SEAL

in documentation, an impression made by the impact of a hard engraved surface on a softer material such as wax or clay, producing a device in relief. Seals have been used from remote antiquity to authenticate documents. The study of seals, known as sigillography (q.v.), is a major historical discipline. in zoology, any of numerous species of carnivorous aquatic mammals that live chiefly in cold regions and have limbs modified into webbed flippers for use in swimming. Together with walruses, seals make up the suborder Pinnipedia of the order Carnivora. Baikal seals (Phoca sibirica), endemic to Lake Baikal, southeastern Siberia. Seals range in size from the freshwater seal (Phoca sibirica; see photograph) of Lake Baikal, which is 1 m (3 feet) long and weighs about 70 kg (150 pounds), to the male elephant seal, which can reach lengths of 6.5 m (21 feet) and weigh 3,530 kg (7,780 pounds). Seals' streamlined bodies are round in the middle and tapered at the ends. Their limbs are short and have long feet that have been modified into flippers. Seals possess a thick subcutaneous layer of fat (blubber), which insulates the animal against the cold, provides a food reserve, and contributes to buoyancy. Seals are found throughout the world, with some species favouring the open ocean and others inhabiting coastal waters or spending time on islands, shores, or ice floes. The coastal species are generally sedentary, but the oceangoing species make extensive, regular migrations. Seals are especially abundant in cold polar seas. All species of seals must come ashore once a year to breed. The gestation period averages about 11 months (including a delay in implantation in many, possibly most, species), and the females (cows) are soon impregnated again after giving birth to their young (pups). One pup is normally born to each cow and is nursed for a few weeks to a few months; during this period the cow remains on land and does not feed. The young gain weight rapidly, for the cow's milk is up to about 50 percent fat. Nearly all seals are gregarious, at least when breeding; some species assemble in enormous herds on sea beaches or floating ice. In species such as the fur seals, the gray seal (Halichoerus grypus), and elephant seals (Mirounga), the bulls (males) take possession of harems of cows and drive rival bulls away from their territory. Seals are excellent swimmers and diversespecially the Weddell seal (Leptonychotes weddelli). Seals cannot swim as fast as dolphins or whales but are more agile in the water. Various species are able to reach depths of 150250 m or more and to remain underwater for 20 to more than 30 minutes. Seals eat mainly fish, and some may also consume large quantities of squid, other mollusks, and crustaceans. The leopard seal (Hydrurga leptonyx) of the Antarctic feeds largely on penguins and other seabirds, as well as fish. The main predators of seals are human beings, killer whales, polar bears, and large sharks. The seals consist of two families: earless, true, or hair, seals; and eared seals. Earless seals lack external ears and are of the family Phocidae (13 genera, 18 species). Depending on the species, the males are smaller to much larger than the females. When swimming, an earless seal uses its forelimbs to maneuver in the water and propels its body forward by side-to-side strokes of its hind limbs. Because the hind flippers cannot be moved forward, the seals propel themselves on land by wriggling on their bellies or pulling themselves forward with their front limbs. Most earless seals form pairs during the breeding season. The pups are weaned after two to four weeks. The principal species of earless seals are the bearded seal, elephant seal, gray seal, harbour seal, harp seal, hooded seal, leopard seal, monk seal, ribbon seal, ringed seal, and Weddell seal (qq.v.). Eared seals possess external ears and are of the family Otariidae (6 genera, 14 species). They consist of various species of sea lion and of fur seal (qq.v.). Eared seals have longer flippers than earless seals, and the males are twice to more than four times the size of the females. Eared seals, when swimming, rely mainly on a rowing motion of their fore flippers for propulsion. Because they are able to turn their hind flippers forward, they can use all four limbs when moving on land. During the breeding season, eared seals gather in large herds. The young are suckled for four to six months. Seals have been hunted for their meat, hides, oil, and fur. The pups of harp seals, for example, are born with fluffy white coats that are of value in the fur trade. The fur seals of the North Pacific and the ringed seals of the North Atlantic have also been hunted for their pelts. Elephant seals and monk seals were hunted for their blubber, which had various commercial uses. Seal hunting, or sealing, was so widespread and indiscriminate in the 19th century that many species might have become extinct if international regulations had not been enacted for their protection. The severe decline of sealing worldwide after World War II and the effects of international agreements aimed at conserving breeding stocks enabled several severely reduced species of seals to replenish their numbers. See also carnivore.