|Sea otters are diurnal mammals that inhabit the Pacific Ocean off the coasts of North America and Asia. These carnivorous creatures are one of the few mammals to exhibit tool use and use rocks to break open prey. As social creatures, sea otters spend much of their time floating on their backs in small groups.|
The sea otter’s pelage is brown or reddish brown. Sea otter fur is the densest of all mammals, with about 100,000 hairs per square centimeter. Because sea otters do not have any insulating fat, the fur is responsible for heat maintenance. The fur consists of two layers: a dark undercoat and longer, lighter-colored guard hairs, which trap a layer of air next to the skin to keep it dry.
Sea otters have circular, furry faces with short noses, rounded eyes and ears, and long whiskers that assist in foraging for food. Sea otters use their sensitive whiskers to locate small creatures in the dense kelp beds and crevices.
The sea otter’s 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 use their small, agile forepaws to capture the prey and rub, roll, twist, and pull the prey apart.
Sea otters are the only carnivores with just four lower incisors.
Sea otters display sexual dimorphism as males are larger than females. Males measure 1.2 to 1.5 meters in length, while females measure 1 to 1.4 meters. Individuals can weigh as much as 45 kilograms. The tail comprises less than a third of the body length, measuring 25 to 35 centimeters, or 10-14 inches.
Sea otters are long-lived. The maximum estimated lifespan of sea otters is 23 years in the wild.
The word otter derives from the Old English word otor or oter. This, and cognate words in other Indo-European languages, ultimately stem from the Proto-Indo-European language root *wódr̥, which also gave rise to the English word water.
Sea otters are social creatures and congregate in groups known as bevies, families, lodges, rafts, romps, or pods, when resting. Romp is descriptive of their playful nature, while raft is descriptive of their water-floating habits.
Three regional subspecies of the sea otter have been confirmed and are recognized with distinct geographical distributions.
Enhydra lutris lutris, the Asian sea otter, is the nominate species and ranges from the Kuril Islands north of Japan to Russia’s Commander Islands and the Kamchatka Peninsula in the western Pacific Ocean. It was first described by Linnaeus in 1758. In the eastern Pacific Ocean, Enhydra lutris kenyoni, the northern sea otter, is found throughout the Aleutian Islands and southern Alaska and was described by Wilson in 1991. It’s been reintroduced to various locations from south of Prince William Sound, Alaska along the Pacific coast of Canada and into Oregon state in the continental USA. Enhydra lutris nereis, the southern sea otter, is native to central and southern California and was described by Merriam in 1904.
The Asian sea otter is the largest subspecies and has a slightly wider skull and shorter nasal bones than both other subspecies. Northern sea otters are slightly larger than Californian otters. Adult male northern sea otters weigh 27 to 39 kilograms, while females weigh 16 to 27 kilograms. Adult male southern sea otters average 29 kilograms in mass, while females average 20 kilograms. Northern sea otters possess longer mandibles (lower jaws) while southern sea otters have longer rostrums and smaller teeth.
Sea otters are found in two geographic regions on the Pacific Coast: along the Kuril and Commander Islands off the coast of Russia, the Aleutian Islands below the Bering Sea, and the coastal waters off the Alaskan Peninsula to Vancouver Island, Canada; and along the central California coast from Ano Nuevo to Point Sur.
Sea ice limits the sea otter’s northern range to below 57 degrees North latitude, and the distribution of giant kelp (Macrocystis pyrifera) forests limits the southern range to about 22 degrees North latitude.
Sea otters inhabit marine neritic and marine oceanic habitats. They can be found in temperate coastal waters with rocky or soft sediment ocean bottom and live in offshore forests of giant kelp (Macrocystis pyrifera).
They spend most of their active time foraging below the canopy and eat, rest, and groom themselves at the water surface.
While sea otters are capable of diving to depths of at least 45 meters, they prefer coastal waters up to 30 meters deep. The shallower the water, the less time is spent diving to reach food. Pups start diving at 2 months of age.
Sea otters are social creatures and congregate in groups known as bevies, families, rafts, romps, or pods, when resting. They commonly feed in small groups. Because males steal food from females if they get a chance, females tend to avoid males except when mating and forage in separate areas.
Sea otters are diurnal with crepuscular peaks in foraging activity. Sea otters spend 15 to 55% of their time foraging, depending on food availability. Foraging dives usually last 50 to 90 seconds, but otters can remain submerged for nearly 6 minutes. The sea otter feeding process, including foraging, eating, and cleaning their fur after a meal, lasts 2 to 3 hours and sea otters tend to eat 3 to 4 times a day.
Sea otters are long-lived and typically remain in the same area for years. Male sea otters have larger home ranges than females. The home range of a male may overlap with that of several females. Many males actively defend their territories. Disputes are usually settled with splashing and vocal displays, and fighting is rare. Same-sex territories do not overlap and are defended by their owners.
Sea otters communicate through body contact, scent, 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 otters spend the majority of their time in the ocean, but rest on land when the population density is high or during stormy weather. When resting or sleeping, sea otters float on their backs 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. While a mother sea otter is foraging, she also wraps her pup in kelp at the water surface to keep it from drifting away.
Sea otters utilize vertical undulations of the body to swim, tucking in the forelimbs and using the hind limbs and tail to control their motion. Otters can swim as fast as 9 kilometers per hour, or 6 miles per hour, underwater.
Sea otters diligently groom and clean their fur to maintain its insulating ability. Female sea otters groom their pups extensively for 3 months as their coat develops.
