Otters have the thickest fur of all animals with at least 250,000 hairs per square inch. Some species have up to 1 million hairs per square inch.
They are the only known marine animals that are missing the “blubber” layer of fat under the skin. Their fur retains heat instead.
Sea otters are meticulously clean. After eating, they wash themselves in the ocean, cleaning their coat with their teeth and paws. They have good reason to take care of their coats — it helps them to remain waterproof and insulated against the cold. Sea otters have thick underfur that traps air to form an insulating layer against the chilly waters. This coat is invaluable to otters, but it has worth to some humans as well.
Sea otters were hunted for their fur to the point of near extinction in the early 20th century. Today, they are protected by law.