Sea otters are well adapted to their marine environment. Their nostrils and ears can close when diving, and they are able to change the refractive power of their lenses so they can see clearly in water as well as in air. The skeleton of a sea otter is loosely articulated and has no clavicle, allowing the animal a great deal of flexibility when swimming and grooming. A sea otter’s hind feet are flattened and webbed much like flippers, and the fifth digit on each foot is elongated, allowing the otter to swim more efficiently on its back. The front paws are short and have extendable claws and tough pads on the palms, enabling the otter to grip slippery prey, and the teeth are adapted for crushing hard-shelled invertebrates. Sea otters have large, lobulated kidneys that allow them to conserve water and maintain water balance while living in a saltwater environment, Their kidneys efficiently absorb water and eliminate excess salt in urea, a waste product more concentrated than sea water. Sea otters are very buoyant due to their large lungs, which are two-and-one-half times bigger than those of a similar-sized land mammal.
Sea otters are not particularly streamlined, and because of this, they are the slowest swimming of all marine mammals. Top speed for a sea otter is 5.6 mph (9 km/hr), but speeds of 2 to 3 mph (3.2 to 4.8 km/hr) are more common. Sea otters usually swim on their backs while paddling with their hind flippers, but when an otter needs to travel quickly, it swims on its stomach and undulates its entire body. Sea otters are graceful in the ocean, but they aren’t built to travel on land. It is rare to see a sea otter on land, but some like to haul out on rocks, and on Kodiak, we occasionally see sea otters resting on blocks of ice in the winter. When they do travel on land, they travel at a clumsy, rolling gait or run in a bounding motion.
Otters generally dive and feed in fairly shallow water, less than 60 ft.(18.3 m), and they normally only stay under water for one to two minutes, but they have been known to dive as deep as 330 ft. (100.58 m) and remain submerged for as long as four to five minutes. They are able to stay under water this long because of their large lungs that can store an abundant supply of oxygen, and because of their flexible ribs that allow their lungs to collapse under pressure.
A marine mammal must maintain a body temperature near 100° F (37.8° C), and in Alaska, where the water drops as low as 35° F (1.67° C), this can be a challenge. Other marine mammals have a thick layer of blubber to insulate themselves from the cold, but sea otters have very little fat and depend mainly on their fur to keep them warm. Sea otters have the thickest fur of any animal, with 850,000 to one million hairs per square inch (up to 150,000 per square centimeter). It is their dense, beautiful fur that made them so valuable to fur traders in the 1700’s and 1800’s.
The fur consists of two layers. Long guard hairs form the outer layer, and these provide a protective coat that keeps the underfur dry. It is this extremely dense underfur that keeps the otter warm, but to insulate efficiently, the fur must be clean, so sea otters spend a large portion of each day grooming and cleaning their fur. In addition to cleaning his fur, an otter will somersault in the water and rub his body to trap air bubbles in his fur. These bubbles not only provide insulation but also help to keep the skin dry. Since sea otters must have clean fur to stay warm, they are particularly susceptible to the ravages of an oil spill. If their fur becomes oiled, it loses its insulating properties. And when an otter tries to clean his oiled fur, he ingests the toxins from the oil. An otter’s underfur ranges from brown to black, with guard hairs that may be light brown, silver, or black. Alaskan sea otters often have lighter fur on their heads, and the fur usually lightens as an otter ages.
In addition to their warm fur, sea otters maintain their body heat by burning calories at a rapid rate. A sea otter’s metabolism is two to three times higher than that of a similar-sized land mammal. Because its metabolic rate is so high, a sea otter must eat 23 to 33 percent of its body weight each day. That means that a fifty-pound otter will eat 11 to 16 lbs. (5 to 7.3 kg) of food every day. Sea otters also maintain their body heat by keeping their forepaws out of the water and their hind flippers folded over their abdomens when resting and floating. An otter’s paws are covered by very little fur and lose heat rapidly when submerged in cold water.