Trichechidaemanatees

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The three species that make up this family are placed in one genus, Trichechus. Manatees are found along the coast of the southeastern US, in the West Indies and adjacent parts of South America, in the Amazon and Orinoco drainages of South America, and tropical west Africa.

Manatees are very large. Their maximum length may exceed 4 m, and their weight 1000 kg. They have a streamlined body; small and rounded head with a square, boxy snout; forelimbs flattened and webbed to form flippers, and a large, dorso-ventrally flattened tail fin or fluke. This fin has a single lobe. Their eyes are small, and the numerous large and thick vibrissae probably serve an important role in helping the animal detect its surroundings in the muddy waters that manatees seem to prefer. The flippers bear vestigial nails. Manatees are excellent swimmers, able to stay submerged for more than 15 minutes, but they are incapable of travelling on land.

The lips of a manatee are deeply divided right from left, and the two sides work together during feeding, grasping vegetation (aided by the short, coarse bristles that line the outer surface of the lips) to bring in food.

The skulls of manatees can easily be distinguished from those of dugongs by a number of characters. A few of the more conspicuous ones are listed here. Manatee skulls usually possess nasals, which are absent or vestigial in dugongs. The premaxillae are relatively small compared to those of dugongs, and only slightly deflected ventrally. In addition, the jugal is expanded behind the orbit, but does not reach the premaxilla. The supraorbital processes of the frontals are strongly developed and form a ledge over the orbits, and the frontal region is especially long. The teeth also are unmistakeable. Adults have no incisors or canines. Incisors are present as milk teeth, but buried in the premaxillae and covered by a horny plate. There are a large and possibly indefinite number of cheek teeth in manatees, an unusual condition in mammals, but these teeth migrate forward in the jaw as the animal ages and rarely are more than 6 present at any time. The teeth are brachydont and strongly lophodont.

Manatees, like dugongs, have an unusually dense postcranial skeleton, especially the ribs. They are unusual in that they have only 6 cervical vertebrae, instead of the 7 seen in most other mammals.

Manatees live in loose aggregations, sometimes alone and sometimes in groups of over 200 individuals. These aggregations often form around artificial sources of warm water, such as power plant outflows (which may be responsible for manatees' expanding their ranges northward along the east coast of the United States in recent years). When in groups, manatees are in frequent contact with each other, touching, rubbing, and rolling against their neighbors' bodies. They feed on aquatic plants of a number of species.

Manatees are frequently hunted for their hides and mildly flavored, fatty meat. They are also often involved in accidents with boats. Habitat destruction, however, may provide the greatest threat to their existence.

Fossil manatees are known from the Early Miocene of South America.

Literature and references cited

Nowak, R.M. and J.L. Paradiso. 1983. Walker's Mammals of the World, Fourth edition. John Hopkins University Press, Baltimore, London.

Rathbun, G. B. 1984. Sirenians. Pp. 537-547 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.

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

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

uses smells or other chemicals to communicate

endothermic

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

having the capacity to move from one place to another.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate