River otters are nocturnal (most active at night). During the day, they rest in underground dens.

Anatomy: River otters are from 3 to 4 feet (0.9-1.3 meters) long and weigh about 44-82 pounds (20-37 kg). The tail is 1 to 1 1/2 feet (30-45 cm) long. Webbed feet help the otter swim. These sleek mammals have dense fur, short legs, and semi-retractible claws. Both the ears and the nostrils close when the animal is under water.

Fur: River otters are kept warm by their dense fur and high metabolism. They are active all year long, even through very cold winters. Careful grooming with their forepaws keeps the fur waterproof.

Diet: River otters are carnivores (meat-eaters). They eat crustaceans (like crayfish), slow-swimming fish (like trout), amphibians, insects, small mammals and birds. They hunt for their prey in the water and on the land. These intelligent mammals have a keen sense of smell; they use their sensitive whiskers to help find prey in muddy water.

Predators: River otters are preyed upon by the bald eagle, some bears and coyotes.

Classification: kingdom Animalia, Phylum Chordata, Class Mammalia (mammals), Order Carnivora, Family Mustelidae (weasels, ferrets, minks, skunks, otters, badgers), Genus Lontra.