clams out of fields of eelgrass (Zostera marina), leaving divots in otherwise dense mats of the aquatic vegetation. In meadows that otters inhabit compared with those they don’t, the eelgrasses are more genetically diverse and the plants more resilient, according to a study published in the journal Science. That’s because by foraging and disturbing the seabed, otters prompt the plants to flower and produce seeds, and their digging provides more space and sunlight for seeds to settle and germinate.” [Source: Douglas Main, February 3, 2022; U.S. Department of Interior, September 25, 2017]

Sea Otter Habitat, Range and Subspecies

sea otter range

Sea otters are found along the Pacific Coast in the Kuril and Commander Islands off the coast of Russia, the Aleutian Islands below the Bering Sea, the coastal waters off the Alaskan Peninsula to Vancouver Island, Canada; and along the central California coast from from San Mateo County to Santa Barbara County, and San Nicolas Island. Sea ice limits their northern range to below 57 degrees N lattitude, and the distribution of kelp forests limits the southern range to about 22 degrees N lattitude. Hunting during the 18th and 19th centuries greatly reduced the distribution of sea otters. [Source: Joe Allegra; Rhiannon Rath; Aren Gunderson, Animal Diversity Web (ADW) |=|]

Most sea otters live in Alaska. Approximately 90 percent of the world’s sea otters live in coastal Alaska. Many live in the waters surrounding public lands including Kodiak National Wildlife Refuge, Kenai Fjords National Park and Glacier Bay National Park. Southern sea otters range along the mainland coastline of California [Source: U.S. Department of Interior, September 25, 2017]

Sea otters inhabit temperate and subarctic coastal waters with rocky or soft sediment ocean bottom. They live in offshore forests of giant kelp (Macrocystis pyrifera), and spend most of their active time foraging below the canopy. They eat, rest, and groom themselves at the water surface. While sea otters are capable of diving to depths of at least 45 meters (147 meters), they prefer coastal waters no more than 30 meters deep. The shallower the water, the less time is spent diving to reach food.

Three subspecies of the sea otter are recognized, each with a distinct geographical range. 1) Asian sea otters (Enhydra lutris lutris, nominate) ranges across Russia's Kuril Islands northeast of Japan, and the Commander Islands in the northwestern Pacific Ocean. 2) Northern sea ottera (E. l. kenyoni) are found from Alaska's Aleutian Islands across Alaska and British Columbia to Oregon in the northern Pacific Ocean; and 3) southern sea otters (E. l. nereis) are native to central and southern California. Asian sea otters are the largest subspecies and has a slightly wider skull and shorter nasal bones than both other subspecies. Northern sea otters possess longer mandibles (lower jaws) while southern sea otters have longer rostrums (structures projecting out from the head) and smaller teeth. [Source: Wikipedia]

Sea Otter Characteristics

Sea otters range in weight from 14 to 45 kilograms (31 to 99 pounds) and are one to 1.5 meters (3.3 to 4.9 feet) long, including their 25-to-35-centimeter (9.8-to-13.8 inch) -tail, which makes up less than a third of the body length. Their average basal metabolic rate is 98.479 watts. Alaskan sea otters are slightly larger than Californian otters. Adult male Alaskan otters weigh 27 to 39 kilograms (60 to 86), while females weigh 16 to 27 kilograms (35 to 60 pounds). Adult male California sea otters average 29 kilograms (64.4 pounds) in weight, while females average 20 kilograms (44 pounds). measuring 25 to 35 centimeters. [Source: Wikipedia, Joe Allegra; Rhiannon Rath; Aren Gunderson, Animal Diversity Web (ADW) |=|]

Sexual Dimorphism (differences between males and females) is present: Males are larger than females. Male sea otters usually weigh 22 to 45 kilograms (49 to 99 pounsa) and are 1.2 to 1.5 meters (3.9 to 4.9 feet) in length, though individuals up to 54 kilograms (119 pounds) have been recorded. Females weigh 14 to 33 kilograms (31 to 73 pounds) and measure one to 1.4 meter (3.25 to 4.6 feet) in length. |=|

Sea otters have circular, furry faces with short noses, rounded eyes and ears. Their hind legs are long and the paws are broad, flat and webbed. The forelimbs are short and have retractable claws, which help with grooming and eating. Sea otters are the only carnivores with just four lower incisors. Females have two mammae. Sea otter fur is brown or reddish brown and consists of two layers: a dark undercoat and longer, lighter-colored guard hairs.

