Sirenia

"Seacow" redirects here.

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The Sirenia (commonly referred to as sea cows) are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. Four species are living, in two families and genera. These are the dugong (one species) and manatees (three species). Sirenia also include Steller's sea cow, extinct since the 18th century, and a number of taxa known only from fossils. The order evolved during the Eocene, more than 50 million years ago.

Sirenia, commonly sirenians, are also referred to by the common name sirens, deriving from the sirens of Greek mythology. This comes from a legend about their discovery, involving lonely sailors mistaking them for mermaids.

"Sea cow" (seekoei) is also the name for a hippopotamus in Afrikaans. In Germanic languages, the word See can mean either a body of fresh or salt water, so this follows from the species inhabiting lakes in southern Africa rather than the sea itself.

Overview

Manatees, which sometimes are known as sea cows, are part of the order Sirenia along with dugongs. Sirenians are one of four groups of marine mammals, the others being cetaceans (whales, dolphins, and porpoises), sea otters, and pinnipeds (walruses, earless seals, and eared seals). The sirenians and cetaceans are completely aquatic, while pinnipeds spend considerable time on land, including giving birth and raising their young, and sea otters can mate and raise their young entirely at sea. Sirenia is thought to have evolved from four-legged land mammals over 60 million years ago, with the closest living relatives being the Proboscidea (elephants) and Hyracoidea (hyraxes) (Domning 1994).

The dugong is a large marine mammal which, together with the manatees, is one of four living species of the order Sirenia. Sirenians are one of four groups of marine mammals, the others being cetaceans (whales, dolphins, and porpoises), sea otters, and pinnipeds (walruses, earless seals, and eared seals). The sirenians and cetaceans are completely aquatic, while pinnipeds spend considerable time on land, including giving birth and raising their young, and sea otters can mate and raise their young entirely at sea. Sirenia is thought to have evolved from four-legged land mammals over 60 million years ago, with the closest living relatives being the Proboscidea (elephants) and Hyracoidea (hyraxes) (Domning 1994). The dugong is the only strictly-marine herbivorous sirenian, as all species of manatee utilize freshwater to some degree.

Like all modern sirenians, the dugong has a fusiform body with no dorsal fin or hindlimbs, instead possessing paddle-like forelimbs used to maneuver itself. The main visual difference between manatees and dugongs are the tails. A manatee tail is paddle-shaped, while a dugong tail is forked, similar in shape to that of a whale of dolphin. The paddle-like forelimbs aid in movement and feeding, while its fluked tail provides locomotion through vertical movement. The dugong also possesses a unique skull and teeth (Myers 2002).

Unlike the manatees, the dugong's teeth do not continually grow back via horizontal tooth replacement (Self-Sullivan 2007). The dugong has two incisors (tusks) which grow posteriorly until puberty, after which they first erupt in males. The female's tusks continue to grow posteriorly, often not externally evident, but sometimes erupting later in life after reaching the base of the premaxilla (Marsh 1989). The full dental formula of dugongs is:

2.0.3.3

3.1.3.3

Like other sirenians, the dugong experiences pachyostosis, a condition in which the ribs and other long bones are unusually solid and contain little or no marrow. These heavy bones, which are among the densest in the animal kingdom (Waller et al. 1996), may act as a ballast to help keep sirenians suspended slightly below the water's surface (Myers 2000).

Dugongs are generally smaller than manatees (with the exception of the Amazonian manatee), reaching an average adult length of 2.7 meters (8.9 feet]]) and a weight of 250 to 300 kilograms (550 to 660 pounds]) (IFAW 2008). An adult's length rarely exceeds 3 meters, and females tend to be larger than males (Marsh 1989). The largest known dugong was an exceptional female landed off the Saurashtra coast of west India, measuring 4.03 meters (13.3 feet) and weighing 1,018 kilograms (2,240 pounds) (Wood 1982).

Description

Sirenians have major aquatic adaptations: the forelimbs have modified into arms used for steering, the tail has modified into a paddle used for propulsion, and the hindlimbs (legs) are but two small remnant bones floating deep in the muscle. They appear fat, but are fusiform, hydrodynamic, and highly muscular. Their skulls are highly modified for taking breaths of air at the water's surface, and dentition is greatly reduced. The skeletal bones of both the manatees and dugong are very dense, which helps to neutralize the buoyancy of their blubber. The manatee appears to have an almost unlimited ability to produce new teeth as the anterior teeth wear down. They have only two teats, located under their forelimbs, similar to elephants. The elephants are thought to be the closest living relatives of the sirenians.

The lungs of sirenians are unlobed. In sirenians, the lungs and diaphragm extend the entire length of the vertebral column. These adaptations help sirenians control their buoyancy and maintain their horizontal position in the water.

Living sirenians grow between 2.5 and 4.0 meters long and can weigh up to 1,500 kg. Hydrodamalis gigas, Steller's sea cow, could reach lengths of 8 m.

