Manatees: Supremely Adapted, Seriously Endangered
by David Salvesen
Manatees: Supremely Adapted, Seriously Endangered
by David Salvesen
It's hard to imagine how a manatee--a 1,000-pound creature with a body like a fat cucumber and a face packed with wrinkles and whiskers--could be mistaken for a mermaid. But apparently some European sailors did just that, perhaps suffering from the deprivations of prolonged sea voyages, or maybe from a lack of spectacles. The legendary alluring qualities of manatees are also reflected in the name of the order to which they belong: Sirenia, after the sea nymphs of Homer's Odyssey, whose mesmerizing songs lured wayward seafarers to their deaths. Unfortunately, now and in the past, seafarers have often been the ones killing manatees, a situation conservationists are struggling to control before these graceful, gentle mammals are gone.
Manatees, along with their cousin the dugong, are also known as sea cows, and this name is apt. Like cows, manatees spend most of their time eating and resting, inhabiting shallow, slow-moving rivers, estuaries, bays, and canals rather than grassy pastures. For six to eight hours each day, manatees graze on floating vegetation, sea grasses, and freshwater plants. In that time, a manatee consumes an amount of vegetation equivalent to about 11 percent of its body weight. With the usually 10- to 12-foot-long manatees weighing anywhere from 800 to 3,500 pounds, that's between 88 and 385 pounds of greens a day.
Agile Underwater Behemoths
For all their ponderous appearance, manatees are well adapted to life underwater. They can remain under for about 15 minutes when resting and about five minutes when active. Nostrils on the upper surface of the snout close tightly like valves when submerged. Most of the manatees' bones lack marrow, resulting in a dense, heavy bone mass that helps them remain submerged, much like scuba divers carry weights to keep from bobbing to the surface. To adjust their depth, manatees change the volume of air in their submarine-shaped lungs, which lie lengthwise just below the surface of the back and run about two-thirds of the body length. (This is a design flaw for manatees in the modern world, as even minor collisions with boats can easily damage lungs.)
Manatees are also surprisingly agile, sometimes somersaulting and barrel-rolling in the water. While mostly reclusive and slow moving, they can be very playful and, when surprised, quite unpredictable: When startled, they have been known to flip alligators and tip canoes. Powered by a fan-shaped tail and steering with two small forelimbs, or flippers, manatees can maintain a cruising speed of 16 miles per hour for several hours, with a top speed of about 40 miles per hour. Their flippers are also used to walk along the bottom, to root for rhizomes in the mud, and to scoop food into their mouths. Flexible upper lips that split down the middle permit each side of the manatee's mouth to move independently, like an elephant's trunk, enabling them to manipulate their food efficiently.
Manatees can live for 60 to 70 years, but reproduce slowly. Females reach sexual maturity as young as three years of age and thereafter produce a single calf every three to five years, although twins occur occasionally. Gestation lasts 13 months. Calves, which average about 60 pounds at birth, can be born at any time during the year. And because they are born with teeth, young manatees begin eating plants within a few weeks of birth, although they will nurse for two years or so. Manatees' large ear bones are highly developed in calves. Vocalizations, which to the human ear sound like chirps or squeaks, allow mother and calf to communicate and keep track of one another. Adults do not seem to communicate much vocally.
Danger in the Water
Although rather sluggish, manatees present a challenge to would-be predators. Their large size, thick, tough skin, and lack of fins make it hard for predators to get a grip on them. "It's like trying to take a bite out of a big, rolling tire under water," explains Thomas O'Shea, a marine biologist with the United States Fish & Wildlife Service (USFWS). Nonetheless, O'Shea suspects that a calf may be taken now and then by crocodiles or sharks. "But out of hundreds of manatees we photographed in Florida, only one showed evidence of a shark bite," he remarks.
While manatees rarely have to grapple with reptile or fish predators, their leisurely lives of grazing are often interrupted by the fast-paced activities of humans. Boats pose a great danger to these shallow-water creatures. For example, in Florida and other places, many manatees bear the scars from close encounters with motorboats, whose slashing propellers lacerate the animals' hides. Many manatees die from such encounters.
Native Americans hunted manatees for their flesh, bones, and hides. Early colonists used manatees' fat for lamp oil, bones for medicines, and hides for leather. Today, manatee hunting has all but ceased in the United States, but continues in the Caribbean, West Africa, and Central and South America, where in some areas a tradition of eating manatees apparently dates back to the Catholic conquistadors, who called manatees fish so they could be eaten during Lent. Conservationists believe this practice is now dying out.
Some hunters have changed their ways in order to cash in on ecotourism. In Belize, which is trying to build a tourism industry around manatees, some reformed poachers are capitalizing on their knowledge of manatees and charging tourists for the chance to get a glimpse of the reclusive creatures. Local residents can pocket more from guiding tourists to manatee hangouts than from killing the animals. Hunting remains a serious threat, however, and other dangers have arisen: Manatees drown in fish nets, and pollution and habitat destruction further threaten them throughout their ranges.
The Power of Moving Teeth
The aquatic sirenians probably originated in the Old World, where their predecessors were land animals that also gave rise to elephants, hyraxes, and perhaps the aardvark. Fossil evidence suggests that sirenians reached the New World in South America about 35 million years ago. Today, the sirenian order includes three species of manatees: the West Indian manatee (Trichechus manatus), which occurs from the southeastern United States to northern Brazil; the West African manatee (Trichechus senegalensis), found in coastal waterways of western Africa; and the Amazonian manatee (Trichechus inunguis), which lives in the fresh waters of the Amazon River and its tributaries. A relative, the dugong (Dugong dugon), inhabits the coastal waters of the Indian and Pacific oceans.
