The sea otter (Enhydra lutris) is a marine mammal native to the coasts of the northern and eastern North Pacific Ocean. Adult sea otters typically weigh between 14 and 45 kg (31 and 99 lb), making them the heaviest members of the weasel family, but among the smallest marine mammals. Unlike most marine mammals, the sea otter’s primary form of insulation is an exceptionally thick coat of fur, the densest in the animal kingdom. Although it can walk on land, the sea otter is capable of living exclusively in the ocean.
The sea otter inhabits nearshore environments, where it dives to the sea floor to forage. It preys mostly on marine invertebrates such as sea urchins, various mollusks and crustaceans, and some species of fish. Its foraging and eating habits are noteworthy in several respects. Its use of rocks to dislodge prey and to open shells makes it one of the few mammal species to use tools. In most of its range, it is a keystone species, controlling sea urchin populations which would otherwise inflict extensive damage to kelp forest ecosystems.
Its diet includes prey species that are also valued by humans as food, leading to conflicts between sea otters and fisheries.
It is a universally accepted truth that sea otters are adorable. But did you know the wettest of weasels are extremely hardy little critters too? They live among the kelp forests that grow off the west coast of North America, Alaska and Russia – where the water temperature can be as low as one degree Celsius (33.8 degrees Fahrenheit) – and almost never come ashore. Heat loss in water is 27 times faster than in air of the same temperature, meaning sea otters face a constant battle to maintain their core body temperature and stay alive.
Unlike other marine mammals, otters don’t have a layer of insulating blubber to keep them warm in chilly waters. Instead, they rely entirely on their velvety fur. At up to a million hairs per square inch (for comparison, you probably have as many hairs on your entire head), it is officially the densest fur in the world.
Sea otters’ coats are composed of two layers: long waterproof guard hairs and a fluffy underlayer. The guard hairs form a waterproof barrier and are kept oiled with sebaceous secretions from glands in the otter’s skin. Bundled around each guard hair are ten to 100 underhairs. These hairs are covered in microscopic barbs that enable the hairs to tangle together and trap a layer of insulating air next to the otter’s skin, providing four times the amount of insulation as the same amount of blubber.