Manatee

Manatees (family Trichechidae, genus Trichechus) are large, fully aquatic marine mammals sometimes known as sea cows. The name manatí comes from the Taíno, a pre-Columbian people of the Caribbean, meaning "breast". They contain three of the four living species in the order Sirenia, the other being the dugong, which is native to the Eastern Hemisphere. The Sirenia are 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).

Physical characteristics

Manatees are mainly herbivores, spending most of their time grazing in shallow waters and at depths of 1-2 meters (3-7 ft). Much of the knowledge about manatees is based upon research done in Florida and cannot necessarily be attributed to all types of manatees. Generally, manatees have a mean mass of 400-550 kg (900-1200 lb), and mean length of 2.8-3.0 m (9-10 ft), with maximums of 3.6 meters and 1,775 kg seen (the females tend to be larger and heavier). When born, baby manatees have an average mass of 30 kg.

On average, most manatees swim at about 5 km/h to 8 km/h (1.4 m/s to 2.2 m/s; 3 to 5 miles per hour). However, they have been known to swim at up to 30 km/h (8 m/s; 20 miles per hour) in short bursts. Manatees inhabit the shallow, marshy coastal areas and rivers of the Caribbean Sea and the Gulf of Mexico (T. manatus, West Indian Manatee), the Amazon Basin (T. inunguis, Amazonian Manatee), and West Africa (T. senegalensis, West African Manatee). A fourth species, the Dwarf Manatee (T. bernhardi) was recently proposed for a population found in the Brazilian Amazon, although some have questioned its validity, instead believing it is an immature Amazonian Manatee. The coast of Georgia is usually the northernmost range of the West Indian Manatee as their low metabolic rate makes cold weather endurance difficult. They may on occasion stray up the mid-Atlantic coast in summer. Half a manatee's day is spent sleeping in the water, surfacing for air regularly at intervals no greater than 20 minutes.

Florida Manatees (T. m. latirostris) have been known to live up to 60 years, and they can move freely between different salinity extremes; however, Amazonian Manatees (T. inunguis) never venture out into salt water. They have a large flexible prehensile upper lip that acts in many ways like a shortened trunk, somewhat similar to an elephant's. They use the lip to gather food and eat, as well as using it for social interactions and communications. Their small, widely spaced eyes have eyelids that close in a circular manner. Manatees are also believed to have the ability to see in color.

They emit a wide range of sounds used in communication, especially between cows and their calves, yet also between adults to maintain contact and during sexual and play behaviors. They may use taste and smell, in addition to sight, sound, and touch, to communicate. Manatees are capable of understanding discrimination tasks, and show signs of complex associated learning and advanced long term memory. They demonstrate complex discrimination and task-learning similar to dolphins and pinnipeds in acoustic and visual studies.

Manatees typically breed only once every other year, since gestation lasts about 12 months, and it takes a further 12 to 18 months to wean the calf. Only a single calf is born at a time and aside from mothers with their young or males following a receptive female, manatees are generally solitary creatures.

The clearest visible difference between manatees and dugongs is in the shape of the tail; a manatee tail is paddle-shaped, while a dugong tail is fluked, similar in shape to a that of a whale.

Diet

Manatees are herbivores and eat over 60 different plant species such as mangrove leaves, turtle grass, and types of algae, using their divided upper lip. An adult manatee will commonly eat up to 9% of its body weight (approx 50 kg) per day. Manatees have been known to eat small amounts of fish from nets.

Like horses, they have a simple stomach, but a large cecum, in which they can digest tough plant matter. In general, their intestines are unusually long for animals of their size. The adults have no incisor or canine teeth, just a set of cheek teeth, which are not clearly differentiated into molars and premolars. Uniquely among mammals, these teeth are continuously replaced throughout life, with new teeth growing at the rear as older teeth fall out from further forward in the mouth. At any given time, a manatee typically has no more than six teeth.

