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Seal (mammal), large aquatic mammals distinguished by having all four legs fully adapted into flippers. There are three families of seals: the true seals, the eared seals, and the walrus. True seals lack external ears and have relatively short flippers that are nearly useless for walking on land. The eared seals, including the sea lions and fur seals, sport tiny external ears and can lift their bodies off the ground with their flippers. The stout-bodied walrus, with its wrinkled skin and unique tusks, is the only member of its family.
While the limbs of seals have developed into flippers, the tail has practically disappeared, making seals different from other marine mammals, such as whales, dugongs, and manatees, which have lost their hind legs and use their powerful tails for swimming. In addition to their flippered limbs, seals have streamlined and flexible bodies, adaptations that make them excellent and efficient swimmers. Sea lions are the fastest seals and can swim at top speeds of nearly 40 km/h (25 mph). These seals are so flexible that they can nearly touch their rear flippers with their nose when bending backward.
Seals range in size from the male southern elephant seal, which reaches more than 3600 kg (8000 lb), and the male walrus, which grows more than 3.5 m (11.5 ft) in length and reaches weights of over 1700 kg (3700 lb), to the diminutive ringed seal-with adults averaging about 50 to 60 kg (about 110 to 130 lb) and a little more than 1 m (3 ft) in length.
Seals make and hear sounds underwater. Their whoops, screams, barks, moans, and wails are used in simple communication, such as mating calls and territorial defense. Some species, including bearded seals, Weddell seals, and walruses, sing complex songs that may last more than a minute. Inuit seal hunters listen for bearded seal songs by placing one ear against a kayak paddle handle while the blade is held underwater. It is still unknown if seals use reflected underwater sound waves, or echolocation, to navigate through the depths or to track prey.
II PHYSICAL DESCRIPTION
Although similar in appearance, it is easy to distinguish differences between eared seals (sea lions and fur seals), true seals, and walruses. Sea lions and fur seals have tiny external ears while only the ear openings are visible in true seals and walruses. On land, sea lions, fur seals, and walruses rotate their large flippers back and forth to function as legs, enabling them to walk clumsily on land or ice. The short flippers of true seals do not turn under the body to act as legs. These seals move on land mainly by flexing their bodies, although on snow and ice they can also use their foreflippers as paddles to reach surprising speeds. The crabeater seal, for instance, can cruise at 25 km/h (16 mph) on level Antarctic ice.
Seals have many adaptations to life in the water. External ears are greatly reduced or absent, and in many species, testicles and mammary glands are located in slits or pockets under the skin, features that streamline the seal body for more efficient swimming. When seals submerge underwater their nostrils close automatically. The pupils of their eyes expand widely to capture light in near darkness. This ability is important for finding prey at night or in deep water.
Seals conserve oxygen for long periods of time, enabling them to stay submerged at great depths, much longer than humans can. As a seal starts to dive, its heart rate slows to about one-tenth of its heart rate at the water surface. At the same time, the arteries, which transport oxygen-carrying blood to most of the animal's body, constrict or squeeze shut so that only the sense organs and nervous system continue to receive a normal flow of blood. Seal muscles also store oxygen, and the spleen, an organ that stores oxygen-rich blood, is exceptionally large in seals, serving as a kind of biological scuba tank.
The deepest-diving seals can descend hundreds of meters and stay underwater for one to two hours. During a dive, carbon dioxide builds up in the blood and the lack of oxygen causes lactic acid levels to rise in the muscles. Unlike most animals, seals are able to resist pain and fatigue caused by lactic acid accumulation. But once seals return to the water surface, they need a recovery period to bring their body chemistry back to normal. Rapid blood circulation through very large veins leading to the lungs helps to rid the seal's body of carbon dioxide. The big-branched veins carrying blood out of the walrus's lower body are so large that a person could pull them over their legs like pants.
Keeping warm is important for seals since water quickly conducts heat away from their bodies. Adult seals produce a thick layer of fat, called blubber, under their skin, which is an excellent insulator against the cold. Blubber is also used to store energy for times when food is scarce; seals can live off the stored fat in blubber for weeks to months.
While most newborn seals have little or no blubber, many seal species develop a fur coat during infancy that traps air next to the skin for an extra layer of insulation. The beautiful white coat of the infant harp seal, born on the Arctic ice, may actually set up a small greenhouse effect, trapping the energy of sunlight as heat near the skin. Many species shed this fur coat as they grow older, replacing it with blubber for insulation. Fur seals, however, keep a dense coat of fur throughout their lives, made up of about 120,000 hairs per sq cm (about 800,000 hairs per sq in). By contrast, an entire human head contains only about 100,000 hairs.
