One of the most valuable commodities in the fur industry, the fur of the otter consists of two different layers. The outer layer is composed of tightly knit guard hairs that form a watertight barrier between the animal and its environment. The underfur of the otter is even denser than the guard hairs and, in fact, at as many as one million hairs per square inch, is believed to be the thickest fur of any mammal on the planet. When this underfur is in good condition and well groomed, it is capable of trapping an insulating layer of air, keeping the otter’s skin dry and protected from the cold of surrounding water.