Pinnipeds have streamlined bodies, rounded in the middle and tapered at the ends, with a thick layer of fat beneath the skin. Their limbs are short and their feet are long and webbed, forming flippers. The sea lions and fur seals (family Otariidae) and the walrus (family Odobenidae) are able to turn their hind flippers forward for walking on land; they swim chiefly by a rowing action of the long front flippers. The true seals (family Phocidae) are unable to rotate the hind flippers. They progress on land by wriggling on their bellies, pulling themselves with the short front flippers; in the water they are propelled by a side-to-side sweeping action of the hind flippers.
Nearly all pinnipeds are marine, and most inhabit cold or temperate regions. Some spend most of the year in the open ocean, while others inhabit coastal waters and spend varying amounts of time on shores, islands, or ice floes. Occasionally they ascend rivers. All pinnipeds leave the water at least once a year, at breeding time. In nearly all species the females give birth a year after mating, so that the births take place on land, just before breeding begins. The pups are nursed during the period, usually of several months duration, spent on land. Some species spend most of the year far from their breeding grounds; the northern fur seals make particularly lengthy migrations each year. Most pinnipeds have diets of fish and shellfish; many are bottom feeders, with physiological adaptations for deep diving. They have acute hearing and some, if not all, make use of echolocation (sonar) for underwater navigation.
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.
Seals have been used by the Eskimo and other northern hunting peoples for food, oil, and hides. Commercial sealing has been largely confined to a few species, most notably the fur seal. Commercially important species of true seals are the harp seal, whose pups are valued for their fluffy white coats, and the ringed seal. The hunting of these seals is regulated by international treaties, and they are not in danger of extinction. The elephant seals were formerly hunted for oil and almost exterminated, but they are now protected and are stabilized or increasing in numbers. The monk seals have been greatly depleted by hunting in past centuries and their survival is threatened, although they are no longer of commercial importance. The Caribbean monk seal is believed by some authorities to be extinct. The ribbon seal and Ross seal are not much hunted; estimates of their populations have varied considerably, but they are not thought to be endangered.
Seals are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Carnivora, suborder Pinnipedia.
See B. Davies, Savage Luxury: The Slaughter of the Baby Seals (1971); V. B. Scheffer, The Year of the Seal (1972); J. E. King, Seals of the World (2d ed. 1983); R. L. Gentry and E. L. Kooyman, Fur Seals (1986).