Seal

Seal — The true seals are a diverse and widely distributed group of mostly marine, aquatic mammals. They are also called the earless seals because they lack external ears, having only a tiny, wrinkled ear opening on each side of the head. The true seals, family Phocidae, are classified with the eared seals (sea lions and fur seals), family Otariidae, and the walruses, family Odobenidae, in the Pinnipedia–the pinnipeds are regarded as either a suborder of the order Carnivora or a separate order. The true seals comprise 18 living species grouped into 13 genera; 10 extinct genera are also known. The most numerous seal is the crabeater, Lobodon carcinophagus, estimated at 14 million individuals.

The true seals are predominately a marine group, but two species, the Caspian Sea seal, Pusa caspica, and the Lake Baikal seal, P. sibirica, inhabit large inland bodies of water. The Baikal seal is the only completely freshwater species, but various populations of the ringed seal, P. hispida, and the harbor seal, Phoca vitulina, also live in freshwater lakes or rivers. The true seals are especially numerous in the colder waters (above 40 deg latitude) of both hemispheres, with concentrations in the polar regions, but the monk seals, Monachus, are a tropical group, formerly widespread in the Caribbean and Mediterranean seas and around Hawaii.

Seals range in size from 125 cm to 6.5 m (4 ft to 21 ft) in length and from 90 kg to 3.5 metric tons (200 lb to about 8,000 lb) in weight. The smallest are the ringed seals, Pusa, and the largest, the elephant seals, Mirounga. The hind limbs of the true seals are turned permanently backward and cannot be pivoted forward under the body to help propel the animals across land. The true seals must therefore employ a wriggling or hunching movement, rolling or sliding whenever possible, when progressing over land. Although this method of travel is generally ineffective and slow, the crabeater seal has been known to outrace men. In the water, seals swim gracefully, using side-to-side undulations of the hind limbs. The best diver is the Weddell seal, Leptonychotes weddelli, which has been observed to remain submerged for 70 minutes and to dive to depths of 600 m (2,000 ft). Fish, crustaceans, and squid are the most important food items, but the leopard seal, Hydrurga leptonyx, feeds regularly on warm-blooded animals, including penguins and other seals.

Although the evidence is inconclusive, it is thought that at least some seals have an echolocation system akin to that of bats, porpoises, and shrews; this notion appears probable because some seals must hunt food and find breathing holes during months of constant polar darkness.