Females are slightly smaller. An otter's tail is long and tapered, thickest where it joins the body and furred its entire length. The face is broad, and the eyes protrude slightly. In autumn, the normally thick fur grows in even thicker for extra cold resistance. All four feet are wide and webbed between the toes, although the hind pair are used more in swimming than the front pair.
Otters obtain most of their food from the water.
An otter's hearing is acute, its eyesight adequate above water and superb below. An otter is a fast, graceful swimmer, probably the most adept in water of all the land mammals. While underwater, valve-like structures seal an otter's ears and nose, and its pulse rate drops, slowing blood and oxygen circulation and making possible long submersion. An otter's top swimming speed is about seven miles per hour. Otters den on the edges of lakes, rivers or streams, or occasionally on islands or patches of high ground in marshes.