Difference between revisions of "Eared seal" - New World Encyclopedia

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==Behavior, feeding, and reproduction==
 
==Behavior, feeding, and reproduction==
  
All otariids breed on land during well-defined breeding seasons.  Except for the [[Australian Sea Lion|Australian sea lion]], which has an atypical 17.5 month breeding cycle, they form strictly annual aggregations on beaches or rocky substrates, often on islands.  All species are [[polygynous]], i.e. successful males breed with several females. In most species, males arrive at breeding sites first and establish and maintain territories through vocal and visual displays and occasional fighting. Females typically arrive on shore a day or so before giving birth.  While considered social animals, there are no permanent hierarchies or statuses established on the colonies. The extent to which males control females or territories varies between species.  Thus northern fur seals and [[South American sea lion]]s tend to herd specific harem-associated females, occasionally injuring them, while Steller sea lions and [[New Zealand sea lion]]s control spatial territories but do not generally interfere with the movement of the females. 
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All otariids breed on land during well-defined breeding seasons.  Except for the [[Australian Sea Lion|Australian sea lion]], which has an atypical 17.5 month breeding cycle, they form strictly annual aggregations on beaches or rocky substrates, often on islands.  All species are [[polygynous]], i.e. successful males breed with several females.  
  
Otariids are carnivorous, feeding on [[fish]], [[squid]] and [[krill]]. Sea lions tend to feed closer to shore in [[upwelling]] zones feeding on larger fish while the smaller fur seals tend to take longer, offshore foraging trips and can subsist on large numbers of smaller prey items.   They are visual feeders and some females are capable of dives up to 400 m (1300 ft).  
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In most species, males arrive at breeding sites first and establish and maintain territories through vocal and visual displays and occasional fighting. Females typically arrive on shore shortly before giving birth to pups from the mating of the previous year. Females go into [[estrous]] sometime after giving birth, perhaps a week or two weeks later, and they breed again, but implanting of the embryo is delayed, allowing an annual cycle in most species.
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 +
While considered social animals, there are no permanent hierarchies or statuses established on the colonies. The extent to which males control females or territories varies between species. Northern fur seals and [[South American sea lion]]s tend to herd specific harem-associated females, occasionally injuring them, while Steller sea lions and [[New Zealand sea lion]]s control spatial territories but do not generally interfere with the movement of the females. 
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Otariids are carnivorous, feeding on [[fish]], [[squid]], and [[krill]]. Sea lions tend to feed closer to shore in [[upwelling]] zones feeding on larger fish while the smaller fur seals tend to take longer, offshore foraging trips and can subsist on large numbers of smaller prey items. They are visual feeders and some females are capable of dives up to 400 meters (1300 feet).
  
 
==Evolution and taxonomy==
 
==Evolution and taxonomy==

Revision as of 17:57, 3 July 2008

Eared seals
An Australian Sea Lion
An Australian Sea Lion
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Suborder: Caniformia
Superfamily: Pinnipedia
Family: Otariidae
Gray, 1825
Genera

Arctocephalus
Callorhinus
Eumetopias
Neophoca
Otaria
Phocarctos
Zalophus

Eared seal is the common name for any of the marine mammals comprising the pinniped family Otariidae, characterized by presence of a pinna (external part of ear), the ability to invert their hind-flippers under the body, aiding land movement, and a swimming motion using their long front flippers to propel them through the water. These characteristics help distinguish otariids from the earless seals of the family Phocidae. Extant eared seals comprise 16 species in seven genera commonly known either as sea lions or fur seals.

Otariids are adapted to a semi-aquatic lifestyle, feeding and migrating in the water but breeding and resting on land or ice. They reside in subpolar, temperate, and equatorial waters throughout the Pacific and Southern oceans and the southern Indian and Atlantic oceans. These marine mammals are conspicuously absent in the north Atlantic.

Overview

Eared seals are one of the three main groups of mammals within the taxonomic group Pinnipedia. Pinnipeds are aquatic (mostly marine) mammals that are characterized by having both front and hind limbs in the form of flippers. In addition to eared seals, other pinnipeds are walruses and earless seals.

Eared seals are one of two groups of seals (any pinniped other than walruses): earless seals, comprising the taxonomic family Phocidae (phocids), and eared seals comprising the family Otariidae (otariids). Walruses generally are considered a separate family of pinnipeds, the Obobenidae, although sometimes they are included with the phocids (McKenna and Bell 1997).

One way of differentiating between the two main groups of seals is by the presence of the pinna, a small furry earflap (external ears), found on the otarids and missing from phocids. Phocids are referred to as "earless seals" because their ears are not easily seen, while otarids are referred to as "eared seals." The name otariid comes from the Greek otarion meaning "little ear," referring to the small but visible external ear flaps (pinnae).

