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Manatees — Gentle Giants of the Sea

Manatee photo by Jim Reid, United States Fish and Wildlife ServiceIt's hard to imagine anyone mistaking a manatee for a mermaid. But in 1493, when Columbus's ships were taking on fresh water at the island known as Hispaniola, shared today by Haiti and the Dominican Republic, he recorded in his journal that his men "saw three sirenas (mermaids) who rose very high from the sea, but they were not as beautiful as they are painted." It is generally agreed that the creatures were manatees.

Columbus and his men were not the only ones to mistake manatees, or their close cousins, dugongs, for mermaids. Writer Tim Dietz tells of the French priest Pére Labat, who in 1732, reported on a captured "mermaid" in Africa. Labat's description could easily have been that of a dugong: a creature possessing a humanlike upper body with two breasts and two short arms [flippers], and a lower body of a fish with a long, forked tail.

Neither mermaids nor fish

Not everyone mistook manatees and dugongs for mermaids. Many people thought the creatures were huge fish. But they are neither mermaids nor fish. Both manatees and dugongs are mammals that belong to the scientific order Sirenia. Like all sea mammals, they must rise above the surface of the water to breathe air into their lungs. Sirenians are tropical animals that must live in waters at or above sixty-eight degrees Fahrenheit. They can live in either salt water or freshwater, but scientists believe that manatees must, at least occasionally, find fresh water to drink. Manatees near boating docks are often observed drinking from water hoses. Dugongs, however, spend their entire lives in salt water. Experts believe the dugongs' kidneys have the ability to filter salt from their food, so they apparently have no need for freshwater.

More than just blobs in the sea

At first glance, these huge mammals look most like big, shapeless blobs floating beneath the surface of the water. When resting on the bottom, which they frequently do, sirenians can easily be mistaken for large rocks. Their bodies are oval shaped with no discernible neck, so their heads appear to be bulbous extensions of their upper bodies. A pair of short front flippers are used for locomotion and for manipulating food into their mouths. Daniel Hartman, who made the first long-term study of manatees, noted that "the flippers are held in close to the chest while swimming, but they are [also] used in 'walking' on the bottom." Females have teats just under their flippers.

Though all species of Sirenia are similar, there are some major differences between them. The manatee's body is rotund, narrowing slightly before it flares into a broad, paddle-shaped tail. The dugong's body is more streamlined, tapering gradually into the tail, which flares out into two flukes (like a dolphin's). The manatee has rough, wrinkled skin that is brownish gray in color, while the dugong has smooth skin that is dark gray on the topside and light gray on the underside. The manatee's lips are very large and the upper lip is divided so each side can move independently to push food into its mouth. The dugong's upper snout is also divided, with each side capable of independent movement, but its lower snout slants downwards, ending in a flattened area called the rostral disk. This disk is used to dig up rhizomes (underground plant growth) and roots from the sea bottom. Most species of manatees have a feature unique to sea mammals -- three or four fingernails at the ends of the flippers. And dugongs have small tusks that erupt through the upper jaw at around nine or ten years of age. All sirenians have very sparse body hair and long whiskers on either side of the snout.

Manatees can grow as long as thirteen feet and weigh up to thirty-five hundred pounds, although the average is ten feet long and weighs around twelve hundred pounds. Dugongs are somewhat smaller, with the average adult reaching a length of nine and one-half feet and weighing around nine hundred pounds.

Sirenian senses

Contrary to earlier belief, sirenian senses are well developed. Although their external ear openings are either not present at all or are very tiny, their internal ear bones are large and they are thought to hear well. Tests have determined that they are able to hear sounds at lower frequencies than humans can. Some scientists think that sound reception is located near the large cheekbones that are in direct contact with the ear bones rather than the tiny ear openings. This arrangement is similar to that of dolphins, "in which it is theorized that sound enters the fat-filled lower jaw and is conducted to the ear bones, apparently bypassing the tiny ear canal."

Because sirenian eyes are tiny and set deep and wide apart, it used to be thought that they had poor eyesight. However, more rigorous testing has determined that they can see well in both dim and bright light and may see color, but their vision may sometimes be limited because of the murkiness of the water in which they swim.

