manatee

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Manatee
Antillean Manatee
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Sirenia
Family:	Trichechidae Gill, 1872
Genus:	Trichechus Linnaeus, 1758
Species
Trichechus inunguis Trichechus manatus Trichechus senegalensis Trichechus "pygmaeus" (validity questionable)

Manatees (family Trichechidae, genus Trichechus) are large, fully aquatic marine mammals sometimes known as sea cows. They are noted for their rather friendly nature, large size (up to 4 metres) and paddle-like flippers. The name manatí comes from the Taíno, a pre-Columbian people of the Caribbean, meaning "breast".^[1] 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).^[2]

Physical characteristics

Manatees are mainly herbivores, spending most of their time grazing in shallow waters and at depths of 1-2 metres (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 to 550 kilograms (880 to 1,200 lb), and mean length of 2.8 to 3 metres (9.2 to 9.8 ft), with maximums of 3.6 metres (12 ft) and 1,775 kilograms (3,910 lb) seen (the females tend to be larger and heavier). When born, baby manatees have an average mass of 30 kilograms (66 lb).

On average, most manatees swim at about 5 to 8 kilometres per hour (3.1 to 5.0 mph). However, they have been known to swim at up to 30 kilometres per hour (19 mph) 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. "pygmaeus") was recently proposed for a population found in the Brazilian Amazon,^[3] although some have questioned its validity, instead believing it is an immature Amazonian Manatee.^[4] 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.^[5] They demonstrate complex discrimination and task-learning similar to dolphins and pinnipeds in acoustic and visual studies.^[6]

Manatees typically breed only twice a biennial, 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.^[7]

The clearest visible difference between manatees and dugongs is in the shape of the tail;^[8] a manatee tail is paddle-shaped, while a dugong tail is fluked, similar in shape to a that of a whale. Manatees also have shorter snouts than dugongs.

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.^[9]

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 farther forward in the mouth. At any given time, a manatee typically has no more than six teeth.^[7]

Population

Approximate distribution of Trichechus; T. manatus in green; T. inunguis in red; T. senegalenis in orange

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.^[10] 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.^[11] Manatee counts are highly variable without an accurate way to estimate numbers:^[12] 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.^[6] 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^{[citation needed]} 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^{[citation needed]}. 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

A group of 3 manatees

A manatee at SeaWorld, Florida.

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.^[13] 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.

Manatees can also be viewed in a number of European zoos, such as the Tierpark in Berlin, the Nuremberg Zoo, and in Beaval Park Zoo in France.

They are also included within the new plans for a National Wildlife Conservation Park in Bristol, England which is due to open in 2010 with the manatees as an addition in 2015.

Vulnerability

Antillean Manatee

Although manatees have few natural predators (sharks, crocodiles, orcas, and alligators), all three species of manatee are listed by the World Conservation Union as vulnerable to extinction. The current main threat to manatees in the United States is being struck by boats or slashed by propellers. Sometimes manatees can live through strikes, and over fifty deep slashes and permanent scars have been observed on some manatees off the Florida coast.^[6] However, the wounds are often fatal, and the lungs may even pop out through the chest cavity.^[6] It is illegal under federal and Florida law to cause the manatees injury or harm.^[6]

According to marine mammal veterinarians, "The severity of mutilations for some of these individuals can be astounding - including long term survivors with completely severed tails, major tail mutilations, and multiple disfiguring dorsal lacerations. These injuries not only cause gruesome wounds, but may also impact population processes by reducing calf production (and survival) in wounded females - observations also speak to the likely pain and suffering endured".^[6] In an example, they cited one case study of a small calf "with a severe dorsal mutilation trailing a decomposing piece of dermis and muscle as it continued to accompany and nurse from its mother...by age 2 its dorsum was grossly deformed and included a large protruding rib fragment visible."^[6] These veterinarians go on to state that "the overwhelming documentation of gruesome wounding of manatees leaves no room for denial. Minimization of this injury is explicit in the Recovery Plan, several state statutes, and federal laws, and implicit in our society's ethical and moral standards."^[6]

Young Manatees can be curious— this one is checking out a kayak

Manatees occasionally ingest fishing gear (hooks, metal weights, etc.) while feeding. These foreign materials do not appear to harm manatees, except for monofilament line or string. This can clog the animal's digestive system and slowly kill the animal.

Manatees can also be crushed in water control structures (navigation locks, floodgates, etc.), drown in pipes and culverts, and are occasionally killed by entanglement in fishing gear, primarily crab pot float lines. Manatees are also vulnerable to red tides—blooms of algae, often caused by pollution, which leaches oxygen from the water.

