There are 13 species of otter in four genera, all of which are relatives of the weasel, but there is only one species of sea otter. The more diverse river otters live primarily in freshwater rivers and streams, but are also found in coastal saltwater areas. Sea otters only exist in oceanic environments.
River otters are typically about one-quarter the size of sea otters, and they have tails that are longer and more rounded. While sea otters have rear flippers that are much larger than their front feet, a river otter’s limbs are of roughly equal size. A sea otter’s fur is denser than a river otter’s, but all species spend most of their time grooming their fur so it maintains its ability to insulate the animal in water.
Sea otters rarely venture on land, where their movements are awkward and clumsy. In contrast, the agile river otter is frequently ashore. River otters maintain at least one permanent den on land, near the water, as well as several temporary shelters. While all otters consume fish and other aquatic animals, river otters also forage for food along the shore, and they prefer to eat on land rather than in the water.
River otters swim on their bellies with most of their bodies submerged. While river otters paddle through the water using all four of their webbed feet, otters swim primarily with their rear flippers, using their tails as rudders. Sea otters spend nearly all of their time in the water, swimming on their backs with their bodies high in the water. They even eat and sleep in the water, while floating on their backs. Sea otters dive deeper and can stay under water longer than their freshwater cousins.
Sea otters are primarily active during the day, while many river otters are nocturnal. Sea otters congregate in small to large groups, although males and females keep separate, coming together only to mate. In contrast, most river otters are more solitary. Those that are seen together are small family groups. Male and female adult river otters have separate dominance hierarchies, and although they tolerate each other they do not travel together. River otters are generally more territorial than sea otters, but they practice avoidance rather than confronting or challenging each other.
River otters reach sexual maturity at a younger age than sea otters. They tend to have one litter per year of up to four pups, while sea otters only have one pup at a time, with a longer interval between births. Freshwater otter pups are born in dens on land and are blind, toothless and completely helpless at birth. Sea otter pups are born in the water. Although they remain dependent on their mothers, sea otter pups are more developed than river otter pups at birth, with open eyes and emerging teeth.
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