In addition to the presence of the pinna, there are other obvious differences between otarids and phocids. Otarids have hind-flippers that can be inverted under the body, aiding their movement on land, while the hind-flippers of phocids cannot be turned forward under the body, causing their movement on land to be slow and awkward (Riedman 1990). Otarids also swim using their long front flippers to move themselves through the water, while phocids swim by using their rear flippers and lower body in a side-to-side motion (Riedman 1990). There are also behavioral differences, including the breeding systems.

The eared seals include both fur seals and sea lions, with the fur seals traditionally placed in the otariid subfamily Arctocephalinae and the sea lions in the subfamily Otariinae. However, some taxonomists suggest that the differences between the fur seals and the sea lions are not great enough to separate them into these two subfamilies (Riedman 1990).

There are no otariids in the extreme polar regions; among seals, only earless seals live in the Antarctic and Arctic (Riedman 1990). On the other hand, a number of fur seals and sea lions live in tropical and subtropical areas, while only two species of phocids (the endangered Hawaiian and Mediterranean monk seals) are found in the tropics and these are small populations; fur seals also range widely into colder climates as well (Riedman 1990).

Anatomy and Appearance

Otariids have proportionately much larger fore-flippers and pectoral muscles than phocids, and have the ability to turn their hind limbs forward and walk on all fours, making them far more maneuverable on land. They are generally considered to be less adapted to an aquatic lifestyle, since they breed primarily on land and haul out more frequently than true seals. However, they can attain higher bursts of speed and greater maneuverability in the water. Their swimming power derives from the use of flippers more so than the sinuous whole body movements typical of phocids and walruses.

Otariids are further distinguished by a more dog-like head, sharp, well-developed canines, and the aforementioned visible external pinnae. Their postcanine teeth are generally simple and conical in shape. The dental formula for eared seals is:

3.1.4.1-3
2.1.4.1

In general, fur seals have a more pointed snout and longer fore-flippers than sea lions, and they have a thick, luxuriant coat of fur (pelage) (Riedman 1990). The underfur, which is waterproof, is covered with long, "guard" hairs that give them a "somewhat grizzled appearance" (Riedman 1990). The thick underfur of fur seals have historically made them the objects of commercial exploitation. Sea lions, which generally are larger than fur seals, have a more rounded muzzle and shorter fore-flippers than fur seals, and their pelage is more short and coarse (Riedman 1990).

Male otariids range in size from the 70 kilograms (150 pounds) Galapagos fur seal, smallest of all pinnipeds, to the over 1000 kilogram (2200 pound) Steller sea lions. Mature male otariids weigh two to six times more than females with proportionately larger heads, necks, and chests, making them the most sexually dimorphic of all mammals (Weckerly 1998).

Behavior, feeding, and reproduction

All otariids breed on land during well-defined breeding seasons. Except for the Australian sea lion, which has an atypical 17.5 month breeding cycle, they form strictly annual aggregations on beaches or rocky substrates, often on islands. All species are polygynous, i.e. successful males breed with several females.

In most species, males arrive at breeding sites first and establish and maintain territories through vocal and visual displays and occasional fighting. Females typically arrive on shore shortly before giving birth to pups from the mating of the previous year. Females go into estrous sometime after giving birth, perhaps a week or two weeks later, and they breed again, but implanting of the embryo is delayed, allowing an annual cycle in most species.

While considered social animals, there are no permanent hierarchies or statuses established on the colonies. The extent to which males control females or territories varies between species. Northern fur seals and South American sea lions tend to herd specific harem-associated females, occasionally injuring them, while Steller sea lions and New Zealand sea lions control spatial territories but do not generally interfere with the movement of the females.

Otariids are carnivorous, feeding on fish, squid, and krill. Sea lions tend to feed closer to shore in upwelling zones feeding on larger fish while the smaller fur seals tend to take longer, offshore foraging trips and can subsist on large numbers of smaller prey items. They are visual feeders and some females are capable of dives up to 400 meters (1300 feet).

Evolution and taxonomy

Along with the Phocidae and Odobenidae, the two other members of Pinnipedia, Otаriidae are descended from a common ancestor most closely related to modern bears.[1] There remains debate as to whether the phocids diverged from the otariids before or after the walruses.

Otariids arose in the late Miocene (10-12 million years ago) in the North Pacific, diversifying rapidly into the Southern Hemisphere where most species now live. The Callorhinus (northern fur seal) genus is considered to have the oldest lineage.

