About Seal
Pinnipeds (pronounced /ˈpɪnɪˌpɛdz/), commonly known as seals,[a] are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly 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 seals, or true seals), with 34 extant species and more than 50 extinct species described from fossils. While seals were historically thought to have descended from two ancestral lines, molecular evidence supports them as a monophyletic group (descended from one ancestor). Pinnipeds belong to the suborder Caniformia of the order Carnivora; their closest living relatives are musteloids (weasels, raccoons, skunks and red pandas), having diverged about 50 million years ago.
Seals range in size from the 1 m (3 ft 3 in) and 45 kg (100 lb) Baikal seal to the 5 m (16 ft) and 3,200 kg (7,100 lb) southern elephant seal. 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 primarily use their front limbs to propel themselves through the water, while phocids and walruses primarily use their hind limbs for this purpose. 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. Otariids 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 have an advanced tactile system in their whiskers or vibrissae. Some species are well adapted for diving to great depths. They have a layer of fat, or blubber, under the skin to keep warm in cold water, and, other than the walrus, all species are covered in fur.
The meat, blubber and skin of pinnipeds have traditionally been used by indigenous peoples of the Arctic. Seals have been depicted in various cultures worldwide. They are commonly kept in captivity and are even sometimes trained to perform tricks and tasks. Once relentlessly hunted by commercial industries for their products, seals are now protected by international law. The Japanese sea lion and the Caribbean monk seal have become extinct in the past century, while the Mediterranean monk seal and Hawaiian monk seal are ranked as endangered by the International Union for Conservation of Nature. Besides hunting, pinnipeds also face threats from accidental trapping, marine pollution, climate change and conflicts with local people.
Locomotion
Pinnipeds have two pairs of flippers on the front and back, the fore-flippers and hind-flippers. Their elbows and ankles are not externally visible.[35] Pinnipeds are not as fast as cetaceans, typically swimming at 5–15 kn (9–28 km/h; 6–17 mph) compared to around 20 kn (37 km/h; 23 mph) for several species of dolphin. Seals are more agile and flexible,[39] and some otariids, such as the California sea lion, can make dorsal turns as the back of their heads can touch their hind flippers.[40] Pinnipeds have several adaptions for reducing drag. In addition to their streamlined bodies, they have smooth networks of muscle bundles in their skin that may increase laminar flow and cut through the water. The hair erector muscles are absent, so their fur can be streamlined as they swim.[41]
When swimming, otariids rely on their fore-flippers for locomotion in a wing-like manner similar to penguins and sea turtles. Fore-flipper movement is not continuous, and the animal glides between each stroke.[42][43] Compared to terrestrial carnivorans, the fore-limb bones of otariids are reduced in length, giving them less resistance at the elbow joint as the flippers flap;[44] the hind-flippers maneuver them.[45] Phocids and walruses swim by moving their hind-flippers and lower body from side to side, while their fore-flippers are mainly used for maneuvering.[43][46][16] Some species leap out of the water, and “ride” waves.[47]
Pinnipeds can move around on land, though not as well as terrestrial animals. Otariids and walruses are capable of turning their hind-flippers forward and under the body so they can “walk” on all fours.[48] The fore-flippers move along a transverse plane, rather than the sagittal plane like the limbs of land mammals.[49] Otariids create momentum by laterally swaying their heads and necks.[50][49] Sea lions have been recorded climbing up flights of stairs. Phocids lack the ability to walk on their hind-flippers, and must flop and wriggle their bodies forward as their fore-flippers keep them stable. In some species, the fore-flippers may act like oars pushing against the ground. Phocids can move faster on ice, as they are able to slide.[51]