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More About Seals and Sea Lions ...
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Pinnipeds are 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.
It 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.
The true seals or earless seals are one of the three main groups of mammals within the seal suborder, Pinnipedia. All true 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.
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. 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. (It should be noted that the common seal (harbor seal in the U.S.), Phoca vitulina, does not separate foraging and maternal investment; instead, it displays a reproductive strategy similar to those of otariids, in which the mother makes short foraging trips between nursing bouts.)
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 postweaning fast ranges from 2 weeks in the Hooded Seal to 9-12 weeks in the Northern Elephant Seal. 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.
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Taxonmony
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Phylum: chordata
Class: mammalia
Order: Carnivora
Family: Phocidae
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Source: Wikipedia Read more about Seals and Sea Lions
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