They seem to have a fairly well-developed sense of smell, which scientists think they use to recognize one another and for males to know when a female is in estrus (ready to mate). A sense of taste is demonstrated by their preferences for certain plants. Their sense of touch is also highly acute and is one of the sirenians' most important senses.

Special adaptations

Because they live in the water rather than on land, sirenians have developed a number of special adaptations. Manatee nostrils have a special membrane that opens above the water and closes when the animal submerges. The dugong's nostrils do not close, but internal muscles contract to close the passage when the animal is submerged. Both manatees and dugongs have built-in eye protection in the form of nictitating membranes, transparent tissue that covers the eyeballs, enabling the animals to swim with their eyes open. Another adaptation to their aquatic environment is the sirenians' ability to replace 90 percent of the air in their lungs in one breath, thus enabling them to remain submerged for up to fifteen to twenty minutes at a time. By comparison, humans can only replace 10 percent of the air in their lungs with each breath.

Sirenians are unusual, as well, in their ability to rise vertically to the surface and sink back down again, like a helicopter or an elevator. The animals rise straight up, breathe, then sink straight down. There are no muscle or body movements during these surfacings. Scientists think sirenians can do this because their bones lack marrow, a soft connective tissue that is filled with air pockets. This lack of marrow makes the bones extremely dense and heavy. Reynolds and Odell, in their book, Manatees and Dugongs, suggest that the heavy bones "may have a very important function as ballast to offset the [animal's] positive buoyancy [tendency to float]."

Marching teeth

Another special manatee adaptation, related to their diet of fibrous grasses and plants, was the subject of a paper published by Daryl Domning when this world-renowned expert was studying Amazonian manatees in 1975. It was always known that sirenians have thick, ridged pads instead of front teeth and that the ridges help to break vegetation into small pieces that are then pushed to the back of the mouth where the chewing molars are located. This rough diet, plus the grit that is often mixed with it, wears down the molars quite regularly. Domning explained that as a manatee's rear teeth wear down, they move to the front and fall out. As the old teeth fall out, new molars grow to replace them. Only manatees have these "marching teeth." Dugongs' teeth do not replace themselves. Once a dugong's original molars wear out, the roots remain in place, forming smooth pegs that function as teeth -- though not very efficient ones.

Evolution of sirenia

These adaptations have come about over millions of years. Scientists believe that sirenians evolved around fifty-five million years ago from small, hoofed animals. Though it seems unlikely, their closest living relatives today are elephants and small Asian mammals called hyraxes. Although there are vast differences between elephants, hyraxes, and sirenians, fossil evidence shows that all three evolved from a common ancestor. The manatees' toenails, skin color and texture, the absence of body hair, and the shape of their skulls are visible remnants of their ancient heritage. The dugongs' tusks are another link to elephants. And like elephants, sirenians are enormous animals.

Today, there are five species of Sirenia, but scientists believe that there were many more in the past. Early forms of manatees are thought to have originated near the Amazon basin in South America. Some remained there to become the Amazonian manatee (Trichechus inunguis), while others migrated up through the Caribbean, giving rise to the Antillean (Trichechus manatus manatus) and Florida manatees (Trichechus manatus latirostris). Another group managed to swim or were carried on currents across the Atlantic and became the West African manatee (Trichechus senegalensis).

Dugongs, known by the scientific name as Dugong dugong, are thought to have evolved along with manatees, and they once ranged from Europe to Africa, and along the east and west coasts of the Americas. At the present time, they are found only in the Eastern Hemisphere in the Indian and Pacific Oceans. Steller's sea cow, another species of Sirenia, became extinct in the 1700s.

Dwindling populations

Several thousand years ago, manatees were plentiful along the coasts of northern South America, the Caribbean Sea, and the southeast coast of Florida. Today in all areas, the numbers of manatees are vastly diminished.

Manatees, which at the time were not known in Europe, might have been a new sight to Columbus's men, but the natives who lived on the islands were well acquainted with this animal, which they hunted for both its meat and its hide. Soon after Columbus's arrival, other European explorers and settlers arrived in the New World, and they, too, began hunting the manatee. The dugong has also been a victim of thousands of years of hunting.

