They have dense, oily fur which traps air for insulation, and a heavier layer of fat for further insulation. They are dark brown on top, pale brown to gray below. River otters are found throughout North America except for the desert southwest and Mexico. They live in streams, rivers, and lakes, all of which are frequently bordered by woods. Their den is a burrow in the bank, usually the former home of a muskrat or beaver.
Otters are mostly nocturnal, and very powerful and graceful swimmers.
They also appear to enjoy sliding down steep slopes, such as the banks of rivers or slopes covered by ice and snow. They tend to do this in groups. They also play with rocks and mussel shells in the water, tossing and diving for them. Unlike other weasels, otters are very social and tend to live in family groups. Otters primarily feed on aquatic animals such as fish, crayfish, frogs, and other organisms.