Sea lions are seven species of marine mammals within the family of eared seals (a family of sixteen species which include sea lions and fur seals and, together with  true seals and walruses, form the group of marine mammals known as Pinnipeds.)

Physical Appearance

Sea lions differ from the true seals in having small external earflaps and hind flippers that can be turned to face forwards. Together with strong front flippers, this gives them extra mobility on land and an adult sea lion can move extremely fast across a beach if it has to. They also use their front flippers for swimming, whereas true seals use their hind flippers.

Like other Eared seals, the males are considerably larger than the female, in some instances, up to five times as large. This makes them among the most sexually dimorphous mammals.

Sea lions differ from fur seal in that they have coarse, short fur in contrast to a dense underfur characteristic of fur seal. Sea lions are also generally larger than fur seals.

Species

The seven species of sea lions are grouped in five genera:

• Eumetopias
  1. Steller sea lion (also Northern sea lion, Sea king, Stellar sea lion, and Steller's sea lion) (Eumetopias jubatus)
• Neophoca
  2. Australian sea lion (also White-capped sea lion) (Neophoca cinerea)
• Otaria
  3. South American sea lion (also Southern Sea Lion, Patagonian Sea Lion, and Maned sea lion.  (Otaria flavescens)
• Phocarctos
  4. New Zealand sea lion (Phocarctos hookeri)
• Zalophus
  5.  California sea lion (Zalophus californianus)
  6.  Japanese sea lion (Zalophus japonicus)
  7.  Galapagos sea lion (Zalophus wollebaeki)

Some species of sea lions, particularly California sea lions, are intelligent, docile and trainable and frequently used to educational purposes in zoos and aquariums, and for entertainment in marine parks and circuses. Other species, like the Steller Sea Lion, are quite antisocial and can be aggressive.

Stellar Sea Lion. Source: Tom Early/BioLib/Encyclopedia of Life	Australian Sea Lion. Source: Cody Pope	New Zealand sea lion. Source: Roger Kirkwood/WoRMS/Encyclopedia of Life
California sea lion. Source: David Corby/Wikipedia	Galápagos sea lion. Source: Kelley Kane/Wikipedia	Japanese sea lion (stuffed specimen at Tenn?ji Zoo, Osaka, Japan.) Source: Nkensei/Wikipedia based on Wolf et. al. (further reading #10)
South American sea lion. Source:Biopix/Encyclopedia of Life

Reproduction

Sea lions are polygynous, meaning that males will establish territories (often created and protected through fighting or shows of aggression) within which they establish a harem and breed with a number of females (the range varies with species). Males will come ashore and establish their territories at the beginning of the mating season.

Females typically arrice a few weeks after the males and select their mates for the coming season. Before mating, the females will first give birth to a pup conceived during the mating season of the prior year. Mating occurs shortly after the birth of the pup.

Distribution

Conservation Status

Sea lions have been significantly impacted by human activities. The populations of only two of the seven species are considered to be doing well. Three species (Steller Sea Lion, Australian sea lion and Galápagos sea lion)  are endangered and one (Japanese sea lion) is believed to have become extinct in the 1950s.

Further Reading

1. Zalophus japonicus Aurioles, D. & Trillmich, F., 2008, IUCN (accessed April 8, 2009)
2. wollebaeki Sivertsen, 1953 Encyclopedia of Life (accessed April 8, 2009)
3. Zalophus californianus (Lesson, 1828) Encyclopedia of Life (accessed April 8, 2009)
4. Zalophus, Seal Conservation Society (accessed April 8, 2009)
5. [1], MarineBio.org (accessed, April 8, 2009)
6. Galápagos and Californian sea lions are separate species: Genetic analysis of the genus Zalophus and its implications for conservation management, Wolf, JB; Tautz, D; Trillmich, F, Frontiers in zoology, 2007.
7. Archaeology and holocene sand dune stratigraphy on Chatham Island, B. G. McFadgen, Journal of the Royal Society of New Zealand, 1994. 
8. The Pinnipeds: Seals, Sea Lions, and Walruses, Marianne Riedman, University of California Press, 1991 ISBN: 0520064984 
9. Encyclopedia of Marine Mammals, Bernd Wursig, Academic Press, 2002 ISBN: 0125513402
10. Marine Mammal Research: Conservation beyond Crisis, edited by John E. Reynolds III, William F. Perrin, Randall R. Reeves, Suzanne Montgomery and Timothy J. Ragen, Johns Hopkins University Press, 2005 ISBN: 0801882559
11. Walker's Mammals of the World, Ronald M. Nowak, Johns Hopkins University Press, 1999 ISBN: 0801857899