Sea Otter

The Sea otter (Enhydra lutris) is a large otter native to the North Pacific, from northern Japan and Kamchatka east across the Aleutian Islands south to California. The heaviest of the otters, sea otters are the only species within the genus Enhydra.

Hunted extensively for their luxurious fur, the densest of all mammals with up to 394,000 hairs per square centimeter; from 1741 onwards, sea otter populations were greatly reduced to the point of extermination in many parts of their historic range. By 1911 the world population was estimated to be just 1,000-2,000 individuals in 13 colonies. Its estimated that a half million to a million otters were killed over time and over hunted, and the population is thought to have been 150,000 to 300,000 historically before the years of the great hunt. Although several subspecies are still endangered, the otters have since been legally protected, and reintroduction efforts have shown positive results in some areas.

Physical description

With long, streamlined bodies, sea otters are built for life at sea. They have exceptionally thick brown fur with densities of 26,000 to 165,000 hairs/cm2, insulating the animal and retaining heat. Sea otters have sebaceous gland secretions of squalene, which are normally found only in minor concentrations in other mammals. This creates an effective barrier between the water and the skin and acts as a substitute for subcutaneous insulating fat, as the otters have only 1cm of it. Underneath each powerful front paw is a pouch of skin used to temporarily store food collected during extended dives to the bottom. The front paws also have retractable claws, while the hind flippers are long & broadly flattened and webbed. The fifth digit on the hind flipper is the longest, unlike that of any other mammal and this makes walking on land difficult. Sea otters have a fairly short, thick, muscular tail. They have no scent glands.

They have specially adapted spinal columns and bone structures to allow great flexibility. Sometimes the bones will be dyed pale violet from eating purple sea urchins and absorbing polyhydroxynaphthoquinone. They have 38 chromosomes. Sea otters have large lung capacity compared to pinnipeds: 2 to 4 times greater in size. Sea otters store 66% of their oxygen in their lungs, so the large lungs are well suited for their brief shallow dives. This also helps with buoyancy.

Sea otters have a highly unique eye development for mammals, leading to an accommodation at least 3 times greater than any other mammal. This enables them to see clearly and focus on objects above and below water. They are roughly emmetropic in both conditions.

Sea otters have compact molars with smooth cusps; they are the only carnivore with no more than four lower incisors. Male sea otters may reach a maximum weight of 45 kilograms (nearly 100 pounds) and a length of up to 1.5 metres (nearly 5 feet). Females are smaller. Males are generally 35% heavier and 8% longer and have heavier heads and necks.

In the wild the sea otters live perhaps 15-20 years, and can live longer than 20 years in captivity.

Subspecies

• Enhydra lutris kenyoni (Wilson, 1990)
• Enhydra lutris lutris (Linnaeus, 1758)
• Enhydra lutris nereis (Merriam, 1904)

Habitat and diet

Generally sticking to shallow coastal waters of no more than about 55 metres in depth, sea otters are found most often in areas with rocky coastlines and thick kelp forests; barrier reefs and intertidal areas are also inhabited. These otters may be considered a keystone species; they control the population of certain invertebrates which would otherwise run amok if unchecked. Chief among these invertebrates are sea urchins and abalone, a favourite prey of the otters. To eat prey in shells they often use rocks, which sit on their stomach, to break open the shell in order to get the animal inside.

Crabs, mussels, scallops, cephalopods, fish, chiton, and snails are also prey to the sea otter. Individuals may show finicky preferences; despite this, they require 20-25% of their body weight in food each day, and they may forage for prey as often as every 5 hours. Their metabolism is higher than that of otters in captivity, and is 8 times the level in comparatively sized terrestrial mammals. Part of this may be because they have the lowest assimilation efficiency of carnivores (82%), and their gut transit time is only 3 hours. Also these small mammals lose heat easily to their aqueous environment because of water's high thermal conductivity. Sea otters consume more seawater than most other marine mammals. This is likely because of high urea production from a high protein diet, as well as their prey having a higher electrolyte composition than fish. Their kidneys are comparatively larger to other marine mammals in order to offset this.

