Sea otters are highly specialized marine mammals, and they possess the densest fur of any mammal. Except for its nose and pads of its paws, the sea otter's body is covered in thick fur. The coat consists of two layers. The short brown underfur can be as dense as one million hairs per square inch, making it the densest fur of any mammal.
A top layer of long, waterproof guard hairs helps to keep the underfur layer dry by holding cold water away from the skin. This layer is typically deep brown with silver-gray highlights. The color of the head and the neck are lighter than the rest of the body. Sea otters depend upon their fur to stay warm.