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Sea Otter
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Physical Description
The Sea otter or Kalan (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.

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

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 to assist in 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 about 15–20 years, and can live longer than 20 years in captivity.

Habitat and Diet
Generally sticking to shallow coastal waters of no more than about 55 meters 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.

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 and 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 or 2 kilometers of the previous day's location.

They are diurnal animals, and 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 and 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.

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