Otters are taught feeding practices by their mothers, and groups of otters often show matrilineal differences in food preferences. Some lineages of otters are noted for their preference for gulls or sand crabs as food sources.

The otters themselves are threatened mostly by humans, sharks and orcas.

Otters frequently eat while floating on the surface of the water. Otters like to relax and groom themselves this way after a hunt, and also use this method to fill up their fur with warm air bubbles for insulation.

Behavior

Sea otters may be either solitary or may live in groups called rafts, and when resting, they cling to kelp so they don't float away while they are taking a nap. Females avoid males outside of breeding periods, and the otters segregate into male & female areas. Females may be seen in matrilineal groups with grand dams, adult daughters, and then youngest offspring sharing an area. Males sometimes concentrate in "bachelor groups" near the female areas. Conflicting data regarding home range and migration of sea otters suggests their movements are dependent on availability of resources. Home ranges may be as large as 5.4 square kilometers, with most animals traveling found within 1-2 kilometers of the previous day's location.

They are diurnal animals, & most of a sea otter's day is taken up by foraging and grooming. Feeding activity peaks in the early morning and in the evening (crepuscular feeding); dives are fairly short, typically lasting no more than about 90 seconds. The otters wrap themselves in mats of kelp, securing them from the sway of currents while resting and feeding. Floating on their backs, the otters wash, and (if necessary) pry open their prey with a favored rock they keep in their pouch. Sea Otters present a rare example of mammalian tool use.

Breeding & Reproduction

No set breeding season has been observed, but peaks occur from May to June in northern populations and January to March in southern populations. Males reach sexual maturity around 5-6 years and have been known to still sire offspring at 19. Females come into estrus at 4-5 yet sometimes as early as 2-3 years of age. Sea otters are polygynous: that is, males have multiple female partners. Females in captivity are seen to be polyestrus, coming into estrus in late winter/spring and then again in late summer/autumn. Bonding does occur between the sexes during the female's estrus, which lasts 3-5 days. During these periods males will defend their territories; there is very rarely actual fighting involved, with most disputes being settled by raucous posturing. Females have characteristic scars on the nose from the males' habit of holding the formers' muzzles in their jaws during copulation.

Courtship between otters is very playful. When males and females are courting they swim and dive together, with the male twisting and doing corkscrews in the water to let the female know he is interested. The male will also swim facedown and they will swim more quickly than they would usually. The actual mating takes place in the water and can be rough, the male biting the female on the side of the face or on the muzzle, sometimes holding her head under water, on occasion this can be even fatal.

There is a delayed implantation, (where the fertilized egg doesn't immediately implant in the uterine wall, the endometrium of the uterus, and remains unmoving), this delayed implantation in California otters lasts 2-3 months with gestation taking 6 months or so. In Alaska gestation can be longer and around 7-8 months, with 3.5-4.5 months of this in an unimplanted phase. Gestation usually ends in a single birth; twins are a rarity, and usually only one of them survives. The brownish yellow pups are born at around 1-2kg, with a natal-pelage (baby fur) that last for 3 or 4 months. They are totally dependent on the mother during this time for food and grooming, and she usually carries them around constantly on her upturned belly, nursing for 4 weeks or so, and then offering bits of prey. The milk is more similar in composition to other marine mammals than to mustelids, with 23% fat, 13% protein, and only 1% lactose. Sometimes she will place the pup on a haul out area or floating on the water while she searches for food or grooms, during which the pup calls out loudly for her return. Pups are dependent for 5-15 months (averaging 6) and many times only 30% of pups survive their first year. Experienced mothers have the best success. Occasionally adoptions have been seen. The mother is responsible for teaching the pups how to hunt, dive, and groom effectively.

