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South African fur seal
(Arctocephalus pusillus pusillus)
Southern elephant seal
(Mirounga leonina)

Pinnipeds (from Latin pinna, wing or fin, and ped-, foot) or fin-footed mammals are a widely distributed and diverse group of semi-aquatic marine mammals comprising the families Odobenidae (the walrus), Otariidae (eared seals, including sea lions and fur seals), and Phocidae (earless seals).

Pinnipeds are typically sleek-bodied and barrel-shaped. Their bodies are well adapted to the aquatic habitat where they spend most of their lives. Their limbs consist of short, wide, flat flippers. The smallest pinniped, the Baikal Seal, weighs about 70 kg  on average when full-grown and is 1.3 m ( long; the largest, the male Southern Elephant Seal, is over 4 meters  long and weighs up to 4,000 kilograms

Earless seals, also called “true seals" or "phocids", are the most diverse and widespread pinnipeds. They lack external ears, have more streamlined snouts, and are generally more aquatically adapted than otariids. They swim with efficient, undulating whole-body movements using their more-developed rear flippers. The swimming efficiency and an array of other physiological adaptations make them better built for deep and long diving and long distance migration. These mammals are, however, very clumsy on land, moving by wriggling their front flippers and abdominal muscles. The two back flippers form a tail-like structure which does not aid walking on land. True seals generally communicate by slapping the water and grunting, rather than vocalizing.

California Sea LionsEared seals, also called "walking seals" and "otariids", include the animals commonly known as sea lions and fur seals. These are vocal, social animals that are somewhat better adapted to terrestrial habitats with rear flippers that can turn forward so that they can move on all fours on land. Their foreflippers are larger than those of earless seals and are used as a primary source of maneuverability in the water. Eared seals have external ears, as their name suggests, and more dog-like snouts, further distinguishing them from true seals. While sea lions are generally larger than fur seals and lack the dense underfur of the latter, the long-standing division into subfamilies (Arctocephalinae and Otariinae for fur seals and sea lions respectively) is unjustified in light of genetic evidence suggesting that several fur seal species are more closely related to some sea lions than other fur seals. The iconic ball-balancing circus seal is generally some species of sea lion, most commonly a California Sea Lion.

A walrusThe walrus is an exclusively Arctic species—the sole surviving member of the once diverse and widespread Odobenidae family. They are easily recognized by their long tusks and great bulk 2,000 kilograms . While they share with otariids the ability to turn their rear flippers forward, their swimming is more reminiscent of that of true seals, relying more on sinuous whole body movements. They also lack external ears. Unlike eared seals and true seals, which feed primarily by hunting fish and squid in the water column, walrus generally prefer benthic invertebrates, in particular clams. The unique squirt and suck method of feeding on mollusks first differentiated the original walrus ancestor from other pinniped lineages. There remains debate as to whether the walrus diverged from the eared seals before or after the true seals.

Pinniped limbs, or flippers, are proportionally shorter than those of most other mammals. Their fingers are bound together by a web of skin (as are its toes). They also have claws either on the front flippers (earless seals) or back flippers (eared seals). Because water has a much higher density than air, their flippers can be much smaller proportionately in relation to their size than bird or bat wings. Additionally, pinnipeds are essentially weightless in water, allowing them to come to a standstill, and perform aquabatic feats in water that would be impossible for flying creatures.

Fur seals have both blubber and a specially adapted fur coat, including outer guard hairs that repel water and a layer of insulating underfur. For this reason they were particularly prized by sealers. Many species were nearly hunted to extinction.

For most pinniped species molting is an annual process of replacing worn fur (and in some cases, skin) that temporarily grounds them. While molting, thermoregulation can be compromised, so some species, such as Elephant seals, fast and remain onshore for a month or more.

In many species, pups are born with a natal coat of a different length, texture and/or color than adults. This coat is adapted for the terrestrial, pre-weaning period, either a thick pelage to keep them warm in arctic environments, or a thin layer of fur to keep them cool on summer sands. During their first molt (about 11 days after birth for harp seals) the pups replace this with an adult coat better suited to life at sea. Until this age, pups risk hypothermia and drowning if they spend much time in the ocean.

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