Sea otters have the densest fur of any mammal, with the ability to sport as many as 1 million hairs per square inch of fur, and as many as 800 million hairs covering its body. This thick fur is made up of a combination of short undercoat hairs and longer fur made up of what are known as guard hairs, which may be lighter in color than the undercoat.
This fur is perfect for the sea otter, which spends a lot of time in cold water but, as a warm-blooded animal, must stay warm. The sea otter's dense fur helps to insulate the animal's skin by trapping air inside the hairs, forming a protective layer of insulation around the skin. Other marine mammals have a layer of blubber that helps them stay warm, but otters lack that layer of fat and must rely on their thick fur.