World’s Thickest Fur and Sea Otter Adaptions to Cold Water

Sea otters are good swimmers and well-adapted for life in cold seas. They have long whiskers that assist in foraging for food and webbed on their forelimbs and powerful hind. They have patches of loose skin under the forearms that they use to help store tools (usually a rock) so they can have free “hands” while eating, and to transport food during diving. Sea otters can hold their breath a fairly long time. An otter’s lung capacity is 2.5 times greater than that of similar-sized land mammals. Sea otters have been known to stay submerged for more than five minutes at a time. River otters, however, can hold their breath for up to eight minutes. The increased time underwater improves otters’ opportunity to sense prey and forage for food. [Source: U.S. Department of Interior, September 25, 2017]

Sea otters swim with vertical undulations of their body, while tucking their forelimbs and using the hind limbs and tail to control their motion. Otters can swim as fast as nine kilometers per hour under water. Sea otters forage during the day and particularly around dawn and dusk. Foraging dives usually last 50 to 90 seconds. [Source: Joe Allegra; Rhiannon Rath; Aren Gunderson, Animal Diversity Web (ADW) |=|]

Sea otters have the thickest fur of any animal, with between 100,000 and 150,000 hair follicles per square centimeter (600,000 to 1,000,000 per square inch). Unlike most other marine mammals, otters lack a blubber layer. Instead they depend on their dense, water-resistant fur to provide insulation. To keep warm, sea otters spend a large portion of their days grooming and conditioning their fur. This traps air and heat next to their skin.

David Attenborough wrote: “Sea otters have the thickest fur of any mammal, with a million filaments to the square inch. All the hairs on a human being’s head number only about a tenth as many. To exploit the insulating potential of this fur to the full, sea otters spend time each day lying on the surface of the sea, blowing to their under-fur to ensure that it is always fully topped up with air. Their skin fits so loosely around their body that they can pull almost every part of within their mouths, Even so, living permanently in the waters saps a great deal heat from an otter’s body and it has to eat prodigiously in order to remain warm. Every day it consumers about a third of its own body weight. It is as if a human being, in order not to starve, has to eat a hundred hamburgers daily. [Source: “Life of Mammals” by David Attenborough]

The scent glands that the river otter and weasels use are of no value to an animal that lives permanently in the water and the sea otter has lost them. Its hind legs have become so large and flipper-like that on the few occasions that one does haul itself out on to the rocks, it clambers about on a very clumsy way, But the modification it has had to make in changing from a part-time swimmer to a full-time inhabitants of the ocean has been largely behavioral.

Sea Otters’ Supercharged Metabolism

Dealing with cold can be difficult for animals. As the body chills, organs including the brain and muscles slow down. Mammals can increase their metabolism, using energy to warm their body. Sea otters especially are very good at this nad have been described as being with a supercharged metabolism. According to researchers at Texas A&M: “It is especially difficult for water-living mammals to stay warm because water conducts heat away from the body much faster than air. Most marine mammals have large bodies and a thick layer of fat or blubber for insulation. Sea otters have world’s densest hair as described above. [Source: Traver Wright, Randall Davis, Melinda Sheffield-Moore, professors of Health and Kinesiology, Texas A&M University, The Conversation, December 2, 2021]

southern sea otters in Morro Bay, California

Dense fur is not enough, by itself, to keep sea otters warm. To generate enough body heat, their metabolic rate at rest is about three times higher than that of most mammals of similar size. This high metabolic rate has a cost, though. To obtain enough energy to fuel the high demand, sea otters must eat more than 20 percent of their body mass in food each day. In comparison, humans eat around two percent of their body mass — about three pounds (1.3 kilograms) of food per day for a 155-pound (70 kilograms) person.

“Where does the heat come from? When animals eat, the energy in their food cannot be used directly by cells to do work. Instead, the food is broken down into simple nutrients, such as fats and sugars. These nutrients are then transported in the blood and absorbed by cells. Within the cell are compartments called mitochondria where nutrients are converted into ATP — a high-energy molecule that acts as the energy currency of the cell. The process of converting nutrients into ATP is similar to how a dam turns stored water into electricity. As water flows out from the dam, it makes electricity by spinning blades connected to a generator — similar to wind turning the blades on a windmill. If the dam is leaky, some water — or stored energy — is lost and cannot be used to make electricity.

“Similarly, leaky mitochondria are less efficient at making ATP from nutrients. Although the leaked energy in the mitochondria cannot be used to do work, it generates heat to warm the sea otter’s body. All tissues in the body use energy and make heat, but some tissues are larger and more active than others. Muscle makes up 30 percent of the body mass of most mammals. When active, muscles consume a lot of energy and produce a lot of heat. You have undoubtedly experienced this, whether getting hot during exercise or shivering when cold.

