Sea otters, Enhydra lutris, 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 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.
Three subspecies of E. lutris are recognized today. Enhydra lutris lutris ranges from the Kuril Islands north to the commander islands in the western pacific. Enhydra lutris nereis is found off the coast of central California. Enhydra lutris kenyoni is distributed throughout the Aleutian Islands and southern Alaska, and has been reintroduced to various locations from south of Prince William Sound, Alaska to Oregon. (Estes, 1980; Lockwood, 2006; Nowak, 1999; Wilson, et al., 1991)
Sea otters inhabit temperate 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, they prefer coastal waters up to 30 meters deep. The shallower the water, the less time is spent diving to reach food. (Estes, 1980; Nowak, 1999; Paine, 1993)
Alaskan sea otters are slightly larger than Californian otters. Adult male Alaskan otters weigh 27 to 39 kg, while females weigh 16 to 27 kg. Adult male California sea otters average 29 kg in mass, while females average 20 kg. Individuals can weigh as much as 45 kg. Males measure 1.2 to 1.5 m in length, while females measure 1 to 1.4 m. The tail comprises less than a third of the body length, measuring 25 to 35 cm.
The pelage is brown or reddish brown. 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 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.
Sea otters have circular, furry faces with short noses, rounded eyes and ears, and long whiskers that assist in foraging for food. The 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 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 are the only carnivores with just 4 lower incisors. Females have two mammae. (Estes, 1980; Nowak, 1999; Paine, 1993; SeaWorld Parks & Entertainment, 2011)
Sea otters are polygynous, with males having multiple female partners throughout the year. 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 3 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. (Estes, 1980; McShane, et al., 1995; Nowak, 1999; OceanLink, 2011; SeaWorld Parks & Entertainment, 2011)
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. 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. Females usually give birth about once a year, though many females experience longer breeding intervals, giving birth every 2 years. If a pup does not survive, the mother may experience postpartum estrus.
Orientation of the fetus may be either caudal or cephalic, although cephalic orientation is more common near birth. A single pup is born weighing 1.4 to 2.3 kg. Twins occur in 2% of births, but only one pup can be raised successfully. Pups typically remain with their mother for 5 to 6 months after birth. 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. (Estes, 1980; McShane, et al., 1995; Nowak, 1999; OceanLink, 2011; Riedman, et al., 1994; SeaWorld Parks & Entertainment, 2011)
Male sea otters do not provide any care to their offspring. Pups are weaned at around 6 months of age but start to eat solid foods shortly after birth. Females carry their pups on their bellies while they nurse. Their milk is 20 to 25% 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 3 months as their coat develops. A pup’s coat traps air, which keeps the animal afloat. Pups start diving at 2 months of age. The pup remains dependent on the mother for about 6 to 8 months. (Lockwood, 2006; Paine, 1993; SeaWorld Parks & Entertainment, 2011)
The maximum estimated lifespan of sea otters is 23 years in the wild. (Nowak, 1999)
Sea otters congregate in groups known as rafts or pods when resting. Females tend to avoid males except when mating. Sea otters are long-lived and typically remain in the same area for years. They spend the majority of their time in the ocean, but rest on land when the population density is high or during stormy weather.
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 km per hour under water. Sea otters are diurnal with crepuscular peaks in foraging activity. Foraging dives usually last 50 to 90 seconds, but otters can remain submerged for nearly 6 minutes. Sea otters spend 15 to 55% of their time foraging, depending on food availability.
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. (Cohn, 1998; Estes, 1980; Estes, et al., 1986; Fisher, 1939; Limbaugh, 1961; Nowak, 1999; Paine, 1993)
Male sea otters have larger home ranges than females. The home range of a male may overlap with that of several females. Same-sex territories do not overlap and are defended by their owners. (SeaWorld Parks & Entertainment, 2011)
Sea otters 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. (SeaWorld Parks & Entertainment, 2011)
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. 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. Otters consume 20 to 25% of their body weight each day. They obtain most of their water from prey but also drink seawater to satisfy thirst. (Cohn, 1998; Estes, 1980; Estes, et al., 1986; Limbaugh, 1961; Nowak, 1999; Paine, 1993; SeaWorld Parks & Entertainment, 2011)
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 2 to 3 hours. Sea otters usually eat 3 to 4 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. (Cohn, 1998; Estes, 1980; Estes, et al., 1986; Fisher, 1939; Limbaugh, 1961; Nowak, 1999; Paine, 1993; SeaWorld Parks & Entertainment, 2011)
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. (Estes, 1980; Estes, et al., 1998; Kuker and Barrett-Lennard, 2010; Limbaugh, 1961; McShane, et al., 1995)
Sea otters are vital to the overall health and diversity of the kelp forest ecosystem. They 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. The presence of sea otters is believed to be important in the evolution of kelp forest ecosystems.
Two apicomplexan protozoan parasites, Sarcosystis neurona and Toxoplasma gondii infect the sea otter causing encephalitis. An acanthocephalen worm (Profilicollis) has also been linked to mortality and decline in the population. (Cohn, 1998; Estes and Duggins, 1995; Estes and Palmisano, 1974; Estes, 1980; Estes, et al., 1978; Jessup, et al., 2004)
The fur of sea otters was of great importance in the fur trade from the mid 1700s to 1911. Their 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. (Cray, 2006; Nowak, 1999)
Sea otters feed on shellfish, sea urchins, and crabs, competing with commercial fisheries. (Nowak, 1999)
Sea otters were hunted to near extinction (1000 to 2000 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. The Exxon Valdez oil spill of 1989 had a dramatic effect on the Alaskan sea otter population, killing approximately 5,000 individuals.
