The seals are marine mammals. They have been classified as a suborder in the order carnivora, but some zoologists consider the seals a separate order, Pinnipedia. The name comes from Latin words meaning “fin-footed.”
The order is divided into three families the earless seals of the family Phocidae; the sea lions and fur seals, also known as the eared seals, of the family Otariidae; and the walrus, of the family Odobenidae. Seals are found along most coasts but some of the biggest concentrations are in Arctic and Antarctic waters.
Pinnipeds spend most of their lives in water, although they go on land to bear and rear their pups and to molt. Like the cetaceans, they are well adapted for an aquatic life. Their bodies are streamlined and torpedo-shaped. They are padded with blubber that acts as an energy store and that provides insulation. Their limbs have been modified into flippers for swimming. Pinnipeds have either tiny external ears or none at all, and slitlike nostrils. The ears and nostrils are closed when the animals are submerged, but the eyes remain open and are efficient under water. Whereas walruses are almost bare-skinned, most pinnipeds are covered with short hair.
All pinnipeds are well adapted for diving. When they plunge, their heart rate immediately drops from 50 to 100 beats per minute to 10 or less. The remaining blood flow is directed mainly to the brain. Like whales, pinnipeds can survive without breathing for much longer periods than land mammals. The record for diving is probably held by the Weddell seal, recorded at a depth of some 2,360 feet (719 meters) for up to 43 minutes.
Male pinnipeds are much larger than the females. A large bull southern elephant seal weighs up to 8,000 pounds (3,600 kilograms), four times more than the cow. The life span varies according to species, from about 25 years to 40 years or more.
Most pinnipeds feed on fish, but eared seals also eat crustaceans and marine mollusks, such as squid and octopus. Other pinnipeds, such as the leopard seals also eat the pups of other seals, and a genus of sea lion occasionally eats penguins. The diet of the walrus consists chiefly of clams and oysters.
Cows give birth to one pup a year, usually at a traditional breeding space called a rookery, where large colonies of seals gather. Most rookeries are on islands, but rookeries of northern fur seals are large beach areas. More than 150,000 seals may gather at a single rookery. The bull seals at these rookeries are usually polygamous—that is, one bull mates with several cows—and arrive there before the cows do to establish territories, fighting off other males.
After a gestation period of about 12 months, the pup is born. Earless seals suckle their pups for a short time. The harp seal, for example, suckles its young for only nine days after birth. The pups develop very rapidly and during this period the cow does not feed, drawing on her reserves of blubber to produce milk. The pups of some species are able to swim within hours of birth, whereas others take several weeks. Eared seals rear their pups very slowly. Galapagos fur seals, for example, are not weaned until they are two or three years old.
A few days after a cow seal has given birth, she mates again, but the implantation is delayed for a few months. This allows the female to recover from the strain of feeding her pup. It also means that pups are always born at about the same time of year, which is an important consideration for migratory species.
The earless seals
The 18 species of earless seals live mostly in the Arctic and Antarctic, but some inhabit temperate waters. The main exceptions are the monk seals, which live in warm seas, and some seals that inhabit freshwater lakes, such as Lake Baikal in Russia and the Caspian Sea.
Pups of most species are born with a soft, dense, woollike white coat, which is replaced by the stiffer adult coat before they start to swim. The white coat serves as camouflage on the ice and its thickness provides good insulation. Common seal pups, however, are born with their adult coat
The crabeater seal is the most abundant of all earless seal species, with a population of 15 million. Despite its name, it feeds on tiny crustaceans known as krill, which it strains from the water through its teeth. Young crabeaters are hunted by leopard seals and many bear large scars from these attacks.
They are clumsy on land, where they move by pushing the ground with their front flippers. In water, however, they are very graceful, swimming with side-to-side movements of the hind flippers and using the foreflippers for maneuvering at low speed.
The eared seals
Of the 16 species of eared seals, five are sea lions and eleven are fur seals. They are found in polar, temperate, and subtropical areas. Eared seals get their name because of their tiny cartilaginous earflaps, which earless seals do not have. Eared seals are long and slender. Unlike earless seals, which look like they have no tail at all, the eared seals have short, but developed tails.
Fur seals have a thick undercoat of soft fur, protected by longer, coarser hair, whereas sea lions have only one layer of coarse hair. Sea lions are the biggest eared seals, have a heavier muzzle and head, and also a greater disparity in size between the sexes than other seals.
The eared seals and the walrus swim at high speeds by undulating their bodies, with their foreflippers tucked well back. The northern fur seal can swim 10 miles (16 kilometers) per hour. Eared seals are more agile on land than are earless seals because they can turn their hind flippers forward. In this way, they can lift their body off the ground and move forward.
The single species of walrus lives in the Arctic Ocean, ft migrates south in winter, riding on the ice floes, and returns in the spring. It has a thick body and a tough, hairless skin that covers a layer of blubber 1 to 6 inches (2.5 to 15 centimeters) thick. Both sexes have a pair of tusks, which are long upper canines. They use these tusks to stir up food from the seabed, to fight with other walruses, to defend themselves against polar bears, and with the help of the fins, to clamber up onto the ice. Walruses feed on such shellfish as clams and oysters. The bulls can measure 10 to 13 feet (3 to 4 meters) long and weigh as much as 3,000 pounds (1,360 kilograms). Despite their size, they are excellent swimmers and can reach speeds of 15 miles (24 kilometers) per hour.