“To find out if muscle metabolism helps keep sea otters warm, we studied small muscle samples from sea otters ranging in size and age from newborn pups to adults. We placed the muscle samples in small chambers designed to monitor oxygen consumption — a measure of how much energy is used. By adding different solutions that stimulated or inhibited various metabolic processes, we determined how much energy the mitochondria could use to make ATP — and how much energy could go into heat-producing leak.We discovered the mitochondria in sea otter muscles could be very leaky, allowing otters to turn up the heat in their muscles without physical activity or shivering. It turns out that sea otter muscle is good at being inefficient. The energy “lost” as heat while turning nutrients into movement allows them to survive the cold. Remarkably, we found newborn pups have the same metabolic ability as adults, even though their muscles have not yet matured for swimming and diving.

Sea Otter Diet and Picky Eating Habits

Every day sea otters spend 10 to 12 hours hunting and consume between a fifth and a third of their body weight in food every day. Sea otters’ diet includes sea urchins, crabs, mussels, and clams, which they’re known to crack open with a rock and eat while floating in the water. To find food, sea otters may occasionally dive as deep as 78 meters (250 feet) and use their sensitive whiskers to locate small prey inside crevices or employ their strong forepaws to dig for clams. They obtain most of their water from prey but also drink seawater when thirsty. [Source: U.S. Department of Interior, September 25, 2017]

Sea otters are carnivorous. They will eat nearly any fish or marine invertebrate they can find in their kelp forest foraging grounds: mollusks aquatic crustaceans echinoderms (a starfish, sea urchin, or sea cucumber) and other marine creatures. A large part of the diet is made up of sea urchins (Strongylocentrotus purpuratus and Strongylocentrotus franciscanus), sea stars (Pisaster ochraceus), limpets (Diodora aspera), coast mussels (Mytilus edulis), chitons (Katharina tunicata), and purple-hinged rock scallops (Crassadoma gigantea). Otters also eat crabs, octopus, squid, and fish. Individuals tend to be specialized in their choice of prey; one otter may consume only urchins and crabs while another may eat mostly fish, depending on the abilities of the individual and local food availability.[Source: Joe Allegra; Rhiannon Rath; Aren Gunderson, Animal Diversity Web (ADW) |=|]

Research published in 2011 by Tim Tinker, a biologist with the U.S. Geological Survey in Santa Cruz, California, described how sea otters are highly specialized eaters, organizing themselves into groups that target different prey and argues that this behavior helps explain why the sea otter comeback hasn’t been as strong as hoped. Jess Righthand wrote in Smithsonian magazine: Tinker says the California otter’s failure to mount a strong comeback may be tied to pathogens and pollutants in coastal waters. By capturing otters and outfitting them with depth recorders and radio transmitters, Tinker and his co-workers have found they form what he calls “dietary guilds.” Deep-diving otters eat mostly abalone, urchins and Dungeness crabs. Otters diving to medium depths — say, up to 40 feet — forage for clams, worms and smaller shellfish. Still others — the junk food eaters — stay in shallow waters, filling their pouches with black snails. Mothers pass these preferences on to their pups, inducting them into their respective guilds. [Source: Jess Righthand, Smithsonian magazine, September 2011]

Moreover, animals eating mostly snails in the more developed Monterey Bay were more likely than others to contract the parasite Toxoplasma gondii. Researchers don’t know how otters encounter the parasite, but it’s found in cat feces, and they speculate it could wind up in the ocean when cat litter is flushed down the toilet or when rain washes feral or domestic cat waste into storm drains and the bay. In contrast, otters that consumed mostly clams and worms were more likely to be infected with Sarcocystis neurona; the source of this parasite in the otters’ environment is also not known, but it’s found in opossum feces and could possibly be washed into the bay by rainwater. Otters may have little natural immunity to these terrestrial pathogens, which can kill them outright or impair their ability to swim, possibly leaving them more vulnerable to sharks.

Sea Otter Feeding

According to Animal Diversity Web: Sea otters commonly feed in small groups. Hunting occurs on the sea floor. They use their sensitive whiskers to locate small creatures in the dense kelp beds and crevices. They use their small, agile forepaws to capture prey and to rub, roll, twist, and pull apart prey. Sea otters collect invertebrates in loose folds of skin under their armpits and eat at the surface. The feeding process, including foraging, eating, and cleaning their fur after a meal, lasts two to three hours.

