Introduction
Sea otters are fascinating and tremendously appealing animals whose history has been marked by both disaster and recovery. At one time in history, sea otters existed in great numbers along the northern coasts of the Pacific Ocean but due to their more attractive and protective shaggy coat, their discovery exposed them to great danger. An unrelenting onslaught of these sea mammals ensued and within a few decades, great herds of sea otters were killed and the coats used for trade by the Indians.
However, after the species had almost become extinct and their protection began, the species began to recover and towards the close of the 20th century, conservation had given rise to tens of thousands of sea otters. In the 1980s, sea otters were a common occurrence along the beautiful coasts of Alaska, British Columbia, California, and northeast Asia.
But the battle to conserve sea otters was not won yet because over-fishing, whaling, and predator whales during the 21st century once again upset the ecosystem in such a way that only a small fraction of the sea otters that existed twenty years earlier could be accounted for. Governments however stepped in to conserve this sea mammal and today, about 100,000 to 300,000 sea otters now grace the ocean waters under the protection of the law (Kruuk, pp. 3-5; Berta et al., p. 105).
Lifecycle of Sea Otters
The sea otter, Enhydra lutris, is said to be the smallest marine mammal but forms the largest membership in the Family Mustelidae which consists of approximately 70 species of river otters, badgers, skunks, and weasels among others. The sea otters like other otters belong to the weasel family and the largest otter can grow up to 1.8m long from the snout to the tail and weigh about 45 kg. Some sea otters can however weigh as heavy as 40kg. Sea otters have a very long and sleek body with a small head, a short and muscular tail, and sportier limbs than other otters. They have ears and noses that are close to the water.
In terms of their size, sea otters are bigger than other types of otters with adult males weighing about 29kg while females weighing roughly 20kg and have mean total lengths measuring 129cm and 120 cm respectively. Sea otters have shorter tails measuring less than a third of the total body length and have thick fur that gives the mammals a grizzled appearance. Unlike other otters, sea otters have partly retractable and sharp claws on their front feet while the hind feet are flippers.
The sea otter has a larger skull than other otters, using its large jaws to grip struggling prey and crush shellfish. It has long incisors that are used to stab fish and the molars are quite large and flattened being used to crush the food. Sea otters also have five senses like human beings with sensitive whiskers on the snout that help to detect vibrations from swimming creatures as well as find food in clouded muddy waters (Marshall Cavendish Corporation, p. 509; Kruuk, p. 11; Marshall Cavendish Corporation & Gittleman, p. 997).
Sea otters seem to have a good visual acuity both underwater and above the water enabling these creatures to focus clearly on their targets on both surfaces. Unlike most other sea mammals like seals and whales, sea otters do not have fat under the skin to provide heat against the cold water. They are instead helped to keep warm and dry through pockets of air trapped in their glossy magnificent brown fur. The fur is water repellant and they spend a lot of time grooming to keep the fur in good condition.
A sea otter’s coat has two kinds of furs, the outer long and thick guard hairs that are oil-coated and which prevent water from sticking onto the fur, and a finer layer of short hairs underneath. Air trapped between the two layers of hair helps to keep the otter’s skin dry as well as provides insulation from the chilly waters. Sea otters are very clean animals and use their teeth and paws to clean their fur coats and will also wash soon after eating. Swimming and grooming activities as well as feeding also help the sea otter to generate heat (Marshall Cavendish Corporation & Gittleman 995; Berta et al., pp. 140-141, 155).
The normal mating season for sea otters is January to June with a gestation period of 120-360 days (4-12 months) mainly because of a delayed process of implantation. Female otters reach sexual maturity at the age of 3 years while the males take as long as 5 years. Mating takes place in the water and is repeated several times to ensure that all eggs are fertilized. It is usually a very rigorous process that leads to the drowning of the female or she may even be badly bitten by the male.
Male and female sea otters separate soon after courting and mating, each going back to their separate herds. Young pups are usually born in water and very rarely on dry land and the length of time that pups are looked after by their mothers varies between regions. In California for example, mother sea otters spend about 4-9 months looking after their young ones while in Alaska it could be well over 1 year. Mother sea otters carry the pups on their bellies, defending them from any intruders, and only leave them while diving for food.
At birth, pups weigh between 1.5 – 1.8 kg. They are suckled until a time when they can eat solid food. A pup can be able to swim in a few weeks after birth but takes several months to be able to dive. A sea otter gives birth to a single pup about every two years and twins are rare. If twins are born, one is abandoned because it is difficult for a mother sea otter to take care of two pups.
Abandoned and orphaned baby otters are however adopted by other otters, even by males. Sea otters have an average lifespan of about 12 years from birth to death although many of them live up to 20 years (Marshall Cavendish Corporation, pp. 509-510; Kruuk, p. 13; Marshall Cavendish Corporation & Gittleman, pp. 1004-1005).
For food, sea otters mostly depend on large mollusks such as clams and abalones, fish, sea urchins, crabs, octopuses, and squid. The diet however varies between regions and the sea otter itself affects the availability of certain foods within its habitat. Sea otters have for example destroyed entire populations of sea urchins leading to the development of kelp beds that sea otters like to live around.
An otter can carry several abalones and clams at a time under its armpit, often from very great depths and onto the water surface where it opens and consumes them while floating belly-up on the ocean surface. Sea otters are nimble and intelligent creatures, being one of the few world mammals that can make use of tools. They get food from shellfish by smashing them with flat ones. These magnificent sea mammals love foraging in rocky bottoms although, in Alaska, sea otters may be found in large numbers over soft-sediment bottoms. They use their nimble front paws to prize mollusks from rocks as well as dig in the sand for clams.
Tools are also used to hunt by dislodging shellfish from rocks under the water. Sea otters lose a lot of heat in the chilly sea environment which causes heat production at a rate two to three times higher than the heat produced by land mammals of the same size. As a result, food consumption by sea otters is also correspondingly high with an average otter consuming 5-6 kg of food in a single day. An adult sea otter can however consume about 9.5 kg of food in a single day (Marshall Cavendish Corporation & Gittleman, p. 1003; Kruuk, p. 12).
Sea otters spend most of their life out in the sea mainly floating with their belly up on the sea surface and although they seldom venture out into dry land, they occasionally come ashore to rest or sleep. These creatures are very skilled swimmers and divers and push against the water using their webbed hind feet. A sea otter can spend as long as 4 minutes in a single dive.
When at the water’s surface, a sea otter’s locomotion involves pelvic paddling and pelvic undulation when the animal gets submerged. On land, the sea otter moves by either bounding or walking. Walking involves forward and alternate movement of both limbs while bounding occurs when rapid forward locomotion is taking place. They do not move into fresh water and although they have been recorded to dive as far down as 100 meters, they normally go down to a depth of about 30 meters.
In Alaska, sea otters usually forage at 40m deep or more. Sea otters are often to be found along rocky coasts especially where large underwater coral reefs are to be found and occasionally haul out on rocky points on dry land close to the seashore, especially in bad weather. They do not rely on freshwater for washing or even drinking and seawater serves the purpose (Berta et al., p.202, 352; Kruuk, pp. 12-13).
In their daily lives, sea otters are normally lone animals although the males sometimes gather into large social groups numbering about 2000 animals. These large groups are referred to as rafts. Females may also be found in nursing groups of over 100 otters consisting of mothers and pups. These groups are very fluid with sea otters continually joining or leaving the groups. The animals can however be quite social to the extent of defending each other against danger.
Male and female sea otters live in separate groupings and areas although solitary males may be seen in areas where females have gathered together in rafts and try to keep them in the herd. Because sea otters cannot mark their territories using scents, they make large splashes at boundaries to advertise their territories.
At dusk, sea otters gather at the kelp beds where they wrap their bodies in the giant weeds, anchoring themselves in such a way that they are not swept away by ocean currents in their sleep. Sea otters are also very vocal and use different sounds such as shrills for the young, and whine or whistles for the adults especially when they are upset (Marshall Cavendish Corporation, pp. 509-510; Kruuk, pp. 13-14).
During the 18th and 19th centuries, sea otters were killed for their fur in very large numbers. Because of the high quality of their fur, the American Indian tribes slaughtered them in hundreds of thousands and sold their skins to American and British traders. Within a few years, sea otters had almost been drawn to extinction and by the beginning of the 20th century, as few as 1000-2000 sea otters could be found along the east coasts.
In 1911 however, the U.S, Russia, Japan, and Canada entered into an agreement through which hunting of sea otters was stopped and the population started recovering. Sea otter populations revived but soon started declining again due to destruction by predator whales and human activities. In 1989 for example, sea otters were destroyed by oil spills during drilling operations in North America.
More than 2000 otters died when the Exxon Valdez ran aground and spilled its oil cargo off the Alaskan coast of Prince William Sound. Otters have a very slow rate of reproduction that makes them quite vulnerable. The world sea otter population is however gradually increasing and these sea mammals are no longer an endangered species (Kruuk, pp. 11-12; Marshall Cavendish Corporation, pp. 510-511).
Conclusion
Sea otters are spectacular species of otters that are quite different from other species and as a result, have attracted the attention of scientists so that they are also very extensively studied as compared to the other species. This species of sea mammal has characteristics that are rather unique and which continue to excite researchers.
Sea otters are very aquatic as compared to other sea mammals such as seals and are famed for giving birth under the water. They dive to very great depths, drink seawater, and have shaggy and very dense fur that is different from that of other otters. Sea otters access their food using tools and the mothers carry their young ones on their bellies whilst floating on the sea surface. Several hundred sea otters may aggregate into large groups to form what appears like huge rafts (Kruuk, pp. 11-13).
Works Cited
- Kruuk, Hans. Otters: Ecology, Behavior, and Conservation. Oxford University Press, 2006.
- Marshall Cavendish Corporation. Aquatic Life of the World. Marshall Cavendish, 2000.
- Marshall Cavendish Corporation and Gittleman L. John. Exploring Mammals. Marshall Cavendish, 2007.
- Berta, Annalisa, Sumich L James, Kovacs M. Kit, Folkens A. Pieter and Adam J. Peter. Marine Mammals: Evolutionary biology. Academia Press, 2006.