Manatee

Sirenia Fossil range: Early Miocene to Recent
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

Manatee is the common name for large, herbivorous, fully aquatic marine mammals comprising the family Trichechidae, characterized by a nearly hairless body with paddle-like front flippers, small eyes, no external ear, and absence of hind limbs (although there are vestigial pelvic bones). There is one genus recognized in the family, Trichecus, which includes three extant species. Manatees are found in both freshwater and marine environments in parts of the Americas and Africa.

Manatees, which sometimes are known as sea cows, are part of the order Sirenia along with dugongs. Sirenians are one of four groups of marine mammals, the others being cetaceans (whales, dolphins, and porpoises), sea otters, and pinnipeds (walruses, earless seals, and eared seals). The sirenians and cetaceans are completely aquatic, while pinnipeds spend considerable time on land, including giving birth and raising their young, and sea otters can mate and raise their young entirely at sea/

The Sirenia is 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) (Domning 1994). Much of the knowledge about extant manatees is based upon research done in Florida and cannot necessarily be attributed to all types of manatees. The name manatí comes from the Taíno, a pre-Columbian people of the Caribbean, meaning "breast" (Winger 2000).

Description

The body of the manatee is largely hairless. The body is somewhat oval in cross section, with a broad, horizontally flattened tail, short, flexible and paddlelike forelimbs, and no hind limbs, although the pelvic musculature has the presence of vestigial pelvic bones. With the exception of the Amazonian manatee, Trichechus inunguis, there are three or four nails on the end of the forelimbs; Amazonian manatees lack the nails in the forelimbs. There is a small, hard to see external ear opening, but no pinna (external ear). Testes are internal, and mammary glands are paired, with a single nipple in each axilla (Odell 2004).

The upper lips of manatees are split and described as prehensile (Odell 2004). In many ways, the upper lip acts 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.

The main visual difference between manatees and dugongs are the tails. A manatee tail is paddle-shaped, while a dugong tail is forked, similar in shape to a whale's.

A somewhat unique feature of the manatee among mammals is the constant replacement of molar teeth; new teeth enter at the back of the jaw and replace old and worn teeth at the front. Thought to be their close cousins, the elephants also have teeth that get replaced, but they have a limited set of these replacement teeth.

Generally, manatees have a mean mass of 400 to 550 kilograms (900-1200 pounds]]) and mean length of 2.8 to 3.0 meters (9-10 feet), 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 kilograms. The body color generally is gray to brownish, but may have algae and other epiphytes that result in another appearance (Odell 2004).

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.

Distribution and habitat

A group of 3 manatees

A Manatee taken out of his 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). 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. Florida 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.

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.

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

Behavior, diet, and reproduction

On average, most manatees swim at about 5 to 8 kilometers per hour (1.4 to 2.2 meters/second or 3 to 5 miles per hour). However, they have been known to swim up to 30 kilometers/hour (8 meters/second; 20 miles per hour) in short bursts.

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.

Manatees 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.

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. There were near 300 registered and confirmed manatees in Florida killed by human activity in 2006; the majority of these, that happened to be discovered by Florida Fish and Wildlife, have been caused by boat strikes.

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 the 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 Sonrai, the local language, is "ayyu".

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.

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 with boats or slashed with 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. However, the wounds are often fatal, and the lungs may even pop out through the chest cavity. It is illegal under federal and Florida law to cause the manatees injury or harm.

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 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."

One problem is that young Manatees are curious— this one is checking out a kayak

Manatees occasionally ingest fishing gear (hooks, metal weights, etc.) while feeding. These foreign materials do not seem 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 from entanglement in fishing gear, primarily crab pot float lines. Manatees are also vulnerable to red tides—blooms of algae which leach oxygen from the water.

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

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. 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, 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.

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 indigenous hunter would then use various methods of baiting in order to attract a manatee close enough to hit the animal near the head with an oar-like pole, temporarily stunning the manatee. 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 a number of violent collisions with propellers from fast moving recreational motor boats, leading frequently to maiming, disfigurement, and even death. As a result, a large portion of manatees exhibit propeller scars on their backs and they are now even classed by humans from 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. Often the cuts lead to infections, which can prove fatal. Internal injuries stemming from hull impacts have also been fatal.

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.”

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.

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, it was sacred and thought to have been once human. Killing one was taboo and required penance.

See also

• USS Manatee (AO-58)
• Evolution of sirenians

References

ISBN links support NWE through referral fees

• Template:MSW3 Shoshani

Jennifer Winger ZooGoer 34(6) 2000. Copyright 2000 Friends of the National Zoo. All rights reserved. Smithsonian National Zoological Park .

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External links

• 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
• A website dedicated to helping save the manatee
• Some more information on manatees
• Report on Florida manatees by primary school students

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