Sea otters usually eat three to four times a day. Sea otters break open prey items with hard shells or exoskeletons with a rock. Some otters hold the rock on their chest and drive the prey into the rocks. Others leave the prey on their chests and hit the prey with the rocks. The same rock is kept for many dives. Otters often wash their prey by holding it against their body and turning in the water. Males steal from females if they get a chance. For this reason, females tend to forage in separate areas. [Source: Joe Allegra; Rhiannon Rath; Aren Gunderson, Animal Diversity Web (ADW) |=|]

David Attenborough wrote: “Individual sea otters vary a good deal in their tastes. Most. However, live on creatures that they can collect from the sea floor — sea urchins, clams, abalone, crabs and oysters. They bring them up from considerable depth — 60 feet or so — and sometimes will make repeated dives to detach one form its moorings. If necessary one can descend as deep as 130 feet. When an otter finally brings up something edible, it also carries with it, packed into a pouch formed by skin on the armpit, a rock. Once on the surface it lies on its back, puts the rock on its stomach and then uses it like an anvil on which to smash open crabs and shellfish, [Source: “Life of Mammals” by David Attenborough]

Sea Otter Predators and Defenses

Sea otters are occasionally eaten by coyotes after taking refuge on the sand during stormy weather. Young pups left alone on the surface while their mothers feed beneath the surface are preyed upon by bald eagles. [Source: Joe Allegra; Rhiannon Rath; Aren Gunderson, Animal Diversity Web (ADW) |=|]

Great white sharks prey on sea otters but not all that often. According to A. Peter Klimley, a marine biologist at the University of California, Davis, white sharks are energy maximizers, so they reject low-fat foods, which explains why they often feed on seals and sea lions but rarely on penguins and sea otters, which are notably less fatty. White sharks also eat other kinds of prey may be low-cal but is easier to find and catch, and thus sometimes energetically more attractive.

Great whites often release animals after biting into them and more likely to do this if they bite into a relatively low fat creature like a sea otter or human than a high-fat seal or sea lion. Klimley told Smithsonian magazine, “It may be a textural discrimination [of fat], more than what we could call taste...We once took a seal and stripped the fat off it and put it all the water. The shark ate the fat but not the rest of the body. They are actually very discriminating predators.” [Source: R. Aidan Martin, Anne Martin, Natural History magazine, October 2006]

In the early 2000s, the populations of sea otters suddenly plummeted in waters off Alaska The number of sea otters in the Aleutians declined from 85,000 in 1990 to 8,500 in the early 2000s. It was first once thought that orcas (killer whales) were responsible for declines — that orcas began seeking them as prey because their main food sources of seals and sea lions had declined — but evidence was not conclusive.

For protection from predators sea otters seek the cover of kelp beds and form groups with individuals alerting the group if danger is sensed. Researchers have found groups of over 1,000 otters floating together. To keep from drifting away from each other, sea otters wrap themselves up in seaweed, forming something that resembles a raft. [Source: U.S. Department of Interior, September 25, 2017]

Sea Otter Behavior

Sea otters are natatorial (equipped for swimming), diurnal (active during the daytime), crepuscular (active at dawn and dusk), motile (move around as opposed to being stationary), sedentary (remain in the same area), solitary, territorial (defend an area within the home range) and social (associates with others of its species; forms social groups). Males have larger home ranges than females. The home ranges of males may overlap with those of several females. Same-sex territories do not overlap and are defended. [Source: Joe Allegra; Rhiannon Rath; Aren Gunderson, Animal Diversity Web (ADW) |=|]

David Attenborough wrote: In California, sea otters live in coastal waters among marine forest of giant kelp. The plants are firmly attached to the sea floor their huge straps rise to the surface and float along it, Here male sea otters establish territories patrolling the borders and driving off intruding males. To prevent themselves being carried away from their patch of ocean by ocean currents while they are asleep, the seize the free end of a kelp strap and then revolve in the water, so that the kelp wraps round them and they are firmly anchored. Then they put their paws over their eyes and go to sleep. [Source: “Life of Mammals” by David Attenborough]

Sea otters are relatively long-lived and tend to remain in the same area for years. They spend the majority of their time at sea but rest on land when the population densities are or during stormy weather. Sea otters spend 15 to 55 percent of their time foraging, depending on food availability. |Sea otters congregate in groups known as rafts or pods when resting. Females tend to avoid males except when mating. When resting or sleeping, sea otters float on their back and wrap themselves in kelp to keep from drifting. Their hind limbs stick out of the water and their forelimbs are either folded on their chest or used to cover their eyes. They diligently groom and clean their fur to maintain its insulating ability. |=|

