Only the hind feet are webbed. On the average, females are about 25 percent smaller than males. When prime, river otter fur appears black-brown, with the belly slightly lighter in color than its back. The chin and throat are grayish. Otter fur consists of a very dense undercoat overlaid with longer guard hairs, which are usually removed by furriers.
River otters appear to have well-developed senses of smell and hearing.
Their vision is not especially good but may be better underwater than above. Several sets of strong whiskers are used by the animal in hunting and avoiding obstructions. May. Mating can take place in or out of the water. The pups are born toothless and blind in a den that is usually a subterranean burrow. Their eyes open 7 weeks later.