Manatees were commonly hunted for their meat by natives of the Caribbean, although this is much less common today.^[14]

On June 8, 2006, The Florida Fish and Wildlife Conservation Commission voted to reclassify the manatee on Florida's list, to a "threatened" status in that state.^[15] While none of the state laws protecting manatees have changed, many wildlife conservationists are not pleased with the removal decision. Manatees remain classified as "endangered" at the federal level.

While humans are allowed to swim with manatees in one area of Florida,^[16] there have been numerous charges of people harassing and disturbing the manatees in various ways, in addition to the concern about repeated motorboat strikes causing the maiming, disfiguring, and death of manatees all across the Florida coast, and this privilege of swimming with wild manatees may be soon repealed.^[17]

Hunting

Trichechus sp.

Manatees were traditionally hunted by indigenous Caribbean people. When Christopher Columbus arrived in the region, manatee hunting was an established trade. Native Americans hunted manatees to make war shields, canoes, and shoes, though the manatee was predominantly hunted for its abundant meat. The primary method of hunting the manatee was somewhat crude, as the hunter would use dugout canoes to approach targeted manatees. The hunter would then use various methods of baiting to attract a manatee close enough to hit the animal near the head with an oar-like pole, temporarily stunning the prey. Many times the creature would flip over, leaving it vulnerable to further attacks.

Manatees were also hunted for their valuable bones, which were used to make "special potions." Up until the 1800s, museums paid as much as $100 for manatee bones or hides. Though hunting manatees was banned in 1893, poaching continues today.

Disposition and boat collisions

A sign advising boaters of no-wake manatee zone

Manatees are slow-moving, non-aggressive, and generally curious creatures. They enjoy warmer waters and are known to congregate in shallow waters, and frequently migrate through brackish water estuaries to freshwater springs.

Their slow-moving, curious nature, coupled with dense coastal development, has led to many violent collisions with propellers from fast moving recreational motor boats, leading frequently to maiming, disfigurement, and even death. As a result, a large proportion of manatees exhibit propeller scars on their backs and they are now even classed by humans by their scar patterns. Some are concerned that the current situation is inhumane, with sometimes upwards of 50 scars and disfigurements from boat strikes on a single manatee.^[6][18] Often the cuts lead to infections, which can prove fatal. Internal injuries stemming from hull impacts have also been fatal.

The two ships MV Freedom Star and MV Liberty Star used by NASA to retrieve the Space Shuttle SRBs from the sea and pull them back to the Kennedy Space Center are equipped with water jets to protect the endangered manatee population that inhabits regions of the Banana River where the ships are based.

In 2003, a population model was released by the U.S. Geological Survey that predicted an extremely grave situation confronting the manatee in both the Southwest and Atlantic regions where the vast majority of manatees are found. It states, “In the absence of any new management action, that is, if boat mortality rates continue to increase at the rates observed since 1992, the situation in the Atlantic and Southwest regions is dire, with no chance of meeting recovery criteria within 100 years.”^[19]

In 2007, a University of Florida study found that more than half of boat drivers in Volusia County, Florida sped through marked conservation zones despite their professed support for the endangered animals, and little difference was found between the driving speeds of ski boats, pontoons, and fishing vessels. In the study, 84 percent of the 236 people who responded said they fully obeyed with speed limits in manatee zones during their most recent boating experience, but observers found that only 45 percent actually complied. "Hurricanes, cold stress, red tide poisoning and a variety of other maladies threaten manatees, but by far their greatest danger is from watercraft strikes, which account for about a quarter of Florida manatee deaths," said study curator John Jett.^[20]

Cultural depictions

The manatee has been linked to folklore on mermaids. Native Americans ground the bones to treat asthma and earache. In West African folklore, they were sacred and thought to have been once human. Killing one was taboo and required penance.^[21]

In the episode of South Park, 'Cartoon Wars Part II' (2006), they were shown as the writers of rival cartoon Family Guy.

Notes

References

• Shoshani, Jeheskel (November 16, 2005). Wilson, D. E., and Reeder, D. M. (eds). ed. Mammal Species of the World (3rd edition ed.). Johns Hopkins University Press. pp. 93. ISBN 0-801-88221-4. http://www.bucknell.edu/msw3. 

External links

Wikimedia Commons has media related to: Trichechidae

• Florida Fish and Wildlife Conservation Commission
• Reuters: Florida manatees may lose endangered status
• A website with many manatee photos
• A coalition of scientists studying and saving manatees around the world
• USGS gallery on manatees
• Report on Florida manatees by primary school students