Parasites and infectious disease contribute to sea otter mortality, specifically Toxoplasma gondii, which infects domestic cats, and Sarcosystis neurona, which infects opossums. 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. 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.
According to the Otter Foundation, the California sea otter population declined from July 2008 to July 2011. Estimates suggest a California population of approximately 2700 individuals. Enhydra lutris 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 2008, E. lutris is considered endangered by the IUCN. Sea otters are vulnerable to large-scale population declines, with oil spills being the greatest anthropogenic threat. (Cohn, 1998; Cray, 2006; Doroff and Burdin, 2011; Hilton-Taylor, 2000; Jessup, et al., 2004; Nowak, 1999)
Joe Allegra (author), San Diego Mesa College, Rhiannon Rath (author), San Diego Mesa College, Aren Gunderson (author), University of Northern Iowa, Paul Detwiler (editor), San Diego Mesa College, Gail McCormick (editor), Special Projects.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
uses sound to communicate
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
an animal that mainly eats meat
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
active at dawn and dusk
having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.
in mammals, a condition in which a fertilized egg reaches the uterus but delays its implantation in the uterine lining, sometimes for several months.
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
parental care is carried out by females
the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
a species whose presence or absence strongly affects populations of other species in that area such that the extirpation of the keystone species in an area will result in the ultimate extirpation of many more species in that area (Example: sea otter).
eats mollusks, members of Phylum Mollusca
having the capacity to move from one place to another.
specialized for swimming
the area in which the animal is naturally found, the region in which it is endemic.
an animal that mainly eats fish
having more than one female as a mate at one time
mainly lives in oceans, seas, or other bodies of salt water.
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
associates with others of its species; forms social groups.
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement
The term is used in the 1994 IUCN Red List of Threatened Animals to refer collectively to species categorized as Endangered (E), Vulnerable (V), Rare (R), Indeterminate (I), or Insufficiently Known (K) and in the 1996 IUCN Red List of Threatened Animals to refer collectively to species categorized as Critically Endangered (CR), Endangered (EN), or Vulnerable (VU).
uses sight to communicate
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
breeding takes place throughout the year
Cohn, J. 1998. Understanding Sea Otters. BioScience, 48: 151-155.
Cray, D. 2006. What's Killing the Sea Otters. Time: 62-63. Accessed September 25, 2012 at http://www.time.com/time/magazine/article/0,9171,1538645,00.html.
Doroff, A., A. Burdin. 2011. "Enhydra lutris" (On-line). In: IUCN 2012. IUCN Red List of Threatened Species. Version 2012.1. Accessed November 12, 2011 at http://www.iucnredlist.org/details/7750/0.
Estes, J. 1980. Enhydra lutris. Mammalian Species, 133: 1-8.
Estes, J., D. Duggins. 1995. Sea otters and kelp forests in Alaska: generality and variation in a communtiy ecological paradigm. Ecological Monographs, 65: 75-100.
Estes, J., J. Palmisano. 1974. Sea otters: their role in structuring nearshore communities. Science, 185: 1058-1060.
Estes, J., N. Smith, J. Palmisano. 1978. Sea otter predation and community organization in the western Aleutian Islands, Alaska. Ecology, 59: 822-833.
Estes, J., M. Tinker, D. Doak. 1998. Killer whale predation on sea otters linking oceanic and nearshore ecosystems. Science, 282: 473-476.
Estes, J., K. Underwood, M. Karmann. 1986. Activity-time budgets of sea otters in California. Journal of Wildlife Management, 50: 626-636.
Fisher, E. 1939. Habits of the southern sea otter. Journal of Mammalogy, 50: 21-36.
Hilton-Taylor, C. 2000. "Enhydra lutris" (On-line). In: IUCN 2000. IUCN Red List of Threatened Species. Version 2000.1. Accessed December 05, 2001 at http://www.redlist.org/search/details.php?species=7750.
Jessup, D., M. Harris, C. Kreuder, J. Ames, P. Conrad, M. Miller. 2004. Southern Sea Otter as a Sentinel of Marine Ecosystem Health. EcoHealth, 1(3): 239-245.
Kuker, K., L. Barrett-Lennard. 2010. A re-evaluation of the role of killer whales Orcinus orca in a population decline of sea otters Enhydra lutris in the Aleutian Islands and a review of alternative hypotheses. Mammal Review, 40(2): 103-24.
Limbaugh, C. 1961. Observations on the California sea otter. Journal of Mammalogy, 42: 271-273.
Lockwood, S. 2006. Sea Otters. Minnesota: Chanhassen.
McShane, L., J. Estes, M. Riedman, M. Staedler. 1995. Repertiore, structure, and individual variation of vocalizations in the sea otter. Journal of Mammalogy, 76: 414-427.
Nowak, R. 1999. Walker's Mammals of the World. Baltimore and London: John Hopkin's University Press.
OceanLink, 2011. "Sea Otters" (On-line). OceanLink - Marine Sciences Education and Fun. Accessed December 01, 2011 at http://www.oceanlink.info/biodiversity/otter/otter.html.
Paine, S. 1993. The World of the Sea Otter. San Francisco: Sierra Club.
Riedman, M., J. Estes, M. Staedler, A. Giles, D. Carlson. 1994. Breeding Patterns and reproductive success of California sea otters. Journal of Wildlife Management, 58: 391-399.
SeaWorld Parks & Entertainment, 2011. "Otters" (On-line). SeaWorld/Busch Gardens ANIMALS. Accessed November 15, 2011 at http://www.seaworld.org/animal-info/info-books/otters/index.htm.
Wilson, D., M. Bogan, R. Brownell, Jr., A. Burdin, M. Maminov. 1991. Geographic variation in sea otters, Enhydra lutris. Journal of Mammalogy, 72: 22-36.