Phocidaeseals
This diverse group contains 19 species in 10 genera. Phocids are broadly distributed along coastlines above 30 degrees N latitude and south of 50 degree S latitude. Some species are also found at intermediate tropical localities, and in a few freshwater lakes and rivers.
Members of this family vary tremendously in size, from small ringed seals, which weigh around 90 kg, to massive elephant seals, the males of which weigh up to 3600 kg and are the largest of the pinnipeds. Their bodies are streamlined ("fusiform"). They lack any external ear. Forelimbs are relatively short, less than 25% of the length of the body and smaller than the hind flippers. They have well developed claws. The large hind flippers extend straight backward and cannot be brought under the body. On land, earless seals are awkward, moving by a combination of sliding and flexing their spines from side to side. Even so, some species are capable of moving faster than a human. Phocids have a short, stubby tail, and males have a well-developed baculum.
Young of many phocids are covered with dense, soft, often white coats. In adults, the fur is often stiff and short, without an appreciable undercoat. A few species are nearly naked. Some have spotted or banded color patterns. A thick, insulating layer of blubber lies beneath the skin; the weight of the blubber may amount to more than 25% of the entire weight of the animal.
The skulls of phocids nearly or completely lack postorbital processes, and the alisphenoid canal is also absent. The bullae are somewhat inflated. The dental formula is 2-3/1-2, 1/1, 4/4, 0-2/0-2 = 26-36. The upper incisors have simple, pointed crowns. The canines are long and pointed, and the cheek teeth usually have 3 cusps, but their structure varies considerably among species. In crab-eating seals, for example, the cheek teeth have complex cusps that make them into sort of a sieve, used for straining plankton.
Most seals feed on fish, squid, octopus, and shellfish, but some take plankton, and one species catches penguins and small seals.
The social structure of phocids varies from species to species. Some are monogamous or associate in small groups, while elephant seals are highly gregarious and polygamous. Most seals differ from sea lions, however, in that they do not congregate in the huge rookeries. Some species are migratory. Seals are accomplished divers. Their ability to reach great depths and stay under water for prolonged periods varies considerably from species to species. The champion diver may be the Weddell seal, which is known to reach depths of 600 m and to stay submerged for more than an hour.
Literature and references cited
Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, and J. F. Merritt. 1999. Mammalogy. Adaptation, Diversity, and Ecology. WCB McGraw-Hill, Boston. xii+563pp.
Paradiso, J. L. 1975. Walker's Mammals of the World, Third Edition. Johns Hopkins University Press, Baltimore.
Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution, an Illustrated Guide. Facts of File Publications, New York. 259 pp.
Stains, H. J. 1984. Carnivores. Pp. 491-521 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.
Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vii+576 pp.
Vaughan, T. A., J. M. Ryan, N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia. vii+565pp.
Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.
Contributors
Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.
Glossary
- bilateral symmetry
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having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
- chemical
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uses smells or other chemicals to communicate
- endothermic
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animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
- motile
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having the capacity to move from one place to another.
- sexual
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reproduction that includes combining the genetic contribution of two individuals, a male and a female
- tactile
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uses touch to communicate