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SEAL,
any carnivorous marine mammal of the order Pinnipedia; members
of the order have fins as feet. The various forms, known also as
fur seals, hair seals, elephant seals, sea lions, and leopard seals, are
widely distributed throughout the marine regions of the frigid and
temperate zones; only the monk seals of the genus Monachus are
tropical. Three families of seals exist: the Otariidae, or eared
seals; the Odobenidae, consisting of the single species of walrus
(for their description; and the Phocidae, or true seals. All three families
represent a reversion to aquatic habitat from ancestral land dwellers;
they have become almost perfectly adapted to life in the water,
and resort to shores or ice floes only to breed and to rear their
young. Seals eat fish, shellfish, and other marine animals.
The eared seals have long, flexible necks, and small external
ears. They have hind flippers that can be turned forward, enabling
them to support the body, and use all four limbs for land travel.
They comprise two groups, the sea lions and the fur seals.
The sea lions are the larger of the eared seals. Steller’s
sea lion, Eumetopias jubata, is found in the North
Pacific Ocean. Adult males attain a length of 3.5 m (12 ft) and
a maximum weight of 1100 kg (2400 lb); females are much smaller,
weighing up to 350 kg (770 lb). The southern sea lion, Otaria
byronia, a smaller species, is found on the coasts of South
America. The seal frequently trained for exhibition in circuses
and zoos is the small California sea lion, Zalophus californianus, found
off the California coast.
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The commercially important fur seals closely resemble the
sea lions anatomically but differ in having a rich, silky undercoat
of fur. Two genera are recognized, Arctocephalus of the
southern hemisphere, and Callorhinus of the North
Pacific Ocean and the Bering Sea. Six to eight species of the southern
fur seal are recognized; the best known is A. pusillus of
Tasmania and the southern coasts of Africa and Australia.
The single northern species is the northern fur seal, C.
ursinus, which breeds in the Pribilof Islands of the Bering
Sea. The male, or bull, is mostly dark brown in color, shading to
gray at the shoulders. Reaching maturity at about the age of seven
years, the bull attains a length of 2 m (6 ft) and a weight of 250
kg (550 lb); the female, or cow, matures at three years and attains
an average weight of 52 kg (115 lb). Large, older bulls have harems
of as many as 40 cows and battle off rival males until defeated.
Immature and bachelor males congregate on beaches removed from the
breeding grounds. As winter approaches, the fur seals migrate southward
to latitudes of Baja California.
In the late 19th century, the indiscriminate slaughter at
sea of the migrating herds for their valuable fur led to a sharp
decline in the fur-seal population. In 1911 an international treaty,
adopted by the U.S., Great Britain, Russia, and Japan, established
effective controls for the preservation of the species. Pelagic
sealing, or the hunting of seals in the ocean, was prohibited by
the agreement, which permitted capture only of immature or bachelor
seals, taken at the outskirts of the breeding grounds under government
supervision. In 1911 the Pribilof herd numbered about 200,000. A
revised treaty (1957) made the Pribilof Islands a government reservation.
Under protection, the herd has increased to an estimated population
of about 2 million, or about 85 percent of the fur seals of the
world.
The 19 species of Phocidae, or true seals, lack external ears
and have shorter, relatively inflexible necks and undeveloped forelimbs;
the forelimbs, however, do bear claws used for crawling up rocks
and ice floes. The hind limbs do not flex forward and are stroked
vertically in swimming like the tail of a dolphin. True seals are
better adapted to life in water than are the longer-limbed eared
seals, but on land they progress laboriously by wriggling and hunching
the entire body.
Many species of Phocidae are hunted for their leather. Among
them is the harbor seal, Phoca vitulina, which
lives in northern oceans, is yellowish white with brown markings,
and grows up to 1.8 m (6 ft) long. The species Pusa caspica, of
the Caspian Sea, and Pusa sibirica, of Lake Baykal,
are smaller. The harp seal, Pagophilus groenlandicus, is
slightly larger, reaching a length of 1.8 m (6 ft). It is extremely
gregarious, and, during the breeding season, is found in great numbers
on the ice floes northward from Newfoundland. The ringed seal, or
floe rat, Pusa hispida, is also Arctic in distribution,
and is about the same size as the harp seal. The gray seal, Halichoerus
grypus, and the hooded seal, Cystophora cristata, are
larger, and are also found in Arctic waters. The monk seal, Monachus
monachus, is found in the Mediterranean and Black seas,
and another species, M. schauinslandi, is found
in the Hawaiian Islands; M. tropicalis, a species
of monk seal in the Caribbean, is probably extinct.
Elephant seals are so called for their trunklike proboscis
and great size. Scientists have discovered that elephant seals migrate
twice a year, the only vertebrates to do so. They were once numerous,
but the oil obtained from their blubber was found to be of high
commercial value, and they were hunted almost to extinction. Survivors
of large herds of two species are still extant. Mirounga
angustirostris, once common in the waters of Southern California,
attains a length of 6.7 m (22 ft), and a single animal has yielded
as much as 1550 liters (200 gal) of oil. A similar species, M.
leonina, formerly found in most regions of the southern hemisphere,
still exists on South Georgia Island and other areas in the South
Atlantic Ocean. The Antarctic seals include the leopard seal, Hydrurga
leptonyx; Weddell’s seal, Leptoncyhotes
weddelli; the Ross seal, Ommatophoca rossi; and
the crabeater seal, Lobodon carcinophagus.