Sea otters are carnivorous. They will eat nearly any fish or marine invertebrate they can find in their kelp forest foraging grounds. Their diet consists of marine invertebrate herbivores and filter feeders such as 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.
Sea otters consume 20 to 25% of their body weight each day and usually eat 3 to 4 times a day. They obtain most of their water from prey but also drink seawater to satisfy thirst.
Sea otter 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.
Sea otter hunting occurs on the sea floor.
Sea otters are one of few mammals that exhibit tool use. Sea otters have patches of loose skin under the forearms that they use to help store tools, such as rocks, so they can free up their paws while eating. They also use these loose folds of skin to collect and transport invertebrates during diving. 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.
Sea otters are polygynous, with males having multiple female partners throughout the year. Males mate with females that inhabit their territory. If no territory is established, they seek out females in estrus.Females reach sexual maturity at 4 years of age. Males reach sexual maturity at 5 to 6 years, but may not mate until much later.
When a male sea otter finds a receptive female, the two bond for the duration of estrus, or 3 days. The couple engages in playful and sometimes aggressive behavior. The male holds the female’s head or nose with his jaws during copulation. Visible scars are often present on females from this behavior.
Sea otters are one of several species of mammals that undergo delayed implantation in which the embryo does not implant during the immediate period following fertilization, but remains in a state of suspended growth allowing for birth to occur under favorable conditions. Delayed implantation produces varied gestation times, which has been reported as 4 to 12 months. Orientation of the fetus may be either caudal or cephalic, although cephalic orientation is more common near birth.
Sea otters can reproduce year round, though there are peaks of birth in May to June in the Aleutian Islands and in January to March in California.
Females usually give birth about once a year, though many females experience longer breeding intervals, giving birth every 2 years. A single pup is born weighing 1.4 to 2.3 kilograms. Twins occur in 2% of sea otter births, but only one pup can be raised successfully. If a pup does not survive, the mother may experience postpartum estrus.
Male sea otters do not provide any care for their offspring.
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. Female sea otters groom their pups extensively for 3 months as their coat develops. A pup’s coat traps air, which keeps the animal afloat. Female sea otters carry their pups on their bellies while they nurse. They have two mammae and their milk is 20 to 25% fat. Pups are weaned at around 6 months of age but start to eat solid foods shortly after birth.
Pups start diving at 2 months of age. The pup typically remains dependent on the mother for about 6 to 8 months after birth.
Sea otters are vital to the overall health and diversity of the kelp forest ecosystem. Their presence is believed to be important in the evolution of kelp forest ecosystems. Sea otters are considered a keystone species and play a major role in the community by controlling herbivorous invertebrates. Sea otters prey on sea urchins, thereby preventing sea urchins from overgrazing the kelp forest. This allows the kelp forest to thrive and contributes to an increase in marine diversity. The variety in the sea otter diet reduces competition between benthic grazers and supports greater diversity in those species.
Great white sharks (Carcharodon carcharias) are one of the primary predators of sea otters. Otters are occasionally eaten by coyotes (Canis lantrans) 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 (Haliaeetus leucocephalus). It was once thought that killer whales (Orcinus orca) were responsible for declines in the sea otter population in Alaska, but evidence is inconclusive.
Sea otters compete with commercial fisheries by feeding on shellfish, sea urchins, and crabs.
The fur of sea otters was of great importance in the fur trade from the mid 1700’s to 1911. Sea otter fur was coveted due to its extreme density and insulating quality. Pelts sold for as much as $1,125 each and were fashioned into hats, coats, and other garments sold in Russia, Canada, and the United States.
Sea otters are vulnerable to large-scale population declines, with oil spills being the greatest anthropogenic threat. The Exxon Valdez oil spill of 1989 had a dramatic effect on the Alaskan sea otter population, killing approximately 5,000 individuals.
According to the Otter Foundation, the California sea otter population declined from July 2008 to July 2011. Estimates suggest a California population of approximately 2,700 individuals.
The sea otter was placed under the Endangered Species Act (ESA) in 1973 and is now listed on CITES Appendix I and II. In Canada, sea otters are protected under the Species at Risk Act. As of 2020, the sea otter is considered Endangered by the International Union for Conservation of Nature and Natural Resources (IUCN) Red List of Threatened Species.
Parasites and infectious disease contribute to sea otter mortality, specifically Toxoplasma gondii, which infects domestic cats (Felis catus), and Sarcosystis neurona, which infects opossums. These apicomplexan protozoan parasites infect the sea otter and cause encephalitis. It is postulated that cat and opossum feces travel to storm drains via runoff and disposal in toilets, eventually coming into contact with sea otters.
An acanthocephalen worm (Profilicollis) has also been linked to mortality and decline in the sea otter population.
The fur of sea otters was of great importance in the fur trade from the mid 1700’s to 1911. Hunting during the 18th and 19th centuries has greatly reduced the distribution of sea otters. Sea otters were hunted to near extinction (1,000 to 2,000 individuals worldwide) at until the turn of the 20th century when the United States, Russia, Japan, and Great Britain reached an agreement in 1911 called the International Fur Seal Treaty, banning the hunting of fur-bearing sea mammals. In 1972, the U.S. Marine Mammal Protection Act offered further protection by banning capture and harassment of sea mammals.
In September 2006, Governor Arnold Schwarzenegger passed a law raising the maximum fine for harming a sea otter to $25,000, and required that all cat litter sold in California display a warning label that advises not to dump cat feces down storm drains or in toilets.