Sirenians have no defense mechanisms. Their only means of protection is to swim away from whatever is threatening them. They have few natural enemies, although the Amazonian manatees are occasionally prey to jaguars and caimans, a type of crocodile. And dugongs are sporadically attacked by sharks. Today, the biggest threat to all sirenians is from humans. All but the Florida manatee are still hunted. All are losing habitat to increased human activity and development of coastal areas. And all fall victim to human-related activities such as being caught in large gill nets or shark nets -- or colliding with boats.

Dugongs

Of the currently existing species of Sirenia, the one with the largest population is the dugong, which swims in the waters of the Indian Ocean, eastern Africa, the Red Sea, the Persian and Arabian Gulfs, southern Asia, the Philippines, Borneo, New Guinea, and Australia. The numbers vary in different locations from under a hundred to several thousand, but in all areas there are far fewer than there were just ten years ago. According to Masaharu Nishiwaki and Helene Marsh, who conducted an extensive study of dugongs in 1985, "The precise extent to which the dugong's range has been contracted is unknown, but over much of its present range it is now represented by relict [remnant] populations separated by large areas where it is close to extinction or extinct."

The largest concentrations of dugongs today are found off the coast of northern Australia in the waters of the Great Barrier Reef. According to Tony Stokes, the coordinator of the threatened species of the Great Barrier Reef Marine Park Authority, "The population is in serious decline in the southern Great Barrier Reef, down by 50-80 percent over the last twenty years." As of early 1998, though dugongs in Australia were not classified as endangered, they were classified as vulnerable, meaning that they are close to becoming endangered. They are listed as endangered throughout the remainder of their range.

Florida and Antillean manatees

The Florida manatee, which makes its home in the waters of the southeastern and gulf waters of the United States, and the Antillean manatee, which swims in the Caribbean Sea around the countries of Central America and northeastern South America, are quite similar. Until 1986 they were considered one species known as the West Indian manatee (Trichechus manatus). The differences are in the shape of their skulls and other minor anatomical features. "It was not until the 1980s that sufficient anatomical material was available to allow scientists to confirm the subspecies distinction on the basis of skull characteristics." The 1997 counts of the Florida manatee indicate between two thousand and twenty-five hundred animals.

Because the Antillean manatee swims in the waters of many different countries and changes locations in the rainy season, often seeking out remote regions, it has been difficult to accurately assess its total population. It is known, however, that its numbers have decreased drastically.

Amazonian and West African manatees

Little research has been done on the Amazonian and West African manatees. The Amazonian manatee, the only one to live completely in freshwater, inhabits the rivers and estuaries of the Orinoco and Amazon River basins in northern South America. As with the Antillean manatee, it has been difficult to determine exact numbers of these species, but again it is known that the population is much reduced from fifty to sixty years ago. At one time there were huge herds of Amazonian manatees; today there are only small groups of four to eight animals.

In some areas, the West African manatee has become extinct.

Protecting sirenians around the world

Because of the shrinking of sirenian populations, international conservation organizations and individual countries have included them among protected species. All species of Sirenia are listed under the U.S. Endangered Species Act, passed by Congress in 1973. That act states that a species is endangered if it is in danger of extinction in all or a large part of its natural habitat. The act makes it a violation to harass or in any way harm endangered species. The World Conservation Union's Red Data Books, which classifies species as endangered (likely to become extinct under current conditions), vulnerable (likely to become endangered under current conditions), or rare (uncommon, but not in imminent danger) lists all manatees and dugongs as vulnerable. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) -- an agreement among 103 countries for the regulation of trade in plants and animals that are or may be threatened with extinction -- lists all dugongs outside of Australia and the Amazonian, Florida, and Antillean manatees as threatened with extinction. Dugongs within Australian waters and the West African manatee are registered as vulnerable. The West African manatee is also protected by the African Convention of Nature and Natural Resources, and all other countries in which sirenians live have protective laws as well. In addition to these listings, the Florida manatee is also listed under other protective acts. But despite all this protection, many manatees and dugongs are killed each year.

Source: Endangered Animals & Habitats: The Manatee, Lucent Overview Series. Lucent Books. 1999.

class="small">Manatee photo by Jim Reid, United States Fish and Wildlife Service.

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