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manatee, sea cow
, the Amazonian manatee. The Amazonian manatee is the smallest of the manatees. Several physical characteristics distinguish it from the other two species. It lacks nails on its pectoral fins, and usually has whitish patches on its underside.
, the West Indian manatee. There are two subspecies of the West Indian manatee: the Florida manatee (
Trichechus manatus latirostris
) and the Antillean manatee (
Trichechus manatus manatus
). These subspecies are distinguished by differences in cranial measurements and by their geographic distribution.
, the West African manatee. About the same size and shape of the West Indian manatee, the West African manatee differs in some important respects: position of the eyes, snout, and cranial bones.
The manatee's body is streamlined - full around the middle and narrowing to a paddle-shaped tail.
The true color of a manatee is gray, although it may appear brownish gray. Amazonian manatees usually have white or pink patches on the belly and chest.
Organisms such as algae, which may grow on the skin of slow-moving individuals, alter the body color and make some manatees look more green or brown.
Adult West Indian and West African manatees average about 3 m (10 ft.) in length. Large individuals may reach lengths of up to 4 m (13 ft.).
Amazonian manatees are the smallest of all three species. They are shorter and more slender. The longest specimen measured 2.8 m (9.2 ft.).
Average adult weights are approximately 363 to 544 kg (800-1,200 lb.). One particularly large Florida manatee weighed 1,655 kg (3,650 lb.). Females are generally larger than males. A particularly large Amazonian manatee individual weighed 480 kg (1,058 lb.).
Manatees are primarily herbivores. They feed on a wide variety of submerged, emergent, floating, and shoreline vegetation. Manatees in Florida feed on more than 60 species of plants including turtle grass, manatee grass, shoal grass, mangrove leaves, various algae, water hyacinth, acorns, and hydrilla. Manatees consume about 4% to 9% (15 to 49 kg or 32-108 lb. for an average adult manatee) of their body weight in wet vegetation daily.
In Sierra Leone, Africa, the West African manatee is considered a pest because they consume fields of planted rice.
Manatees have occasionally been seen to eat foods other than plants. Antillean manatees have been known to eat fish from nets and West African manatees have been known to eat clams.
Some Amazonian manatees living in deep bodies of water apparently fast during dry seasons (November and December) when water levels drop as much as 9 to 15 m (30-50 ft.), eliminating their access to vegetation.
Because manatees have a very low metabolic rate, it is speculated that Amazonian manatees are able to fast for up to seven months if necessary.
The exact gestation period of most manatee species is not known and is poorly studied. It is known, however, that a Florida manatee's gestation is approximately 12 months.
Male manatees (bulls) are sexually mature by about nine to ten years, although some have produced viable sperm at two to three years of age.
Female manatees (cows) are sexually mature as young as three years of age.
All living manatee species are found in warm tropical and subtropical waters.
The West Indian manatee was once abundant throughout the tropic and subtropical western North and South Atlantic and Caribbean waters. However, the manatee's numbers have been greatly reduced and is rarely seen throughout its range.
The range of the Florida manatee is primarily peninsular Florida but extends as far north as Rhode Island. Manatees have been rescued near Houston, Texas, and Mississippi.
Antillean manatees have a patchy distribution throughout the Caribbean, Mexico, and northeastern South America. The southern range extends through Central and South America to Brazil.
West African manatees range from Senegal to Angola, on the west coast of Africa.
Amazonian manatees are the only species of manatee confined to fresh water. They inhabit the Amazon Basin, mostly in Brazil. They are uncommon or close to extinction in Peru, Ecuador, and Colombia.
The West Indian and West African manatees inhabit rivers, bays, canals, estuaries, and coastal areas rich in seagrass and other vegetation. They can live in fresh, saline (salt), and brackish waters. They move freely between extremes.
West Indian manatees may be found in any waterway over 1 m (3.25 ft.) deep and connected to the coast. They prefer waters with temperatures above 21°C (70°F). Florida manatees rarely venture into deep ocean waters. However, manatees have been spotted as far offshore as the Dry Tortugas Islands, approximately 81 km (50 mi.) west of Key West, Florida.
West African manatees live in quiet coastal areas, large rivers, lagoons, and connected lakes, where the water temperature is above 18°C (64°F).
The Amazonian manatee is restricted to fresh water. They are most common in floodplain lakes and channels in white-water river systems with water temperatures ranging from 25°-30°C (77°-86°F).