The three manatee species (family Trichechidae) and the dugong (family Dugongidae) are endangered species. All four are vulnerable to extinction from habitat loss and other negative impacts related to human population growth and coastal development. Steller's sea cow, extinct since 1786, was hunted to extinction by humans. Manatees and dugongs are the only marine mammals classified as herbivores. Unlike the other marine mammals (dolphins, whales, seals, sea lions, sea otters, and walruses), sirenians eat primarily sea grasses and other aquatic vegetation, and have an extremely low metabolism and poor tolerance for especially cold water. Sirenians have been observed eating dead animals (sea gulls), but their diets are made up primarily of vegetation. Like dolphins and whales, manatees and dugongs are completely aquatic mammals that never leave the water—not even to give birth. These animals have been observed eating grass clippings from homes adjacent to waterways, but in this rare occurrence, only the top portion of the sirenian is lifted out of the water. The combination of these factors means sirenians are restricted to warm, shallow, coastal waters, estuaries, and rivers with healthy ecosystems that support large amounts of seagrass or other vegetation.

The Trichechidae species differ from the Dugongidae in the shape of their skull and tails.

Classification

The order Sirenia has been placed in the clade Paenungulata, within Afrotheria, grouping it with two other orders of living mammals: Proboscidea, the elephant families, and Hyracoidea, the hyraxes, and two extinct orders, Embrithopoda and Desmostylia.

Subdivision

• ORDER SIRENIA
  • Family †Anthracobunidae
    • Genus †Ishatherium
      • †Ishatherium subathuensis
  • Family †Prorastomidae
    • Genus †Pezosiren
      • †Pezosiren portelli
    • Genus †Prorastomus
      • †Prorastomus sirenoides
  • Family †Protosirenidae
    • Genus †Protosiren
      • †Protosiren minima
      • †Protosiren sattaensis
      • †Protosiren fraasi
      • †Protosiren smithae
  • Family Dugongidae
    • Genus †Nanosiren
      • †Nanosiren garciae
      • †Nanosiren sanchezi
    • Genus †Sirenotherium
      • †Sirenotherium pirabense
    • Subfamily Dugonginae
      • Genus Dugong
        • Dugong dugon, dugong
    • Subfamily †Hydrodamalinae
      • Genus †Dusisiren
        • †Dusisiren dewana
        • †Dusisiren jordani
        • †Dusisiren takasatensis
      • Genus †Hydrodamalis
        • †Hydrodamalis cuestae
        • †Hydrodamalis gigas, Steller's sea cow
  • Family Trichechidae
    • Subfamily †Miosireninae
      • Genus †Anomotherium
        • †Anomotherium langewieschei
      • Genus †Miosiren
        • †Miosiren canhami
        • †Miosiren kocki
    • Subfamily Trichechinae
      • Genus †Potamosiren
        • †Potamosiren magdalenensis
      • Genus Trichechus
        • T. manatus, West Indian manatee
          • T. m. manatus, Antillean manatee
          • T. m. latirostris, Florida manatee
        • T. senegalensis, African manatee
        • T. inunguis, Amazonian manatee
        • T. "pygmaeus", dwarf manatee – validity questionable
    • Genus †Ribodon
      • †Ribodon limbatus

† extinct

References

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Notes

• Domning, D. P., 1994. Paleontology and evolution of sirenians: Status of knowledge and research needs. Proceeding of the 1st International Manatee and Dugong Research Conference, Gainesville, Florida.

• Grzimek, B., D. G. Kleiman, V. Geist, and M. C. McDade. 2004. Grzimek's Animal Life Encyclopedia. Detroit: Thomson-Gale. ISBN 0307394913.

• Marsh, H. 2008. Dugong dugon. 2008 IUCN Red List of Threatened Species. Retrieved on November 12, 2008.
• Marsh, H. 1989. Chapter 57. Dugonidae. In D. Walton, and B. J. Richardson (eds.), Fauna of Australia. Volume 1B, Mammalia. Canberra: Australian Govt. Pub. Service. ISBN 0644060565. Retrieved November 12, 2008.
• Marsh, H., H. Penrose, C. Eros, and J. Hugues. 2002. Dugong: Status reports and action plans for countries and territories. IUCN. Retrieved November 13, 2008.
• Myers, P. 2002. Dugongidae. Animal Diversity Web. University of Michigan Museum of Zoology. Retrieved November 12, 2008.
• Myers, P. 2000. Sirenia. Animal Diversity Web. University of Michigan Museum of Zoology. Retrieved November 12, 2008.

• Reep, R. L., C. D. Marshall, and M. L. Stoll. 2002. Tactile hairs on the postcranial body in Florida manatees: A mammalian lateral line? Brain, Behavior and Evolution 59: 141-154. Retrieved November 13, 2008.
• Self-Sullivan, C. 2007 Evolution of Sirenia. sirenian.org. Retrieved November 12, 2008.
• Shoshani, J. 2005. Dugong. In D. E. Wilson, and D. M. Reeder (eds.), Mammal Species of the World, 3rd ed. Johns Hopkins University Press. ISBN 0801882214.
• Waller, G., M. Burchett, and M. Dando. 1996. SeaLife: A Complete Guide to the Marine Environment. Washington, D.C.: Smithsonian Institution Press. ISBN 1560986336.

Bibliography

• Daryl P. Domning. Bibliography and Index of the Sirenia and Desmostylia.
• Template:MSW3 Shoshani
• Garrison, Tom. Oceanography, 5th Ed., Brooks Cole, 30 July 2008. ISBN 978-0-495-55531-5
• The Paleobiology Database

External links

Wikispecies has information related to:

Sirenia

Wikibooks Dichotomous Key has a page on the topic of

Sirenia

• Pezosiren portelli, earliest known Sirenian
• Save The Manatee

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