In true seals, the hind flippers provide the thrust for swimming, but they cannot be brought under the body for walking on land. Among eared seals (and walruses), the front flippers are the most important for swimming and the hind flippers can be brought forward under the body for walking.

Terminology

A group of seals is called a herd. A breeding colony is known as a rookery. The young are called pups; a group of pups is called a pod; adult males are bulls, while females are cows.

Species

True seals include the common or harbour seal Phoca vitulina, found in coastal regions over much of the northern hemisphere, with the largest concentration around the Wash, in East Anglia, England. The males may weigh twice as much as the females. They feed on fish and squid. The grey seal Halichoerus grypus, which grows to 2.7 m/9 ft, also has its main population around British coasts, but occurs off the Canadian coasts of Newfoundland, and around Iceland and Scandinavia as well. In 1997 there were approximately 220,000 grey seals in the world.

The largest seal is the southern elephant seal Mirounga leonina, which can be 6 m/20 ft long and weigh 4 tonnes. The male has an inflatable snout (an enlargement of the nasal cavity), which acts as a resonator to amplify its cries. It feeds mainly on squid, and spends 80% or more of its life under water. Of its total population of 600,000, half breed on the island of South Georgia. Elephant seals are polygamous; pregnant females arrive on the breeding ground and gather together, giving birth in September; within three weeks they are ready for mating again. In October a single male, which can be three times larger than the female, will claim a harem of as many as 100 females, and will fight any other male that approaches his territory, proclaiming his ownership with a loud cry. Only fully mature males have the size and strength to keep such a harem. After mating, in November, the female produces a single pup, which is suckled for several weeks without the female leaving to feed, surviving on its blubber. When the pup is weaned, the mother is ready to mate again. Elephant seals travel 21,000 km/13,050 mi a year, between California's Channel Islands and feeding grounds in the Pacific.

The smallest seal is the Baikal seal Pusa sibirica, only 1.2 m/4 ft long and the only seal to live entirely in fresh water.

Eared seals include sea lions and fur seals. The rarest seals are the monk seals, the only species to live in warmer waters. The Caribbean monk seal Monachus tropicalis may already be extinct, and the Mediterranean monk seal M. monachus, found largely in the eastern Aegean Sea, and Hawaiian monk seal M. schauinslandi are both endangered, mainly owing to disturbance by humans. In 1996 the population of Mediterranean monk seals was approximately 400-550. The population of Hawaiian monk seals was around 1,200 in 1998.

The leopard seal Hydrurga leptonyx has distinctive spotted markings on its underside, and is a fast-moving predator of penguins, other species of seal, fish, and krill. It eats penguins in a very distinctive way, shaking the bird with its teeth to make the flesh come off. Males grow to around 3 m/9.8 ft long; females 3.5 m/11.5 ft. The pups are born in November-January.

Despite their name, Crabeater seals Lobodon carcinophagus do not eat crabs but krill, up to 20 kg/44 lb each day, which they sieve from the sea with their teeth. The most abundant of seal species, they number between 15 and 30 million - more than all the other seals put together; they are the most abundant species of mammal after humans. They never come to land, breeding on Antarctic pack-ice. They live in small herds of fewer than ten, and are monogamous. Their coat fades during the year to white, until a fresh coat is produced at the moult in January. At 2.5 m/8.2 ft long and weighing 225 kg/496 lb, the females tend to be larger than the males.

The Antarctic or Kerguelen fur seal Arctocephalus gazella feeds mainly on krill. The male, at 150 kg/330 lb, weighs up to three times more than the female. Like all fur seals, they have a double coat of long guard hair covering a finer insulating layer; to some extent, this fur replaces the thick layer of blubber found in true seals. Like the elephant seal, they breed in harems, with each bull controlling five or ten cows. The pups are fed by the cows for 117 days, the cow returning to feed the pup every few days. Like other fur seals, the Antarctic fur seal was almost wiped out during the late 19th century by sealing expeditions. It remained extremely rare until the 1950s, but today has risen to 1.5 million. Most of the population breeds on the island of South Georgia.

The Weddell seal Leptonychotes weddelli is easily recognizable from its patterned coat with grey and black flecks. These seals are the most southerly-living mammals in the world, remaining during the winter under the fast ice that never breaks up. They achieve this by creating and enlarging holes in the ice with their teeth; however, this results in tooth decay and probably is the cause of their short lifespan of around 10-15 years. They grow to a length of 3 m/10 ft, and weigh 450 kg/990 lb; the females tend to be slightly larger than the males. Each hole in the ice is the preserve of one male, with several females using the hole and mating with the male. Their diet consists mainly of Antarctic cod, which they catch during dives of up to 400 m/1,300 ft deep.

The Ross seal Ommatophoca rossi is the least known of all the seals. It lives in deep pack ice and is very rarely seen. It is thought to feed on squid.

For seal hunting, see sealing.

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