出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/02/10 15:03:47」(JST)
| West Indian manatee[1] | |
|---|---|
| Trichechus manatus | |
| Conservation status | |
|
Vulnerable (IUCN 3.1)[2] |
|
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Sirenia |
| Family: | Trichechidae |
| Genus: | Trichechus |
| Species: | T. manatus |
| Binomial name | |
| Trichechus manatus Linnaeus, 1758 [3] |
|
| West Indian manatee range | |
The West Indian manatee (Trichechus manatus) is a manatee, and the largest surviving member of the aquatic mammal order Sirenia (which also includes the dugong and the extinct Steller's sea cow).
The West Indian manatee is a species distinct from the Amazonian manatee (T. inunguis) and the African manatee (T. senegalensis). Based on genetic and morphological studies, the West Indian manatee is divided into two subspecies, the Florida manatee (T. m. latirostris) and the Antillean or Caribbean manatee (T. m. manatus).[4][5] However, recent genetic (mtDNA) research suggests that the West Indian manatee actually consists of three groups, which are more or less geographically distributed as: (1) Florida and the Greater Antilles; (2) central and northern South America; and (3) northeastern South America.[6][7]
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Like other manatees, the West Indian manatee has adapted fully to an aquatic life style, having no hind limbs. Pelage cover is sparsely distributed across the body, which may play a role in reducing the build-up of algae on their thick skin. The average West Indian manatee is approximately 2.7–3.5 m (8.9–11 ft) long and weighs 200–600 kg (440–1,300 lb), with females generally larger than males.[8] The largest individual on record weighed 1,655 kg (3,650 lb) and measured 4.6 m (15 ft) long.[9][10] This manatee's color is gray or brown. Its flippers also have either three or four nails, so it can hold its food as it is eating.
As its name implies, the West Indian manatee lives in the West Indies, or Caribbean, generally in shallow coastal areas. However, it is known to withstand large changes in water salinity, so has also been found in shallow rivers and estuaries. It can live in fresh water, saline water, and even brackish water. It is limited to the tropics and subtropics due to an extremely low metabolic rate and lack of a thick layer of insulating body fat. While this is a regularly occurring species along coastal southern Florida, during summer, this large mammal has even been found as far north as Dennis, Massachusetts and as far west as Texas.[11]
The Florida manatee (Trichechus manatus latirostris), a subspecies of the West Indian manatee, is the largest of all living sirenians. Florida manatees inhabit the most northern limit of sirenian habitat. Over three decades of research by universities, governmental agencies, and NGOs, has contributed to our understanding of Florida manatee ecology and behavior, which is more than we know about any other sirenian species. They are found in freshwater rivers, in estuaries, and in the coastal waters of the Gulf of Mexico and the Atlantic Ocean. Florida manatees may live to be greater than 60 years old in the wild, and one captive manatee, "Snooty", has lived for 63 years. In captivity, West Indian manatees live up to 28 years.[citation needed] The biggest single threat to Florida manatees is death from collisions with recreational watercraft. Large concentrations of Florida manatees are located in the Crystal River area and the Wakulla Springs regions in central and north Florida. The best time to see the Wakulla Springs manatees is in November and December, and in the spring for the Crystal River manatees.
The other subspecies of the West Indian manatee is sometimes referred to as the Antillean manatee (T. m. manatus). Antillean manatees are sparsely distributed throughout the Caribbean and the northwestern Atlantic Ocean, from Mexico, east to the Greater Antilles, and south to Brazil. They are found in French Guiana, Suriname, Guyana, Trinidad (however there has been a lack of recent sightings), Venezuela, Colombia, Panama, Costa Rica, Nicaragua, Honduras, Guatemala, Belize, Mexico, Trinidad, Cuba, Haiti, the Dominican Republic, Jamaica, and Puerto Rico. Historically, Antillean manatees were hunted by local natives and sold to European explorers for food. Today, they are threatened by loss of habitat, poaching, entanglement with fishing gear, and increased boating activity. Several of Sirenian International's scientists study Antillean manatees in Belize, which may be the last stronghold for the subspecies. Funds for research, education, and conservation projects are desperately needed in other Central American nations.
The West Indian manatee is surprisingly agile in water, and individuals have been seen doing rolls, somersaults, and even swimming upside-down. Manatees are not territorial and do not have complex predator avoidance behavior, as they have evolved in areas without natural predators. The common predators of marine mammals, such as orcas and large sharks, are rarely (if ever) found in habitats inhabited by this species.
The West Indian manatee is an opportunistic feeder, with large adults consuming 10% to 15% of their body weight in food daily. Manatees feed on about 60 plant species, which includes sea grasses as their major food source. They also consume some fish and small invertebrates. Because manatees feed on abrasive plants, their molars are often worn down and are continually replaced throughout life, called "marching molars".
The West Indian manatee has a high casualty rate due to thermal shock from cold temperatures. During cold weather, many die due to their digestive tracts shutting down at water temperatures below 68°F (20°C). Many manatee deaths are caused by large commercial vessels, but are attributed to "recreational watercraft" due to the elimination of that classification.
Although female West Indian manatees are mostly solitary creatures, they form mating herds while in estrus. Most females first breed successfully between ages of seven and nine; they are, however, capable of reproduction as early as four years of age. The gestation period (pregnancy) lasts from 12 to 14 months. Normally, one calf is born, although on rare occasions two have been recorded. The young are born with molars, allowing them to consume sea grass within the first three weeks of birth. On average, manatees that survive to adulthood will have between five and seven offspring between the ages of 20 and 26. When a calf is born, it usually weighs between 60 and 70 pounds and is between 4.0 and 4.5 feet long. The family unit consists of mother and calf, which remain together for up to two years. Males aggregate in mating herds around a female when she is ready to mate, but contribute no parental care to the calf.
The West Indian manatee has been hunted for hundreds of years for meat and hide, and continues to be hunted in Central and South America. Illegal poaching, as well as collisions with speeding motorboats, are a constant source of manatee fatalities.
Due to their low reproductive rates, a decline in manatee population may be hard to overcome. They enjoy protection from the US Endangered Species Act of 1973, and the US Marine Mammal Protection Act of 1972. The West Indian manatee is also protected by the Florida Manatee Sanctuary Act of 1978, the Manatee Recovery Plan, and the Save the Manatee Club. However, in April 2007, the US Fish and Wildlife Service announced the West Indian manatee population of Florida had rebounded. It advised the species be reclassified as threatened rather than endangered. A computer model produced for the federal study showed a 50% chance that the current statewide manatee population of about 3,300 could dwindle over the next 50 years to just 500 on either coast.[12]
The Florida manatee subspecies (T. m. latirostris) was listed in October 2007 as Endangered by the IUCN on the basis of a population size of less than 2,500 mature individuals and a population estimated to be in decline by at least 20% over the next two generations (estimated at about 40 years) due to anticipated future changes in warm-water habitat and threats from increasing watercraft traffic over the next several decades.[13]
During the winter months, manatees often congregate near the warm water outflows of power plants along the coast of Florida instead of migrating south as they once did, causing some conservationists to worry they have become too reliant on these artificially warmed areas.[14] The US Fish and Wildlife Service is trying to find a new way to heat the water for manatees that are dependent on plants that have closed.
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| リンク元 | 「アメリカマナティー」 |
| 関連記事 | 「Trichechus」 |
マナティー、ナティー属