Sea otters are one of few mammals that exhibit tool use and rocks are the tools of choice. Rocks can be used as a hammer or anvil to break open hard-shelled prey. They keep good rocks store them in the loose patch of skin under their armpits. [Source: U.S. Department of Interior, September 25, 2017]

Sea otters sense and communicate with vision, touch, sound and chemicals usually detected by smelling. They communicate through body contact and vocalizations, although they are not overly vocal. Researchers have recognized nine vocalizations. Pups use squeals to communicate with their mothers. Other calls include coos, whines, distress screams, growls, snarls, and whistles. Scent is important in recognition and surveying physiological states. Each sea otter has its own distinct scent that conveys identity, age, and sex. |=|

Sea Otter Mating and Reproduction

Sea otters are polygynous (males have more than one female as a mate at one time) and employ delayed implantation (a condition in which a fertilized egg reaches the uterus but delays its implantation in the uterine lining, sometimes for several months). Females have an estrus cycle, which is similar to the menstrual cycle of human females and breeding can take place year-round. Sea otters breed once every one or two years. The gestation period ranges from four to 12 months depending on the length of delayed implantation. The average gestation period is six months. Usually one offspring is born, sometimes three. [Source: Joe Allegra; Rhiannon Rath; Aren Gunderson, Animal Diversity Web (ADW) |=|]

Many males actively defend territories. Disputes are usually settled with splashing and vocal displays, and fighting is rare. Males mate with females that inhabit their territory. If no territory is established, they seek out females in estrus. When a male sea otter finds a receptive female, the two engage in playful and sometimes aggressive behavior. They bond for the duration of estrus, or three days. The male holds the female's head or nose with his jaws during copulation. Visible scars are often present on females from this behavior.

Delayed implantation is used to make sure young are born under favorable conditions. Delayed implantation produces varied gestation times between four and 12 months. Females usually give birth about once a year, though many females experience longer breeding intervals, giving birth every two years. If a pup does not survive, the mother may experience postpartum estrus. |=| Orientation of the fetus may be either caudal (oriented towards the back of the body) or cephalic (head down), although cephalic orientation is more common near birth.

Sea Otter Offspring and Parenting

Although sea otters can reproduce year round there are peaks of birth in May to June in the Aleutian Islands and in January to March in California. Parental care is provided by females. Male sea otters do not provide any care to their offspring. The average weaning age is six months. The post-independence period is characterized by the association of offspring with their parents. Females reach sexual maturity at four to five years. Males reach sexual maturity at five to eight years, with the average being six years. [Source: Joe Allegra; Rhiannon Rath; Aren Gunderson, Animal Diversity Web (ADW) |=|]

Other otter species give birth on land but sea otters give birth at sea, something that has been been achieved with substantial physical changes. Usually a single pup weighing 1.4 to 2.3 kilograms (3 to 5 pounds) is born. Twins occur in two percent of births, but only one pup is raised successfully. Pups typically remain with their mother for five to six months after birth. They are virtually inseparable for their mothers but remain a the surface while their mothers dive for food.

Southern sea otters breed and pup year-round, while northern sea otter pups in Alaska are usually born in the spring. A newborn pup needs constant attention and stay with its mother until it develops survival skills. A pup’s coat traps air, which keeps the animal afloat. An otter pup’s fur is so dense that it can’t dive underwater until it gets its adult fur. This is useful when mothers leave their pups safely floating on the water’s surface while they forage for food. [Source: U.S. Department of Interior, September 25, 2017]

Even though pups are weaned at around six months they start eating solid foods shortly after birth. Females carry their pups on their bellies while they nurse. Their milk is 20 to 25 percent fat. While a mother is foraging, she wraps her pup in kelp at the water surface to keep it from drifting away. At any sign of a predator, the female clamps onto her pup’s neck with her mouth and dives. Females groom their pups extensively for three months as their coat develops. Pups start diving at two months of age.

Image Sources: Wikimedia Commons

Text Sources: Animal Diversity Web animaldiversity.org ; National Geographic, Live Science, Natural History magazine, David Attenborough books, New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Discover magazine, The New Yorker, Time, BBC, CNN, Reuters, Associated Press, AFP, Lonely Planet Guides, Wikipedia, The Guardian, Top Secret Animal Attack Files website and various books and other publications.

Last updated June 2025