Pinnipeds, often generalized as seals, are a widely distributed and diverse clade of fin-footed, semi-aquatic marine mammals. They comprise the extant families Odobenidae (whose only living member is the walrus), Otariidae (the eared seals; sea lions, and fur seals), and Phocidae (the earless, or “true” seals). There are 33 extant species of pinnipeds, and more than 50 extinct species have been described from fossils. While they were historically thought to have descended from two ancestral lines, molecular evidence supports them as a monophyletic lineage. Pinnipeds belong to the order Carnivora and their closest living relatives are bears and musteloids.
Seals range in size from the 1 m (3 ft 3 in) and 45 kg (99 lb) Baikal seal to the 5 m (16 ft) and 3,200 kg (7,100 lb) southern elephant seal, which is also the largest carnivoran. Several species exhibit sexual dimorphism. They have streamlined bodies and four limbs that are modified into flippers. Though not as fast in the water as dolphins, seals are more flexible and agile. Otariids use their front limbs primarily to propel themselves through the water, while phocids and walruses use their hind limbs. Otariids and walruses have hind limbs that can be pulled under the body and used as legs on land. By comparison, terrestrial locomotion by phocids is more cumbersome. Otarids have visible external ears, while phocids and walruses lack these. Pinnipeds have well-developed senses—their eyesight and hearing are adapted for both air and water, and they an advanced tactile system in their vibrissae. Some species are also well adapted for diving to great depths. They have a layer of fat, or blubber, under the skin to keep warm in the cold water, and, other than the walrus, all species have fur coats.
Although pinnipeds are widespread, most species prefer the colder waters of the Northern and Southern Hemispheres. They spend most of their lives in the water, but come ashore to mate, give birth, molt or escape from predators like sharks and killer whales. They feed largely on fish and marine invertebrates, but a few, like the leopard seal, feed on large vertebrates such as penguins and other seals. Walruses are specialized for feeding on bottom-dwelling mollusks. Pinnipeds are typically polygynous, although the degree of polygyny varies with the species. Land-breeding species tend toward greater polygyny while ice-breeding species are less polygynous. Some species are even serially monogamous. Male pinniped strategies for reproductive success vary between female defense, territorial defense and lekking. Pups are typically born in the spring and summer months. Females bear almost all the responsibility for raising the young. Mothers of some species fast and nurse their young for a relatively short period of time while others take foraging trips at sea between nursing bouts. Walruses are known to nurse their young while at sea. Seals produce a number of vocalizations, notably the barks of California sea lions, the gong-like calls of walruses and the complex songs of Weddell seals.
The meat, blubber and fur coats of pinnipeds have traditionally been used by indigenous peoples of the Arctic, and seals have been depicted in various cultures worldwide. They are commonly kept in captivity and are even sometimes trained to perform tricks and tasks. Seals were at one time relentlessly hunted by commercial industries for their products, but now they are protected by international law. The Japanese sea lion and the Caribbean monk seal have vanished in the past century, while the Mediterranean monk seal and Hawaiian monk seal are ranked Critically Endangered by the International Union for Conservation of Nature. Besides exploitation, pinnipeds also face threats from by catching, marine pollution, and conflicts with local people.