An otter is any of 13 living species of semi-aquatic (or in the case of the sea otter, aquatic) mammals that feed on fish and shellfish, and also other invertebrates, amphibians, birds and small mammals.
The otter subfamily Lutrinae forms part of the family Mustelidae, which also includes weasels, badgers, wolverines, and polecats. Photos of these relatives are below:
Many otters live in cold waters and have very high metabolic rates to help keep them warm. They must eat 15% of their body weight a day. In water as warm as 10 °C (50 °F), an otter needs to catch 100 grams (3.5 ounces) of fish per hour to survive. Most species hunt for three to five hours a day, and nursing mothers up to eight hours a day.
For most otters, fish is the staple of their diet. This is often supplemented by frogs, crayfish and crabs. Some otters are expert at opening shellfish, and others will feed on available small mammals or birds. Prey-dependence leaves otters very vulnerable to prey depletion.
Otters are very active, chasing prey in the water or searching the beds of rivers and lakes. Most species live beside water, but river otters usually enter it only to hunt or travel, otherwise spending much of their time on land to avoid their fur becoming waterlogged.
Otters are playful animals and appear to engage in various behaviors for sheer enjoyment. Different species vary in their social structure, with some being largely solitary, while others live in groups – in a few species these groups may be fairly large.
The range of the North American river otter has been significantly reduced by habitat loss, beginning with the European colonization of North America. In some regions, though, their population is controlled to allow the trapping and harvesting of otters for their pelts. River otters are very susceptible to environmental pollution, which is a likely factor in the continued decline of their numbers. A number of reintroduction projects have been initiated to help stabilize the reduction in the overall population.