III RANGE AND HABITAT
Most seals live in cold waters near the Arctic and Antarctic. Some true seals live under ice for much of the year, finding cracks between ice floes or scattered holes in order to breath. Depending on the species, true seals use strong claws, teeth, or their head to break through new ice that freezes over openings.
A few kinds of true seals are found in warmer regions. In North American waters, harbor seals are found from northern Canada to Georgia on the Atlantic coast-although they are not common south of Massachusetts-and from Alaska to Baja, Mexico, on the Pacific coast. The northern elephant seal remains in strictly ice-free waters. Its range is from Vancouver Island, Canada, to Cedros Island, off Baja, Mexico.
Monk seals are true seals that love warm, clear waters. The Mediterranean monk seal lives around a few islands near Greece and Turkey and also off northwestern Africa. The Hawaiian monk seal concentrates around small islands northwest of the main Hawaiian Islands.
Most eared seals live well away from icebound areas. The exception is the southern fur seal, whose range extends south to the Antarctic coast. Some eared seals migrate seasonally over long distances, usually after their summer breeding season. For example, northern fur seals swim from their summer breeding range in Alaska's Bering Sea to southern California or Honshu, Japan.
Walruses are found only in the northern hemisphere. Atlantic walruses range from Arctic Canadian waters eastward to northern Europe, including western Russia, while Pacific walruses live mainly in the Bering Sea and in the Chukchi Sea off Siberia. Large herds of walruses typically follow the broken edge of the pack ice north and south with the seasons, although some older males do not migrate. Walrus migrations cover distances as great as 3000 km (1850 mi).
The most isolated of all seals live in central Asia in two of the world's largest and oldest lakes. The Baikal seal inhabits Lake Baikal in southern Russia, believed to be the deepest lake in the world, and the Caspian seal lives in the vast Caspian Sea in southwestern Asia. Ancestors of both of these landlocked seals probably reached these remote lakes by swimming thousands of kilometers up rivers from the Arctic Ocean. A few other species such as ringed seals and harbor seals have been found living year-round in lakes and rivers near the coasts of Russia, Canada, and Alaska.
IV DIET OF SEALS
Most seals eat fish and sometimes squid. The leopard seal, an Antarctic species, may have the most diverse diet of all, commonly hunting penguins and other seabirds, smaller seals, as well as fish, squid, krill (small shrimplike crustaceans), and other invertebrates, as well as feeding on carcasses of dead whales. Leopard seals sometimes hunt humans, lunging onto ice floes to chase people who are on foot, and also threatening scuba divers underwater.
Sea lions and walruses may occasionally kill and eat other seals, although more commonly sea lions eat fish, and walruses dive to the bottom and dig for clams, worms, crustaceans, and other organisms from the mud. Walruses have a large "mustache" of especially sensitive whiskers, or vibrissae, that help them detect their food on the dark sea floor. A big walrus can eat about 45 kg (about 100 lb) of shellfish in one day.
Crabeater seals of the Antarctic are known to eat fish but are unusual in that they feed primarily on krill using unique branching teeth. The seal's upper and lower teeth mesh together, forming an efficient sieve that acts similarly to the giant filtering plates of baleen found in krill-eating whales.
Most seals mate on land or ice and, in all species, females give birth out of the water. In most land-breeding seals, such as elephant seals and sea lions, dense temporary colonies called rookeries form in the breeding season. In these species, males tend to grow larger than females, and a harem system prevails in which the strongest males mate with several females and guard them from other males. This harem system ensures that only the strongest males pass their genes to the next generation. A study of northern elephant seals found that only 9 percent of males succeed in mating.
Seals that breed on ice, by contrast, do not typically organize into large harems. These species, including ribbon seals, harp seals, and Weddell seals, form colonies that spread out over wide areas. Most ice-breeding species are monogamous, and the males and females are nearly the same in size and appearance. An exception is the walrus, which breeds on ice islands where large dominant males gather sizeable harems. Walruses use their tusks in threatening displays toward rival males and in courtship rituals with females. Males have large throat pouches that produce bell-like sounds to attract receptive females.
Most females breed every year and on average bear a single pup (or calf in the walrus) 12 months after mating. The seal embryo does not begin development for 3 to 5 months after conception, thus ensuring that young are born when food is plentiful.
