One Of A Kind - The Sea Otter

A Monotypic Species

The sea otter has been held up as a classic keystone species. Keystone species affect an ecosystem much more profoundly than their size or numbers would suggest. The sea otter is an essential element in keeping its habitat healthy. Part of their nourishment comes from benthic (sea floor) herbivores such as sea urchins. Sea urchins graze on the kelp, eating through the lower stems. The kelp then drifts away and dies. When the kelp forest is destroyed in this way, there is an enormous flow-on or cascade effect on the marine ecosystem. The sea otter helps keep a viable balance of kelp forest and herbivores. Other keystone species are the mountain tapir and the prairie dog.

The sea otter is also a monotypic species, the only member of the genus Enhydra. Other monotypic species include the giraffe, okapi and numbat.

Its full taxonomic name is Enhydra lutris. ‘Enhydra’ derives from Ancient Greek for ‘in the water’ and ‘lutris’ is Latin for ‘otter’. There are three recognised subspecies.

It belongs to the family Mustelidae but is unique to that group as it has no functional anal scent glands, does not utilise dens or burrows and can live its entire life in the water.

The fur of the sea otter, thicker than any other mammal, led to the killing of hundreds of thousands of sea otters. Large-scale hunting began in the 18th century and continued virtually unabated until 1911 when it was estimated that only 1,000 to 2,000 remained in the wild.

Distribution Map(88706)Credit: Wikimedia

The sea otter
is found on the coasts of the northern and eastern North Pacific Ocean.

The sea otter is a marine mammal. Adult males weigh between 22 and 45 kg and are the heaviest members of the weasel family but one of the smallest marine mammals. Adult females weigh from 14 to 33 kg. Length varies from 1 to 1.5 metres. Rather than a layer of blubber, the sea otter has exceptionally thick fur, the densest of any mammal, with up to 150,000 hairs per square centimetre. The long guard hairs are waterproof and keep the dense, short underfur dry.

The coat is shed gradually year-round. To repel water effectively the guard hairs must be kept scrupulously clean and the otter can reach any part of its body. An exceptionally supple skeleton and loose skin helps make grooming easier. The pelage may range from yellow- or grey-brown to almost black but is generally a deep brown with silver-grey speckles. The head, throat and chest are lighter in colour.

The male otter has a baculum or penis bone which is massive for the otter’s size. It is some 150mm in length, bent upwards and 15mm wide at the base.

Adaptations to Marine Living
The nostrils and small ears can close tight. The fifth digit on the hind paws is longest, making walking difficult but handy when sculling on its back.

Foot of Sea OtterCredit: Wikimedia

Because the hind paws provide most of the propulsion for swimming, they are flat, long, broad and fully webbed. The muscular tail is reasonably short, thick and somewhat flattened. The claws on the front paws are retractable and short. The palms have tough pads which are well able to grip slippery prey.

A loose pouch of skin under each foreleg extends across the chest and forms a pouch which they use to store food to bring to the surface to consume. Each otter also has its own favourite stone to break open shellfish which it keeps in the pouch. Once back on the surface the otter eats while floating on its back. Otters feed alone but rest together with members of their own sex in rafts of 10 to 100 animals. They may wrap themselves in kelp to keep from drifting.

It is diurnal or crepuscular, foraging in the morning then resting until late afternoon and sometimes feeding again around midnight. The sea otter is meticulous in its grooming.

Pair of Sea OttersCredit: Wikimedia

Sea otters will 'hold hands' so they don't drift too far while they sleep.

It doesn’t just clean the fur but squeezes out water to introduces more air, untangles knots and rakes out loose hair. The front paws and long whiskers are highly sensitive.

Vocalisations include cooing (females) or grunting (males) when content and whistling, hissing or screaming when distressed or frightened. Pups often sound like seagulls.

It forages on the sea floor, preying mostly on marine invertebrates. The sea otter is one of the few mammal species to use tools, making use of rocks to dislodge prey and to open shells. Dives generally last about a minute although it can hold its breath for up to five minutes. It is the only marine mammal to use its forepaws to lift and turn boulders, pluck snails from kelp and dig for clams. It may also catch fish with its forepaws rather than with its teeth. Abalone requires a specialist technique as it clings with a force equal to 4,000 times its own weight. Multiple dives may be necessary and up to 45 blows in 15 seconds may be delivered with a large stone before the abalone is freed.

Over 100 different prey species are consumed by sea otters.

Its metabolic rate is two to three times that of comparative land mammals. To maintain this, it needs to eat 25 to 38% of its own weight daily to counteract heat loss. Digestion can take as little as three hours (compare the sloth with a time of 18 hours!). It needs little water but can drink seawater if necessary. Large kidneys utilise sea water and excrete concentrated urine.

Autumn is the peak breeding season and males may maintain a breeding territory from spring to autumn. Temporary pair-bonding occurs for a few days during oestrous. Mating takes place in the water. The male may bite the female on the muzzle or even hold her head under water.

Delayed implantation means gestation can vary from four to twelve months. A single pup is usual and is born in the water. It weighs 1.4 to 2.3 kg, has its eyes open at birth and has ten teeth. The newborn pup has a thick coat of baby fur which the mother licks and fluffs for hours until the coat retains so much air that the pup bobs on the water like a cork. Adult fur replaces the fluff at about thirteen weeks.

Sea Otter with PupCredit: Wikimedia

Nursing periods vary according to the area – 6 to 8 months in California and 4 to 12 months in Alaska. Independence is usually reached at six to eight months. Mortality of newborns is high, as much as 75% dying in their first year. Mothers are devoted to their pups and carry them on their chests out of the cold water. If a pup dies, the mother may continue to carry it for days.

Sexual maturity is reached around 3-4 years (females) and 5 years (males). Wild sea otters live for 15 -20 years (females) and 10-15 years (males).

The sea otter walks on land but spends most of its time in the ocean. It swims by moving the rear portion of the body up and down, reaching speeds of up to 9 kmh. Underwater, the short forelimbs press close to the body, and the body is elongated and streamlined. On the surface it usually floats on its back and sculls with its feet and tail. Its lungs have a capacity roughly two and half times that of similarly sized land mammals. Because of this it is highly buoyant.

Sea otters have pungent scent glands so are not particularly favoured as prey. Young prey mammals may kill otters but not eat them. Orcas and sea lions are predators and bald eagles may snatch pups from the surface of the water. On land, bears and coyotes may attack pups. Ten percent of otter deaths are believed to be the outcome of shark bites, particularly from great white sharks.

Oil spills are deadly to sea otters. Oil-soaked fur loses its capacity to retain air, resulting in hypothermia and death. Inhaling oil or ingesting it when grooming damages lungs, liver and kidneys.

A reputation of depleting shellfish resources such as abalone, clams and crabs has seen the otter at loggerheads with commercial, recreational and subsistence shellfish harvesters.

Unusually high deaths of otters in California has been attributed to diseases, particularly Toxoplasma gondii and acanthocephalan parasite infection. The former is carried by opossums and cats and may be transmitted through the faeces of domestic cats washed into the ocean through sewage systems.

The reintroduction of sea otters to British Columbia has seen a return to healthy coastal ecosystems. However sea urchin populations seem to have been controlled by other means in some areas. The sea otter’s assistance appears to be of more importance in areas of open coasts.

Rocky areas dominated by mussel beds are also better for the involvement of sea otters as the animals remove mussels from rocks, allowing diversification of species and freeing space for competitive species.

Sea otters are popular in public aquariums and zoos and do well in captivity.