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.