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North American river otters are semi-aquatic mammals, with long, streamlined bodies, thick tapered tails, and short legs. They have wide, rounded heads, small ears, and nostrils that can be closed underwater. The whiskers are long and thick, reflecting their importance in sensory perception. Fur is dark brown to almost black above and a lighter color ventrally. The throat and cheeks are usually a golden brown. The fur is dense and soft, effectively insulating these animals in water. Feet have claws and are completely webbed.
Individuals live alone or in family groups, typically females and their young. They are known as playful animals, exhibiting behaviors such as mud/snow sliding, burrowing through the snow, and waterplay. Many "play" activities actually serve a purpose as some are used to strengthen social bonds, to practice hunting techniques and to scent mark. North American river otters get their boundless energy from their very high metabolism, which also requires them to eat a great deal during the day. They are excellent swimmers and divers, able to stay underwater for up to eight minutes. They are fast on land, able to run up to 18 miles per hour. They normally hunt at night, but can be seen at all times of day. These otters communicate in a variety of ways, including vocalizing with whistles, growls, chuckles, and screams. They also use touch and communicate through posture and other body signals. Their large and abundant whiskers are very sensitive and are important in tactile sensation. These whiskers are used extensively in hunting, as smell, vision, and hearing are diminished in the water.
North American river otters eat mainly aquatic organisms such as amphibians, fish, turtles, crayfish, crabs, and other invertebrates. Birds, their eggs, and small terrestrial mammals are also eaten on occasion. Prey is captured with the mouth, and mainly slow, non-game fish species are taken, e.g., suckers. The otter's long whiskers are used to detect organisms in the substrate and the dark water. Prey is eaten immediately after capture, usually in the water, although larger prey is eaten on land.
In Alaska, river otters breed in the spring, usually in May. Mating can take place in or out of the water. Following a nine to 13 month gestation period, two or three pups. Delayed implantation accounts for this variation in the length of gestation. The pups are born toothless and blind in a den that is usually a subterranean burrow. Their eyes open seven weeks later. Pups are weaned at about five months and will stay with their mother until shortly before her next litter is born.
Populations were once wiped out through many parts of their range, especially around heavily populated areas in the midwestern and eastern United States. Population trends have stabilized in recent years and reintroduction and conservation efforts have resulted in recolonization of areas where they were previously wiped out. Northern river otter populations are still considered vulnerable or imperiled throughout much of their range in midwestern United States and the Appalachian mountains. They are presumed to be wiped out in New Mexico and population status in South Carolina and Florida has not yet been reviewed.