True seals are called earless seals because they lack external ear projections; they have functional inner ears. They have short, coarse hair, usually with a close, dense undercoat. Their color and pattern vary with the species; many are spotted. The pups of most species have fluffy coats of a light color. True seals are generally polygamous and gregarious, but most do not form harems at breeding time, as do the eared seals. Some species have definite migrations, but in most the seals spread out after breeding, singly or in groups, over a wide area of ocean. Some polar species migrate in winter to avoid the advancing ice; members of other species winter under the ice, surfacing through holes to breathe.
Most true seal species fall into one of three geographical groups: northern, antarctic, and warm-water species. Nearly all are marine, but the Baykal seal ( Pusa siberica ) is confined to the freshwater Lake Baykal of Siberia, and the Caspian seal ( P. caspica ) to the brackish Caspian Sea. In addition several populations of the normally marine harbor seals and ringed seals are found in freshwater lakes. The northern seals include two species of temperate coastal waters: the common seal, or harbor seal, of the N Atlantic and N Pacific, and the larger gray seal of the N Atlantic. The former is the only seal frequently seen off U.S. coasts. The Greenland seal, or harp seal, is found in the arctic Atlantic; the ribbon seal in the arctic Pacific. The small ringed seal and the larger bearded seal are circumpolar arctic species. Antarctic seals include the voracious leopard seal, which feeds on penguins and other sea birds, and the Ross, Weddell, and crabeater seals. The warm-water seals are the Mediterranean, Caribbean, and Hawaiian species of monk seal. A fourth group includes the elephant seal and hooded seal. There are two elephant seal species, one of the Northern and one of the Southern Hemisphere. They are distinguished by their immense size and trunklike snouts. The hooded seal, distinguished by an inflatable bladder over the snout, is found in the arctic Atlantic.
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