which keeps them dry, warm, and somewhat buoyant under water. Several otter species live in cold waters and have high metabolic rates to help keep them warm. European otters must eat 15% of their body weight each day, and sea otters 20 to 25%, depending on the temperature. In water as warm as , an otter needs to catch of fish per hour to survive. Most species hunt for three to five hours each day and nursing mothers up to eight hours each day. Feeding For most otters, fish is the staple of their diet. This is often supplemented by frogs, crayfish and crabs. Some otters are experts at opening shellfish, and others will feed on available small mammals or birds. Prey-dependence leaves otters very vulnerable to prey depletion. Sea otters are hunters of clams, sea urchins and other shelled creatures. They are notable for their ability to use stones to break open shellfish on their bellies. This skill must be learned by the young. Otters are active hunters, chasing prey in the water or searching the beds of rivers, lakes or the seas. 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 prevent their fur becoming waterlogged. Sea otters are considerably more aquatic and live in the ocean for most of their lives. Otters are playful animals and appear to engage in various behaviors for sheer enjoyment, such as making waterslides and sliding on them into the water. They may also find and play with small stones. Different species vary in their social structure, some being largely solitary, while others live in groups – in a few species these groups may be fairly large. Species Extant species Extinct species Subfamily Lutrinae Genus Lutra †Lutra castiglionis – Corsica, Pleistocene †Lutra euxena – Malta, Pleistocene †Lutra nippon – Japan, extinct c. 1979 Genus Lutrogale †Lutrogale cretensis Genus Enhydra †Enhydra reevei Genus †Algarolutra – Corsica and Sardinia, Pleistocene Genus †Cyrnaonyx – Europe, Pleistocene Genus †Enhydriodon – Ethiopia, Late Miocene to Pliocene Genus †Enhydritherium – North America, Late Miocene to Early Pliocene Genus †Limnonyx – Germany, Late Miocene Genus †Megalenhydris – Sardinia, Pleistocene Genus †Paludolutra – Italy, Late Miocene Genus †Sardolutra – Sardinia, Pleistocene Genus †Siamogale – eastern Asia, Late Miocene to Early Pliocene Genus †Sivaonyx – Asia and Africa, Late Miocene to Early Pliocene Genus †Teruelictis