Seal (mammal)

I

Introduction

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.

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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.

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