Traditionally, otariids had been subdivided into the fur seal (Arctocephalinae) and sea lion (Otariinae) subfamilies, with the major distinction between them being the presence of a thick underfur layer in the former. Under this categorization, the fur seals comprised two genera: Callorhinus in the North Pacific with a single representative, the northern fur seal (C. ursinus) and eight species in the southern hemisphere under the genus Arctocephalus, while the sea lions comprise five species under five genera.[2] Recent analyses of the genetic evidence suggests that the Callorhinus ursinus is in fact more closely related to several sea lion species.[3] Furthermore, many of the Otariinae appear to be more phylogenetically distinct than previously assumed; for example, the Zalophus japonicus is now considered a separate species, rather than a subspecies of Zalophus californius. In light of this evidence, the subfamily separation has been removed entirely and the Otariidae family has been organized into seven genera with 16 species and two subspecies.[4][5]

Nonetheless, because of morphological and behavioral similarity among the "fur seals" and "sea lions", these remain useful categories when discussing differences between groups of species.


Species

  • SUBORDER PINNIPEDIA
  • Family Otariidae
    • Genus Arctocephalus
      • Antarctic Fur Seal, A. gazella
      • Guadalupe Fur Seal, A. townsendi
      • Juan Fernandez Fur Seal, A. philippii
      • Galapagos Fur Seal, A. galapagoensis
      • Cape Fur Seal, A. pusillus
        • South African Fur Seal, A. pusillus pusillus
        • Australian Fur Seal, A. pusillus doriferus
      • New Zealand Fur Seal or Southern Fur Seal, A. forsteri
      • Subantarctic Fur Seal, A. tropicalis
      • South American Fur Seal, A. australis
    • Genus Callorhinus
      • Northern Fur Seal, C. ursinus
    • Genus Eumetopias
      • Steller Sea Lion, E. jubatus
    • Genus Neophoca
      • Australian Sea Lion, N. cinerea
    • Genus Otaria
      • South American Sea Lion, O. flavescens
    • Genus Phocarctos
      • New Zealand Sea Lion or Hooker's Sea Lion, P. hookeri
    • Genus Zalophus
      • California Sea Lion, Z. californianus
      • †Japanese Sea Lion, Z. japonicus - extinct (1950s)
      • Galapagos Sea Lion, Z. wollebaeki
  • Family Phocidae: true seals
  • Family Odobenidae: Walrus

Notes

  1. Lento, G.M., Hickson, R.E., Chambers, G.K., Penny, D. (1995) Use of spectral analysis to test hypotheses on the origin of pinnipeds. Molecular Biology and Evolution 12(1) : 28-52.
  2. J.E. King (1983) Seals of the World (2nd edition). Cornell University Press, New York. ISBN 978-0702216947
  3. Wynen, L.P. et al. (2001) Phylogenetic relationships within the eared seals (Otariidae: Carnivora): implications for the historical biogeography of the family. Mol. Phylog. Evol. 21, 270–284
  4. Brunner, S. (2003) Fur seals and sea lions (Otariidae): identification of species and taxonomic review. Systematics and Biodiversity 1(3):339-439.
  5. Scientific Name: Otariidae, Integrated Taxonomic Information System-North America - accessed November, 2007

References
ISBN links support NWE through referral fees

  • McKenna, M. C., and S. K. Bell. 1997. Classification of Mammals above the Species Level. New York: Columbia University Press. ISBN 023111012X.
  • McLaren, I. 1984. Phocidae. Pages 270 to 275 in D. Macdonald, The Encyclopedia of Mammals. New York: Facts on File. ISBN 0871968711. >
  • Riedman, M. 1990. The Pinnipeds: Seals, Sea Lions, and Walruses. Berkeley: University of California Press. ISBN 0520064976.
  • Savage, R. J. G., and M. R. Long. 1986. Mammal Evolution: An Illustrated Guide. New York: Facts on File. ISBN 081601194X.

. [1]

  • Wozencraft, W. C. 2005. Order Carnivora. In D. E. Wilson and D. M. Reeder, eds., Mammal Species of the World, 3rd edition. Baltimore: Johns Hopkins University Press. ISBN 0801882214.

Further reading

  • Berta, A., and L. Sumich (1999) Marine Mammals: Evolutionary Biology. San Diego: Academic Press.
  • Gentry, R. L (1998) Behavior and Ecology of the Northern Fur Seal. Princeton: Princeton University Press.
  • Perrin, W. F., B. Würsig, and J. G. M. Thewissen (2002) Encyclopedia of Marine Mammals. San Diego: Academic Press.
  • Riedman, M. (1990) The Pinnipeds: Seals, Sea Lions and Walruses. Berkeley: University of California Press.

External links

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  1. Weckerly, FW (1998). Sexual-size dimorphism: influence of mass and mating systems in the most dimorphic mammals.. Journal of Mammalogy 79 (1): 33–42.