Population

The population of manatees in Florida (T. manatus) is thought to be between 1,000 and 3,000, yet population estimates are very difficult. The number of manatee deaths in Florida caused by humans has been increasing through the years, and now typically accounts for 20%-40% of recorded manatee deaths. There were 417 manatee deaths in Florida in 2006 with 101 attributed to human causes according to the Florida Fish and Wildlife Commission.

Accurate population estimates of the Florida manatee are notoriously difficult and have been called scientifically weak; with widely varying counts from year to year, some areas show possible increases yet others decreases, with very little strong evidence of increases except in 2 areas. However, population viability analysis studies carried out in 1997, found that decreasing adult survival and eventual extinction is a probable future outcome for the Florida manatees, unless they are aggressively protected. Manatee counts are highly variable without an accurate way to estimate numbers: in Florida in 1996, a winter survey found 2,639 manatees; in 1997 a January survey found 2,229; and a February survey found 1,706. Fossil remains of manatee ancestors show they have inhabited Florida for about 45 million years.

The Amazonian Manatee (T. inunguis) is a species of manatee that lives in the freshwater habitats of the Amazon River and its tributaries. Their color is brownish gray and they have thick, wrinkled skin, often with coarse hair, or "whiskers." Its main predator is also man. The Brazilian government has outlawed the hunting of this manatee since 1973 in an effort to preserve the species. Deaths by boat strikes, however, are still common.

The African Manatee (T. senegalensis) is the least studied of the three species of manatees. Photos of African Manatees are very rare; although very little is known about this species, scientists think they are similar to the West Indian Manatees. They are found in coastal marine and estuarine habitats, and in fresh water river systems along the west coast of Africa from the Senegal River south to the Kwanza River in Angola, including areas in Gambia, Liberia, Guinea-Bissau, Guinea, Sierra Leone, Côte d'Ivoire, Ghana, Mali, Nigeria, Cameroon, Gabon, Republic of the Congo, and Democratic Republic of the Congo. Although crocodiles and sharks occasionally kill manatees in Africa, their only significant threats are from humankind due to poaching, habitat loss, and other environmental impacts. They live as high upriver on the Niger as Gao, Mali. Although rare, they occasionally get stranded as the river dries up at the end of rainy season and are cooked for a meal. The name in Songhay, the local language, is "ayyu".

Habitat

Manatees typically inhabit warm, shallow, coastal estuarine waters and cannot survive below 15°C (288 K; 60°F). Their natural source for warm waters during the winter is warm-spring fed rivers. The West Indian Manatee migrates into Florida rivers such as the Crystal River, the Homosassa River, and the Chassahowitzka River. The head springs of these rivers maintain a water temperature of 22°C (299 K; 72°F) year round. During the winter months, November to March, approximately 400 West Indian Manatees (according to the National Wildlife Refuge) congregate in the rivers in Citrus County, Florida.

Manatees have been spotted as far north as Cape Cod, and as recently as the late summer of 2006, one made it up to New York City and Rhode Island's Narragansett Bay, as cited by The Boston Globe. According to Memphis, Tennessee's The Commercial Appeal newspaper, one manatee was spotted in the Wolf River harbor near the Mississippi River in downtown Memphis, Tennessee, on October 23, 2006, though it was later found dead ten miles downriver in McKellar Lake. Manatees often congregate near power plants, which warm the waters. Some have become reliant on this source of artificial heat and have ceased migrating to warmer waters. Some power plants have recently been closing and the U.S. Fish and Wildlife Service is trying to find a new way to heat the water for these manatees. The main water treatment plant in Guyana has four manatees that keep storage canals clear of weeds; there are also some in the ponds of The National Park in Georgetown, Guyana.

Studies in Florida suggest that Florida manatees must have some access to fresh water for proper osmoregulation.

Captivity

The oldest manatee in captivity is Snooty who is held at the South Florida Museum. He was born at the Miami Seaquarium on July 21, 1948 and came to the South Florida Museum in Bradenton, Florida in 1949.

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