|The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. (January 2013)|
Manatees (family Trichechidae, genus Trichechus) are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing three of the four living species in the order Sirenia: the Amazonian manatee (Trichechus inunguis), the West Indian manatee (Trichechus manatus), and the West African manatee (Trichechus senegalensis). They measure up to 13 feet (4.0 m) long, weigh as much as 1,300 pounds (590 kg), and have paddle-like flippers. The name manatí comes from the Taíno, a pre-Columbian people of the Caribbean, meaning "breast".
Manatees comprise three of the four living species in the order Sirenia. The fourth is the Eastern Hemisphere's dugong. 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).
The Amazonian's hair color is brownish gray and they have thick, wrinkled skin, often with coarse hair, or "whiskers". Photos are rare; although very little is known about this species, scientists think they are similar to West Indian manatees.
Manatees have a mass of 400 to 550 kilograms (880 to 1,200 lb), and mean length of 2.8 to 3.0 metres (9.2 to 9.8 ft), with maxima 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). They have a large, flexible, prehensile upper lip. They use the lip to gather food and eat, as well as using it for social interactions and communications. Manatees have shorter snouts than their fellow sirenians, the dugongs. Their small, widely-spaced eyes have eyelids that close in a circular manner. 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 in each jaw of its mouth. Its tail is paddle-shaped, and is the clearest visible difference between manatees and dugongs; a dugong tail is fluked, similar in shape to a that of a whale.
Like horses, they have a simple stomach, but a large cecum, in which they can digest tough plant matter. In general, their intestines have a typical length of about 45 meters, which is unusually long for animals of their size.
Half a manatee's day is spent sleeping in the water, surfacing for air regularly at intervals no greater than 20 minutes. Manatees spend most of the rest of the time grazing in shallow waters at depths of 1–2 metres (3.3–6.6 ft). The Florida subspecies (T. m. latirostris) has been known to live up to 60 years.
On average, manatees swim at about 5 to 8 kilometres per hour (3 to 5 mph). However, they have been known to swim at up to 30 kilometres per hour (20 mph) in short bursts.
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 once every two years, 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.
Range and habitat
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).
West Indian manatees enjoy warmer waters and are known to congregate in shallow waters, and frequently migrate through brackish water estuaries to freshwater springs. They cannot survive below 15°C (60°F). Their natural source for warmth during winter is warm, spring-fed rivers.
The coast of Georgia is usually the northernmost range of the West Indian manatees because their low metabolic rate does not protect them in cold water. Prolonged exposure to water temperatures below 68 °F (20 °C) can bring about "cold stress syndrome" and death.
Florida manatees can move freely between salinity extremes.
Manatees have been spotted as far north as Cape Cod, and as recently as the late summer of 2006, one was seen in New York City and Rhode Island's Narragansett Bay, as cited in 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.
The West Indian manatee migrates into Florida rivers, such as the Crystal, the Homosassa, and the Chassahowitzka Rivers. The headsprings of these rivers maintain a 22°C (72°F) temperature year round. During November to March, approximately 400 West Indian manatees (according to the National Wildlife Refuge) congregate in the rivers in Citrus County, Florida.
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 conservations to worry they have become too reliant on these artificially-warmed areas. The U.S. 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. 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 suggest Florida manatees must have some access to fresh water for proper osmoregulation.
Accurate population estimates of the Florida manatee (T. manatus) are notoriously difficult and have been called scientifically weak; with widely varying counts from year to year, some areas show increases and others decreases, with very little strong evidence of increases except in two areas. 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. A statewide synoptic survey in January 2010 found 5,067 manatees living in Florida, which is a new record count.
Fossil remains of Florida manatee ancestors date back about 45 million years.
The freshwater Amazonian manatee (T. inunguis) inhabits the Amazon River and its tributaries, and never ventures into salt water.
They are found in coastal marine and estuarine habitats, and in freshwater 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.
They emit a wide range of sounds used in communication, especially between cows and their calves. Adults communicate to maintain contact and during sexual and play behaviors. Taste and smell, in addition to sight, sound, and touch, may also be forms of communication.
Manatees are herbivores and eat over 60 different plant species, such as mangrove leaves, turtle grass, and types of algae. Using its divided upper lip, an adult manatee will commonly eat up to 10% of its body weight (about 50 kg) per day. Manatees have been known to eat small amounts of fish from nets.
Overall, predation does not present a significant threat to the survival of any manatee species.
Relation to humans
The main causes of death for the sea cows are human-related issues, such as habitat destruction and human objects, and natural causes, such as temperatures and disease.
Their slow-moving, curious nature, coupled with dense coastal development, has led to many violent collisions with propeller-driven boats and ships, leading frequently to maiming, disfigurement, and even death. As a result, a large proportion of manatees exhibit spiral cutting propeller scars on their backs, usually caused by larger vessels that do not have skegs in front of the propellers like the smaller outboard and inboard-outboard recreational boats have. They are now even identified by humans based on their scar patterns. Many manatees have been cut in half by large vessels like ships and tug boats, even in the highly populated lower St Johns River's narrow channels. Some are concerned that the current situation is inhumane, with upwards of 50 scars and disfigurements from vessel strikes on a single manatee. Often, the cuts lead to infections, which can prove fatal. Internal injuries stemming from being trapped between hulls and docks and impacts have also been fatal.
Manatees hear on a higher frequency than would be expected for such large marine mammals. Many large boats emit very low frequencies which confuse the manatee and explain their lack of awareness around boats. Research indicates that when a boat has a higher frequency the manatees rapidly swim away from danger.
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."
- "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.
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". 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."
These veterinarians go on to state:
- "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."
Another cause of manatee deaths is the red tide, a term used for the proliferation, or "blooms", of the microscopic marine algae of the species Karenia brevis, a member of the dinoflagellates that produces brevetoxins that can have toxic effects on the central nervous systems of creatures in the area of the algae bloom.
In 1996, a red tide was responsible for 151 manatee deaths. The epidemic began on March 5 and continued through April 28, wiping out approximately 15% of the known population of manatees along South Florida's western coast. In 1982, another outbreak resulted in 37 deaths, and in 2005, 44 more deaths were attributed to the blooms.
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, which can clog a manatee's digestive system and slowly kill it.
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.
The African manatee's only significant threats are due to poaching, habitat loss, and other environmental impacts. They occasionally get stranded as the river dries up at the end of rainy season.
All three species of manatee are listed by the World Conservation Union as vulnerable to extinction.
It is illegal under federal and Florida law to injure or harm a manatee. They are classified as "endangered" by both the state and the federal governments.
The MV Freedom Star and MV Liberty Star, ships used by NASA to tow space shuttle solid rocket boosters back to Kennedy Space Center, are propelled only by water jets to protect the endangered manatee population that inhabits regions of the Banana River where the ships are based.
Brazil outlawed hunting in 1973 in an effort to preserve the species. Deaths by boat strikes are still common.
The primary hunting method was for the hunter to approach in a dugout canoe, offering bait to attract it close enough to temporarily stun it with a blow near the head from an oar-like pole. Many times the creature would flip over, leaving it vulnerable to further attacks.
Later, manatees were hunted for their bones, which were used to make "special potions". Until the 1800s, museums paid as much as $100 for bones or hides. Though hunting was banned in 1893, poaching continues today.
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.
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