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. Dens may be excavations under tree roots or rock piles, abandoned beaver, muskrat or woodchuck burrows, or unused beaver lodges. A typical den has an underwater entrance hole, a living space above water level and several air or exit-entry holes to dry ground.
Otters mature sexually by two years of age.
They breed sometime between January and May, mating taking place in the water. As with many other mustelids, otters have delayed implantation. This means that after fertilization, eggs remain dormant in the female's uterus until the following December, January or February, when they attach to the uterine wall and start to develop. Females usually have their first litter at age three. Pups weigh 4-5 ounces and are blind and toothless at birth.