Sea Otter Facts

Scientific Name: Enhydra lutris

Beaver lay on its back floating down a river

 

FRANKHILDEBRAND/Getty Images

Sea otters (Enhydra lutris) are icons for marine conservation on the western coast of the United States. With their furry bodies, whiskered faces, and propensity to lay on their backs and float on the water, they are an easily-recognized and beloved marine mammal. 

Fast Facts: Sea Otters

  • Scientific Name: Enhydra lutris
  • Common Name: Sea otters
  • Basic Animal Group: Mammal
  • Size: 3.3–4.9 feet
  • Weight: 31–99 pounds
  • Lifespan: 10–20 years 
  • Diet: Carnivore
  • Habitat: Coastlines of the North Pacific Rim, from northern Japan to the central Baja peninsula
  • Conservation Status: Endangered

Description

The sea otter belongs to the weasel family.
Rolf Hicker/All Canada Photos/Getty Images

Sea otters are carnivores in the family Mustelidae—a group of animals that also includes 13 terrestrial and semi-aquatic forms such as weasels, badgers, skunks, fishers, minks, and river otters. Sea otters are the only fully aquatic form of otters, but they share features with the others such as thick fur and short ears. This thick fur keeps the animals warm ​but unfortunately has led to the over-hunting of many of these mustelid species by humans. 

Sea otters are the smallest fully marine mammal in the world: males range in length between 3.9–4.9 feet, while females range between 3.3–4.6 feet. The average body mass for males is about 88 pounds, with a range between 49–99 pounds; females range between 31–73 pounds. 

Temperature balance is a significant challenge for sea otters, who lack the blubber of other marine mammals such as pinnipeds. They have a dense fur made up of an undercoat and longer guard hairs that maintains insulation, but it must be almost continuously maintained—fully 10 percent of a sea otter's day is spent grooming its fur. Fur is inflexible insulation, so when necessary, sea otters cool off by exercising their nearly-hairless rear flippers.

Sea Otter in Monterey Bay,CA
Chase Dekker/ Wild-Life Images / Getty Images

Habitat and Distribution

Unlike some marine mammals like whales, who would die if they were on land for too long, sea otters can go up onto land to rest, groom or nurse. They spend most of their lives in the water, however, and can live their entire lives in the water if they need to. Sea otters even give birth in the water.

Although there is just one species of sea otter, there are three subspecies. These are Russian northern sea otter (Enhyrda lutris lutris), which lives in the Kuril Islands, Kamchatka Peninsula, and Commander Islands off Russia; the northern sea otter (Enhyrda lutris kenyoni), which lives from the Aleutian Islands off Alaska, down to Washington state; and the southern sea otter (Enhyrda lutris nereis), which lives in southern California.

Diet

Sea otter eating a crab
Jeff Foott / Getty Images

Sea otters are carnivores, and they eat fish and marine invertebrates like crabs, urchins, sea stars, and abalone, as well as squid and octopuses. Some of these animals have hard shells, making it difficult to get the meat inside. This isn't an issue for the sea otter, which uses rocks as tools to crack the shells of its prey.

To hunt prey, sea otters have been known to dive as deeply as 320 feet; but mostly males forage at depths of around 260 feet and females about 180 feet.

Sea otters have a baggy patch of skin under their forelimbs, and this is used for storage. They can keep extra food in this spot, and also store a favorite rock for cracking the shell of their prey.

Behavior

Sea otters in kelp, Monterey Bay, California, USA
Mint Images - Frans Lanting / Getty Images

Sea otters are social, and hang out together in groups called rafts. Sea otter rafts are made up of either male otters​ or females and their young and can consist of anywhere from two to over 1,000 otters. Only adult males establish territories, which they patrol during mating season to keep out other adult males. Females rove freely between and among male territories, and otters generally rest in sex-segregated groups.

Reproduction and Offspring

Sea otters reproduce sexually when the female is in estrus, in a polygynous system that allows males to monopolize access to females within their breeding territories. The gestation period is six months, and females nearly always give birth to a single pup, although twinning does occur. The pup stays with the mother for an additional six months, learning to forage, and then the females enter estrus again within several days to weeks after weaning. 

Young sea otters have a form of extremely woolly fur. This fur makes an otter pup so buoyant that it can't dive underwater. Before a mother otter leaves to forage, it wraps the pup young in a piece of kelp to keep it anchored in one spot. It takes 8–10 weeks for the pup to shed its initial fur. 

Female sea otters become sexually mature at about 3 or 4 years of age; males at 5 or 6 although most males don't establish a territory until they are 7 or 8. Female otters live between 15–20 years and can have pups every year from the first estrus; males live between 10–15 years.

A Keystone Species

Sea Otter (Enhydra lutris) eating sea urchin, Monterey Bay, California, USA
David Courtenay / Getty Images

Sea otters are a keystone species, that is, they play an important role in the food web of the kelp forest, so much so that even terrestrial species are influenced by sea otter activity. When sea otter populations are healthy, urchin populations are kept in check, and kelp is abundant. Kelp provides shelter for sea otters and their pups and a variety of other marine organisms. If there is a decline in sea otters due to natural predation or other factors, such as an oil spill, urchin populations explode. As a result, kelp abundance decreases and other marine species have less habitat.

A study published in 2008 showed that when sea otter populations were abundant, bald eagles preyed primarily on fish and sea otter pups, but when sea otter populations declined due to predation by an increased population of orcas, bald eagles preyed more on marine birds.

A 2012 study showed the role sea otters can play in reducing carbon dioxide in the atmosphere. It found that if sea otter populations increase, urchin populations will be controlled and kelp forests will thrive. Kelp can absorb carbon dioxide from the atmosphere, and, the study found, that kelp can absorb as much as 12 times the amount of CO2 from the atmosphere than if it were subject to sea urchin predation. 

Conservation Status

Sea Otter Skins, Unalaska,1892
Gulf of Maine Cod Project, NOAA National Marine Sanctuaries / National Archives

Sea otters first became protected from the fur trade by the International Fur Seal Treaty in 1911, when the population had decreased to about 2,000 as a result of unrestrained hunting for furs. Since then, sea otter populations have rebounded, but the International Union for Conservation of Nature (IUCN) lists the species as a whole as Endangered. The ECOS Environmental Conservation Online System lists both northern and southern sea otters as threatened.

Sea otters in the U.S. today are protected under the Marine Mammal Protection Act.

Threats

Sea otters are heavily affected by oil spills due to their dependence on their fur for warmth. If oil covers a sea otter's fur, air cannot penetrate it and the sea otter will get too cold. The infamous Exxon Valdez spill killed at least several hundred sea otters and affected the sea otter population in Prince William Sound for well over a decade, according to the Exxon Valdez  Oil Spill Trustee Council. 

While sea otter populations increased after protection, there have been recent declines in sea otters in the Aleutian Islands (thought to be from orca predation) and a decline or plateau in the populations in California.

Other than natural predators, threats to sea otters include pollution, diseases, parasites, entanglement in marine debris, and boat strikes.

Sources