Newborn seals vary in size according to species-a newborn ringed seal weighs only 12 kg (25 lb) while a newborn walrus can weigh 63 kg (140 lb). Infant seals grow extremely fast and rapidly build up a layer of blubber that is generally lacking at birth. Females nurse young for a few days to two years, depending on the species. The hooded seal nurses for only three to five days and the pups nearly double their weight from 22 to 43 kg (49 to 95 lb) in this time. At the other extreme are sea lions, fur seals, and walruses, which continue to nurse their young for one to two years.
Seal milk is extremely rich in fat and protein. True seals have the highest milk fat levels, averaging 40 to 50 percent, while sea lions and fur seals pack 10 to 14 percent protein into their milk. Elephant seal milk tastes bland and waxy and physically resembles melted vanilla ice cream.
VI SEAL ORIGINS
Seals evolved from bearlike carnivores about 25 to 30 million years ago. Early seal fossils are found in Europe's North Atlantic and Mediterranean regions. By 8 to 10 million years ago seals were well established in the northern hemisphere-numerous seal fossils have been discovered in the Chesapeake Bay region of the United States dating from this time period. Found with these fossils are giant teeth resembling those of the great white shark, perhaps then, as now, an important seal predator.
Scientists debate whether all seals evolved from a single land ancestor, or whether true seals developed independently of the eared seals and the walrus. Recent molecular evidence from deoxyribonucleic acid (DNA), the genetic material found in all living organisms, suggests that true seals, eared seals, and the walrus are all more closely related to each other than to any other mammal. This indicates they all had the same land ancestor.
Seals moved into the southern hemisphere only in the last few million years-long after they had become common and diverse in the north. In Antarctic waters, they evolved into unique species such as the Weddell, crabeater, and leopard seals.
The monk seals remained in the tropics, perhaps lagging behind as their relatives crossed the equator into southern latitudes. Scientists consider the Hawaiian monk seal to be a living fossil. While modern seals have fused tibia and fibula bones in their flippers, the Hawaiian monk seal still has separate tibia and fibula bones in its hind flippers-a condition seen in the earliest fossil seals. Even the structure of the large vena cava blood vessel in the Hawaiian monk seal resembles that of bears and dogs more than that of other seals.
VII ENDANGERED SEALS
Natural predators of seals include large sharks, especially the great white shark, the orca, or killer whale, and other seals such as the leopard seal. Polar bears kill seals on land and ice in the Arctic. There are even reports of eagle attacks on baby Caspian seals.
Perhaps the greatest menace to seals are humans, who have long hunted seals for food and seal skins, used for clothing and even housing and small boats. Commercial sealing developed into a profitable business in Europe and colonial America by the 1700s. Like whales, seals were hunted for their blubber, which was converted to oil for fuel, lubrication, and tanning. Fur seals were especially valued for their luxuriant pelts. Today seals are still killed for their skins in some countries, such as Canada and Russia.
After only fifty years of commercial hunting in the 1800s, the northern elephant seal became nearly extinct. Hawaiian and Mediterranean monk seals live along warm shores and are easily approachable, making them particularly vulnerable to human hunters. Both monk seal species are currently endangered. A third monk seal, once abundant throughout the Caribbean Sea, became extinct by about 1950.
One of the first formal attempts to protect seal populations occurred in 1911, when the United States, Canada, Russia, and Japan agreed to ban open-sea hunting of seals. After Mexico banned seal hunting in 1922, the northern elephant seal population began to grow. In 1910, only 100 seals were found on Isla de Guadalupe; today this species numbers around 150,000 and has recolonized much of its former range from northern Mexico to southern Alaska. However, another species, the Guadalupe fur seal, hunted during the same period, has not recovered its former abundance along the California and Mexican coasts.
Some countries have enacted laws to protect seals and other marine mammals. Sadly, these laws came too late to save the Caribbean monk seal. Even though the hunting of seals is now much less intense than in the past, threats from pollution, especially oil spills, and the accumulation of marine debris such as lost or discarded fishing line and nets still cause many deaths among seals.
Scientific classification: Seals are classified in the suborder Pinnipedia of the large mammalian order, Carnivora. The true seals make up the family Phocidae, and the harbor seal is classified as Phoca vitulina; the northern elephant seal is classified as Mirounga angustirostris; and the Hawaiian monk seal is classified as Monachus schauinslandi. The eared seals make up the family Otariidae and include the northern fur seal, classified as Callorhinus ursinus; and the California sea lion, classified as Zalophus californianus. The walrus is the only member of the walrus family, Odobenidae, and is classified as Odobenus rosmarus.