The otter (Lutra lutra) is a member of the Mustelidae family and the best adapted to life in water. Adult males weigh around 9 kg and are slightly bigger than the females, which usually weigh a third less. The otter's body is covered with thick, dark brown fur, which enables it to stay dry in the water. Otters live in streams, rivers and lakes and, in some parts of Europe, even in the sea. Otters are good swimmers and swim fast underwater to catch fish, their primary source of food. They also feed on freshwater crustaceans and, in the warmer part of the year, on amphibians, when these are found in and by the water.