All three manatee species and the dugong are endangered. A fifth species, the Steller's sea cow (Hydrodamalis gigas), which grew to more than 30 feet in length and weighed several tons, was hunted to extinction by 1768--less than 30 years after it was first described by Western explorers in the Bering Strait. The Steller's sea cow was the only sirenian to inhabit cold water.
Until about 15 million years ago, both dugongs and manatees inhabited the New World. Dugongs colonized the marine waters of the western Atlantic and Caribbean, while manatees were found in the estuaries of South America. About one million years ago, however, manatees evolved a strategic advantage over dugongs: migrating teeth. Manatees grow new teeth that migrate from the back of the mouth to the front, replacing those that are older and worn. This teething process takes place throughout manatees' lives. "It's like continuously having your wisdom teeth come in," observes O'Shea, who was former head of USFWS's Sirenia Project, a program that gathers details on manatee behavior, ecology, mortality, movements, age determination, and other facets of manatee life history.
According to O'Shea, eons ago some dugongs became isolated in an inland sea in what is now Amazonia. Adapting to the high silt levels of the sea and a diet of abrasive grasses, these isolated dugongs developed their unique tooth replacement mechanism and over time evolved into manatees. In contrast, dugongs in other areas lived on a staple of sea grasses that were far less abrasive. Like most mammals, dugongs grow only two sets of teeth. About a million years ago, changes in habitats and food plants enabled manatees to invade the Caribbean and eventually outcompete dugongs by the sheer force of their chewing power. Today, dugongs live only in the Indian Ocean and Western Pacific.
The Florida Manatee: Trying to Keep Warm and Safe
The Florida manatee (Trichechus manatus latirostris), a subspecies of the West Indian manatee that lives in freshwater and saltwater coastal areas of the southeastern United States, is one of the world's most studied, and best-loved, sirenians. Like their cousins, Florida manatees must live in warm waters because their slow metabolism leaves them unable to metabolize energy fast enough to keep warm in water that dips below 68 degrees Fahrenheit. So each fall, Florida manatees--which range far and wide in summer--migrate to the warm waters of Florida.
One of their favorite watering holes is the warm springs in Kings Bay at the mouth of the Crystal River, where about 600 million gallons of fresh water flow daily from more than 30 natural springs. The water temperature there remains a bath-like 72 degrees year-round. In 1983, this area became the Crystal River National Wildlife Refuge, primarily to provide winter habitat for the Florida manatee. Last winter, about 300 manatees--a record number--wintered in the refuge.
Thousands of snorkelers also converge on the refuge each winter to frolic with the ponderous beasts. Last year, more than 30,000 snorkelers and divers--about 400 during a peak-season winter day--cavorted with, and in some cases harassed, the manatees at the refuge. "During the peak season," says the refuge's wildlife manager Eileen Nunez, "one manatee may be surrounded by six to 12 snorklers." To reduce the pressure on the beleaguered animals, the USFWS created three human-free zones in 1980, sanctuaries where manatees can find peace. Four more sanctuaries were added following a study conducted by University of Florida wildlife biology graduate student Cheryl Buckingham in the late 1980s. Based on her recommendations, the refuge added the sanctuaries, hired a full-time enforcement officer, and closed the refuge in winter from 7:00 p.m. to 7:00 a.m.
Using buoys, the refuge now cordons off six sanctuary areas where, from November 15 to March 31, two-legged swimmers are prohibited. Signs warn, "Manatee Sanctuary. Do Not Enter." According to Nunez, "The manatees learned quickly. When snorkelers start arriving, the manatees head right for the buoys." But, she adds, some manatees remain outside the sanctuaries and "people grab them, poke them, and ride them." To help solve this problem, the refuge has produced a video to teach snorkelers proper behavior around manatees, such as not separating a cow from its calf. Local dive shops are required to show the video to those renting diving and snorkeling equipment. "We don't want people diving down and bothering the manatees. If there are [manatees] on the bottom, they're probably feeding, resting, or sleeping," comments Nunez.
By the end of winter, as temperatures warm and their food supply becomes depleted, Florida manatees begin their summer travels, venturing out along the Gulf and Atlantic coasts. "They pretty much clean out Kings Bay to bare sand by the end of winter," says Joyce Kleen, marine biologist at Crystal River. Some disperse north along the Atlantic coast while others head west along the Gulf coast. Most stick close to the shore, venturing up rivers and creeks to feed.
During the summer months, manatees travel as far west as Texas and as far north as Virginia. The most famous manatee, nicknamed "Chessie," went even farther. He was captured in the Chesapeake Bay in the spring of 1994 and transported in a Coast Guard airplane to Florida by the USFWS, which feared that the wayward sea mammal would succumb to the cold waters of the Chesapeake. Fitted with a radio transmitter to track his movements, Chessie was released into the waters off the Merritt Island National Wildlife Refuge near Cape Canaveral, Florida. More determined than ever, he lit out the next summer for parts north and reached as far as Rhode Island before turning back. Interestingly, mid-Atlantic observers calling in sightings of Chessie did not mistake the wandering sirenian for something as enchanting as a mermaid or siren. Rather, the most frequently used description was "a big gray blob," according to Linda Taylor, spokesperson at the USFWS's Maryland Field Office.
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