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SealsA number of seal species live in the waters surrounding Antarctica, including fur seals, elephant seals and leopard seals.
All seals are classified as Pinnipeds - large marine mammals belonging to the Pinnipedia, a suborder of the order Carnivora. The true seals, sea lions, fur seals and walrus are all pinnipeds. Pinnipeds are typically sleek bodied and rather large. The smallest pinniped, the Galapagos Fur Seal weighs about 30 kg when full-grown and is 1.2 metres long; the largest, the male Southern Elephant Seal, is over 4 metres long and weighs up to 2,200 kg. All are carnivorous and live on fish, shellfish, squid, and other marine creatures. The eared seals (or walking seals), family Otariidae, are the fur seals and the sea lions. They are characterized by the presence of external ear pinnae or flaps, and the ability to walk on four flippers on land as their hind limbs can be brought forward under the body to bear the animal's weight. These are marine mammals, adapted to a semi-aquatic lifestyle. They feed and travel in the water but breed and largely rest on land (or, in some cases, on ice). They are slightly less adapted to the aquatic lifestyle than are the true seals. The true seals or earless seals are members of the family Phocidae. They are sometimes called crawling seals, to distinguish them from the fur seals and sea lions of family Otariidae. Phocids are the more highly specialized for aquatic life of the two groups and, unlike otariids, lack external ears and cannot bring their hind flippers under their body to walk on them. They are more streamlined than fur seals and sea lions, and can therefore swim more effectively over long distances than otariids. However, because they cannot turn their hind flippers downward, they are very clumsy on land because they have to wriggle with their front flippers and abdominal muscles; this method of locomotion is called gallumphing. FeedingAll pinnipeds are carnivorous and live on fish, shellfish, squid, and other marine creatures, such as Antarctic krill. While otariids are built for speed and maneuverability in the water, phocids are built for efficient, economical movement. This allows most phocids to make long foraging trips to exploit prey resources that are far from land, whereas otariids are tied to rich upwelling zones close to their breeding sites.ReproductionThe otarid reproductive strategy is for the mother to make short foraging trips between nursing bouts. The phocid reproductive cycle is characterized by temporal and spatial separation between feeding and maternal investment; in other words, a pregnant female spends a long period of time foraging at sea, building up her fat reserves, and then returns to the breeding site and uses her stored energy reserves to provide milk for her pup. Because a phocid mother's feeding grounds are often hundreds of kilometers from the breeding site, this means that she must fast while she is lactating. This combination of fasting with lactation is one of the most unusual and extraordinary behaviors displayed by the Phocidae, because it requires the mother seal to provide large amounts of energy to her pup at a time when she herself is taking in no food (and often, no water) to replenish her stores. Because they must continue to burn fat reserves to supply their own metabolic needs while they are feeding their pups, phocid seals have developed an extremely thick, fat-rich milk that allows them to provide their pups with a large amount of energy in as small a period of time as possible. This allows the mother seal to maximize the efficiency of her energy transfer to the pup and then quickly return to sea to replenish her reserves. The nursing period is ended by the mother, who departs to sea and leaves her pup at the breeding site. Pups will continue to nurse if given the opportunity, and "milk stealers" that suckle from unrelated, sleeping females are not uncommon. (This almost always results in the death of the pup whose mother the milk was stolen from, as any single female can only produce enough milk to provision one pup.) Because the pup receives the milk energy from its mother so quickly, its development is typically not complete enough for it to begin foraging on its own as soon as the nursing period is complete. Seals, like all marine mammals, need time to develop the oxygen stores, swimming muscles and neural pathways necessary for effective diving and foraging. Because of this, most phocids undergo a postweaning fast, in which they remain on or near the breeding site and live off of the fat stores they acquired from their mothers until they are ready to begin foraging on their own. These pups typically eat no food and drink no water during the fast, although some polar species have been observed to eat snow. The physiological and behavioral adaptations that allow phocid pups to endure these remarkable fasts, which are among the longest for any mammal, remain an area of active study and research. EvolutionIt has long been believed that the pinnipeds are descended from a land-based carnivore, something approximately like a dog that has undergone aquatic adaptation. During the 20th Century there was considerable debate about the relationship between them; some taxonomists maintaining the traditional view that they share a common ancestor, others suggesting that the eared seals (sea lions and fur seals) are distinct from the true seals, and that the similarities between the two groups are the result of convergent evolution. If this were so, Pinnipedia would be a paraphyletic grouping with no taxonomic meaning. Recent studies of mitochondrial DNA, however, have strongly supported the monophyletic hypothesis: that is, the evidence is currently on the side of a single-ancestor theory.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Seal" and from http://www.white-on.com
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