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.
Dwindling Habitat
Because so much of their habitat has been destroyed, Florida manatees have had to use new environs. Many have come to rely on the warm discharges of power plants. "We have found 500 manatees at some plants," notes Bob Turner, marine biologist with the USFWS manatee recovery program in Jacksonville, Florida. One manatee hot spot is the Florida Power and Light Company plant across from Cape Canaveral, which offers an ample supply of sea grass plus added protection at nearby Merritt Island National Wildlife Refuge and the John F. Kennedy Space Center, where boat access is limited and wildlife habitat is protected. In other areas, manatees take whatever refuge they can get. Some huddle around paper mills in northeast Florida and southeast Georgia, which discharge warm but often polluted effluent.
Congregating around power plants may pose certain risks to manatees, according to Turner. When manatees live in close quarters, they are more vulnerable to disease. "They're all bunched up and in each other's faces at a time when they're most stressed," explains Turner. Under those circumstances, it is easy for bacteria or viruses to spread.
In addition, if a power plant shuts down or fails to produce sufficient warm water, the water temperature may drop dangerously low, which occurred between December 1989 and January 1990 during a prolonged cold snap. "We lost about 46 manatees along the central east coast of Florida when that happened," recalls Turner.
Florida power companies have been responsive to the needs of manatees. For example, after its Caloosahatchee River plant in Ft. Myers shut down temporarily in the mid-1980s, putting hundreds of manatees at risk, the Florida Power and Light Company drilled wells into the groundwater and installed pumps to bring warmer water to the surface in the event the plant shuts down again in winter. There have been no unscheduled shutdowns since then, but the well and pump system is ready if the water temperature drops below 66 degrees F.
Meanwhile, manatees face other perils. Occasionally, they fall victim to red tide, a large "bloom" of protozoans that secrete toxins that can be ingested by manatees. In early 1996, 149 manatees died from a red tide, according to the USFWS. Manatees also drown in gill nets, or get trapped in canal locks or floodgates. But the biggest threat comes in the form of metal motorboat propellers: More than 90 percent of Florida manatees bear scars from run-ins with boats. "Boats are the number one cause of manatee mortality in Florida," according to Kleen. Scientists use the distinctive and plentiful scars to identify and track individual manatees that have survived such run-ins.
On the Rebound?
Florida initiated manatee protection legislation as early as 1893, and in 1978 enacted the Manatee Sanctuary Act, which designated the entire state as a refuge and sanctuary for manatees. This act makes it unlawful for anyone to annoy, molest, harass, or disturb the animals and allows for enforcement of boat speed regulations in certain areas. Further protection was provided in the early 1970s, when Congress enacted the Marine Mammal Protection Act and the Endangered Species Act—legislation that make it illegal to harass, capture, or kill any marine mammal. Until recently, none of these laws proved particularly effective in stemming the loss of manatees. Recent gains are likely the result of increased public awareness, a better understanding of manatee population ecology, and greater enforcement of manatee protection guidelines, such as restrictions on boat speeds.
Although the manatee is considered endangered, its population appears to be increasing in Florida. In the mid-1980s the lowest count tallied only 800 manatees. In 1991, the population of manatees in Florida was about 1,850. In 1996, its numbers increased to about 2,275, though a dip followed when more than 400 died (149 succumbed to red tide; most of the rest died after collisions with boats). The increase may be due both to a true increase in the manatee population as well as improved surveying techniques. Biologists are getting better at locating and counting manatees, thanks in good part to surveys conducted from planes. One reason for optimism about the Florida manatee is the increased awareness and interest of Floridians, who have adopted the animal as their official marine mammal.
"Floridians love them," says O'Shea. "When I started working in Gainesville in 1979, few people had ever heard of manatees. That has changed." The manatee license plate, much like Chesapeake Bay plates in Maryland, has been enormously successful at raising awareness and money to support Florida's manatee conservation efforts. Sales of the $17 plates raise about $2.4 million each year. About half goes to manatee protection efforts such as censuses and research, and the other half helps fund public education programs. In addition, the official seal of the state's Department of Natural Resources features a manatee.
While it would be premature to declare that Florida manatees have recovered, few marine scientists would dispute the claim that their numbers have at least stabilized, which is more than can be said for most endangered creatures, including the Florida manatee's cousins.
A quick look at the threats facing sirenians in other parts of the world reveals that Florida manatees are doing better than most. Although many countries have enacted protective laws, they are often ignored. For example, fewer than 100 manatees are thought to survive in Panama. Although protected by law, Panama's manatees are still pursued by rifle- and harpoon-toting poachers. The same is true for the few manatees in nearby Colombia. Despite efforts to better protect its sirenians, Belize—one of the best remaining strongholds for the West Indian manatee—suffered a setback in 1996, when the Belize Centre for Environmental Studies reported that many manatees were being illegally killed in southern Belize and evidence of at least 35 slain manatees was found.
Habitat destruction, dams, and hunting have all taken a toll on West African manatee populations in Nigeria, Liberia, Cameroon, Guinea-Bissau, and other African countries, where data on the extent of remaining manatee populations are lacking. And as Amazonian forests are cut, the Amazonian manatee faces threats to its survival. Deforestation clouds waterways with silt, and nearby industry pumps polluting toxic effluents into the water. The silt builds up and can lower water levels, threatening manatees and their favored food plants. Meanwhile, in countries such as Ecuador, heavy hunting pressure has pushed manatee populations to the brink of extinction. And despite a very wide range, from the coastal waters off East Africa to Australia, the dugong has disappeared, or is endangered, over most of its former range due to hunting, oil spills, and capture in fish nets. The largest remaining concentration of dugongs lives in and around the northern coasts and rivers of Australia.
Perhaps like the Florida manatee, these docile creatures can gain a following. Their greatest hope for avoiding the same fate as the Steller's sea cow lies in the efforts of a sympathetic and committed public, whose struggle to save these magnificent creatures from extinction may some day ensure a shining future for these gentle giants.
(ZooGoer 27(1) 1998. Copyright 1998 Friends of the National Zoo. All rights reserved.)
