The sea otter has the densest fur of any animal, with up to about 150,000 hairs per square centimetre.
Unlike seals and whales, sea otters do not rely on a thick layer of blubber for insulation. Instead, their dense double-layered coat traps air next to the skin. They spend much of their time grooming to keep this insulating layer clean and functional, because fouled or damaged fur allows cold water to reach the body.
Sea otters are marine mammals of the North Pacific. They often float on their backs while feeding, and many use stones as tools to crack open shellfish. Their diet includes sea urchins, crabs, clams, and mussels. By eating sea urchins, sea otters can help maintain kelp forests, making them an important predator in coastal ecosystems.