The patchy distribution of manatees throughout all their ranges is due to their search of suitable habitat: plentiful aquatic plants and a freshwater source of water to drink.
Populations of all species of manatees have apparently declined over the past hundred years. These declines are due to such causes as hunting for their meat, destruction of their habitats, boating, pollution, and low reproduction rates. Antillean, Amazonian, and West African manatee populations are not known due to inadequate scientific research.
An aerial census is done at least once a year on the Florida manatee. The Florida manatee census taken in February, 2004, found 2,505 individuals. The 2005 survey was 3,143. These figures represent the minimum amount of manatees known to be in the state of Florida at the time of the census. Detailed Florida manatee mortality statistics can be found at http://www.floridamarine.org/manatees/
All four species of sirenians are endangered or threatened. The Florida manatee is one of the most endangered marine mammals in the United States.
Manatees average 3 m (10 ft.) and 363-544 kg (800-1,200 lb.). Manatees have paddlelike tails and foreflippers, and no hind limbs.
Manatees belong to the mammalian order Sirenia. There are three species of manatee:
, the Amazonian manatee;
, the West African manatee; and
, the West Indian manatee. Scientists also recognize two subspecies of West Indian manatee:
, the Florida manatee, and
the Antillean manatee. The order Sirenia has one other living species, the dugong. A fifth species, Steller's sea cow, was hunted to extinction by 1768.
All manatees inhabit tropical and subtropical waters of North and South America and Africa. The West Indian and West African manatees live in rivers, bays, estuaries, and coastal areas. They can move freely between freshwater and saltwater habitats. The Amazonian manatee is restricted to the freshwater Amazon basin.
All manatees are herbivores. The Florida manatee feeds on more that 60 varieties of grasses and plants. A manatee's only teeth are molars, for grinding vegetation. Some research suggests that manatees periodically require fresh water. West Indian manatees have been seen congregating at river mouths and drinking from hoses, culverts, and sewage outfalls.
A female manatee, called a cow, gives birth about once every three years. Gestation lasts about 12 months. At birth, West Indian manatees measure about 1.2-1.4 m (4-4.5 ft.) and weigh 27-32 kg (60-70 lb.). A calf nurses from teats under the mother's pectoral flippers.
Florida manatees are highly susceptible to cold; it's not unusual for them to die during extremely cold weather. But human activities pose a much greater threat to manatees. Each year, many Florida manatees are killed and injured by watercraft, the greatest human-induced cause of Florida manatee deaths. Other threats include entanglement, poaching, and habitat destruction. All manatees are threatened or endangered. They are protected by national and local legislation in every country in which they are found.
Manatees are nonaggressive, nonterritorial herbivores that spend most of their time feeding (six to eight hours per day) and resting (two to twelve hours per day). The remainder of their day is spent traveling, investigating objects, and socializing by interacting with other manatees.
Marine zoological parks such as SeaWorld Florida and SeaWorld California help conserve manatees. SeaWorld is authorized to rescue and rehabilitate ill, injured, or orphaned manatees. The first priority of the Manatee Rescue and Rehabilitation program is to return healthy manatees to their natural habitat. SeaWorld Florida has successfully rescued and released more than 150 manatees as of January 2005.
Manatees emit sounds under water. The chirps, whistles, or squeaks are probably produced in the larynx. They seem to make these sounds when they are frightened, sexually aroused, or interacting with each other.
Manatees have well-developed eyes. In the water, they can detect objects from tens of meters away. A manatee's retinas contain both rod and cone cells, indicating that they probably have the ability to see both dim and bright light. Recent tests suggest manatees can distinguish between blue and green colors, although the full extent of their color vision is unknown and more studies are needed.
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ECOLOGY AND CONSERVATION
All species of sirenians are protected to some extent by national or local acts in every country they inhabit.
Federal and state laws have been passed that protect Florida manatees. Federal laws prohibit hunting, capturing, killing, or harassing these animals. These laws include the Marine Mammal Protection Act of 1972 and the Endangered Species Act of 1973. Penalties for breaking these federal laws could include a year in prison and a fine up to $20,000.
The Florida Manatee Sanctuary Act of 1978 is state legislation that allows the establishment of manatee sanctuaries in Florida.
West African manatees are protected under Class A of the African Convention of Nature and Natural Resources, signed by 38 African countries.
Amazonian manatee hunting has been prohibited since 1973.
The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) is an international treaty developed in 1973 to regulate trade of wildlife items. All species of sirenians are protected by this treaty.