The Great Hunt

In the 1600s to the 1700s, Russia was heavily involved in the sable fur trade. Peter the Great declared that Russia should have the monopoly in sable furs, and that they should find new populations of sable to hunt. The Russians conquered Kamchatka, in the eastern edge of Russia, where they could harvest sable and sea otter fur. The Russians continued to explore the north Pacific, looking for sea routes to Japan and America. Vitus Bering and Alexi I. Chirikov, two Navy officers, were given the task of mapping the Arctic coast and finding a way to America. Chirikov managed to return to Kamchatka in 1741, after much difficulty. Bering's sailors suffered from scurvy, and were marooned on Bering Island, in the Commander Islands, where Bering died. There, the surviving crew spent the winter hunting sea otters and gambling with otter pelts. They finally returned home in 1742 with 900 sea otter pelts, enough to pay for the entire expedition. It was Bering's expedition that set off the Great Hunt that was to continue for another hundred years.

The Russians sent many ships to harvest otter furs, and soon depleted all of the otter populations in the Commander Islands. They moved on to the Aleutian Islands, which were already inhabited by the Aleuts. Since the Aleuts could not repel the Russians, the Russians did everything they could to get their hands on otter fur, even taking hostages and forcing the islanders to hunt for them. It wasn't only otters that were hunted; the Russians wiped out the local fox population, along with the Aleuts. It wasn't until 1776, when Captain James Cook reached the north Pacific that other nations joined in the hunt. Captain Cook was killed in Hawaii by natives, but his crew sailed on to the lively trading port of Guangzhou in China, where they found merchants offering outrageous amounts of money for even the smallest scrap of otter fur. The sailors almost mutinied in their desire to return for more otters. It was then that the English discovered the value of these skins.

The Great Hunt continued, with Russians, Americans, and other Europeans competing amongst each other for that prized commodity. The Alaskas were almost completely depleted of sea otters, causing the Russians to sell it to the United States in 1867. Now, the sea otters were being slaughtered by the Americans. It wasn't until 1911 that an international treaty was signed to stop the hunting of sea otters. So few otters remained, some estimate only 1,000-2,000 in the wild, that many assumed they would become extinct. In 1938, biologists found a group of sea otters along the coast south of Carmel, California. This group and other surviving groups would form the nucleus for the restoration of the sea otters.

Modern threats

Oil spills

Even though otters are no longer at risk because of hunters, oil threats are increased, and as they are an inshore species this is one of the greatest current threats to these endangered marine mammals. It is estimated that around 5000 otters were killed as a result of the Exxon Valdez oil spill, with roughly 1000 carcasses being found fairly easily. They are particularly vulnerable, as sea otters rely on their fur for heat, and oil spills cause their air pockets to break. This can cause hypothermia and even death. Also, because of otters' constant grooming habits, when they ingest this oil, they can receive damage to the liver, kidneys, eyes, and other organs. The Exxon spill was seen to cause lesions of the kidneys, liver, hemorrhaging, erosion of the stomach and GI tract, stress and shock, and as well emphysema from the volatile fractions of the oil. These all frequently led to many dead sea otters. People made an effort to try and save some of the sea otters during this spill, and they were washed in diluted Dawn dishwashing detergent, given supportive fluids, antibiotics, corticosteroids, vitamins, and oral activated charcoal. They were then rinsed in fresh water and dried off with pet hair dryers, all the while body temperature was carefully adjusted and they were sedated or anesthetized for much of this. They needed to be maintained for long periods to recover well enough to be released back to the wild. This effort helped lower death totals, yet ten years after the spill the otters were still being impacted through lower survival and reproductive rates. Main causes for oil spills are tankers, and the waste that is thrown out after cleaning them.

Medical problems and pollution

Sea otters were killed during nuclear testing near Amchitka Island in Alaska, and were found with lesions in the lung, heart, brain, and ear.(MMM 2001)

References

• Estes (2000). Enhydra lutris. 2006 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on 11 May 2006. Database entry includes justification for why this species is endangered
• Enhydra lutris (TSN 180547). Integrated Taxonomic Information System. Accessed on 18 March 2006.
• Marine Mammal Medicine, Leslie Dierfauf & Frances Gulland,

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