Dugongs, Sea Cows, and Manatees: Sirenia

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DUGONGS, SEA COWS, AND MANATEES: Sirenia

PHYSICAL CHARACTERISTICS

Dugongs, sea cows, and manatees are mammals that vary in length from 9.8 feet (3 meters) to 32.8 feet (10 meters) and weigh anywhere from 992 pounds (450 kilograms) to more than 9,920 pounds (4,500 kilograms). Sirenians (sye-REEN-ee-unz), members of the order Sirenia, are nearly hairless and skin texture varies from smooth to rough. They have no back limbs, only short, flexible forelimbs that they use to help them swim. The tail of the manatee is paddle-shaped while that of the dugong and sea cow is fluked with long, horizontal fins, like a whale. Eyes are small, and their ears are not visible. Sirenians vary in color from gray to brown. The manatee has both upper and lower molars, flat teeth suitable for chewing, which are replaced on a regular basis throughout its lifetime. Male dugongs have tusks, and all dugongs have molars that are not replaced. Sea cows were toothless. All appear to have whiskers.

GEOGRAPHIC RANGE

Sirenians live in tropical, subtropical, and temperate, or mild, regions throughout the world. The exception to this is Steller's sea cow, now extinct, which lived only in the frigid waters of the northwestern Pacific Ocean.

HABITAT

Manatees and dugongs live in shallow, warm coastal waters that contain plentiful vegetation. Some manatees exist in estuaries (EST-yoo-air-eez), mouth of a river where fresh water mixes with salt water, others occupy both sides of the Atlantic Ocean. The Amazonian manatee lives only in freshwater. The dugong lives in the Indian Ocean, Red Sea, Persian Gulf, and the west coast of India, in strictly saltwater habitats. The sea cow preferred an exceedingly cold environment, and history indicates it liked a mix of salt water and freshwater.

DIET

All sirenians are vegetarians, feeding on vegetation such as sea grasses and other marine plants. While the dugong is strictly a bottom-feeder, eating only what lives on the ocean floor, manatees feed from above the water's surface all the way to the bottom. Sirenians use their flippers to uproot vegetation and use their molars to chew or crush food. Although male dugongs have tusks, it is not clear what role these teeth play in feeding, if any. It takes about one week for food to digest. Manatees consume about 10 percent of their body weight every day. Because they need so much food, sirenians spend a great portion of their time feeding.

The toothless sea cow ate algae (AL-jee) and plankton, plants that are easy to digest without chewing.

BEHAVIOR AND REPRODUCTION

Sirenians are semi-social mammals with the primary unit a female and her calf. Dugongs feed in herds of tens or hundreds of individuals. They have been recorded as traveling hundreds of miles (kilometers) in a matter of days, an impressive feat given that they must surface for air every few minutes. Dugongs have poor eyesight but an acute sense of hearing.

TWIN SEA COWS CAUSE FOR CELEBRATION

In December 2003, Beauval Zoo, located in France, celebrated the birth of twin sea cows—the first twin sea cow birth in captivity.

Quito, a male, and Luna, a female, weighed around 44 pounds (20 kilograms) each and measured 3.3 feet (1 meter). Although Daphne, the mother, was watched around the clock during the final days of her pregnancy, no one anticipated that she would give birth to twins.

As of mid-2004, mother and calves were doing well.

Manatees also travel long distances in short amounts of time and have a north-south migratory pattern, the direction or path taken during seasonal movement from one region to another, that keeps them swimming in warmer waters. Although most marine mammals use echolocation (eck-oh-loh-KAY-shun), a sensory system in which high-pitched sounds are used to determine location and distance, sirenians are not known to. Little is known about the behavior of Steller's sea cow.

Manatees reach sexual maturity between the ages of two and eleven years. Gestation, pregnancy, is believed to be twelve or thirteen months. Usually a single calf is born every two-and-a-half to three years. Manatees do not bond, which means they have numerous mates throughout their lifetimes. In fact, when a female is ready to breed, she may mate with as many as twenty males, often at the same time. Calves can swim to the surface at birth, and they are nursed, fed with mother's milk, until around the age of one. Though they have no vocal cords, calves also vocalize at birth, which is an important part of the mother-calf bonding process. The calf remains close to its mother for up to two years.

Pregnancy for the dugong lasts about one year and results in the birth of a single calf, which will nurse from and remain close to its mother for about eighteen to twenty-four months. Birth takes place in shallow water and the calf will rise to the surface to take its first breath. Dugong calves are about 3.3 to 3.9 feet (1 to 1.2 meters) and weigh 44.1 to 66.2 pounds (20 to 30 kilograms). Dugongs can live for seventy years.

Because Steller's sea cow died out so quickly, most of what we know is speculation, an educated guess based on facts. Gestation lasted at least one year, and calves were seen throughout the year, suggesting that there was no specific breeding season. Pregnancy resulted in single births, but physical data is not available. It is believed that the sea cow was monogamous (muh-NAH-guh-mus), having only one mate.

SIRENIA AND PEOPLE

Sirenians have been hunted by humans for food, hides, and bone, a fact that has endangered a number of their species. Steller's sea cow lived for just a few decades before hunting caused its extinction. Manatees and dugongs help balance the marine ecosystem by recycling nutrients in sea grass beds and keeping the plants in a continual state of growth. Without them, the biodiversity, variety, of marine life would be in danger.

Manatees are being closely studied by scientists in hopes that their immune systems can provide clues as to how humans can fight cancer. Because their immune systems, which protect against disease, are very powerful, doctors are looking for tips on how to boost human immune systems. Specifically, they are studying a manatee population that has become infected with papillomavirus (pap-ih-LOH-mah-vye-rus), a virus that develops into cervical cancer in humans. The manatees became infected in the wild and seem to fall victim to cancers as a result. Researchers study tumor tissue and blood samples taken from the infected population, which live in a rehabilitation tank at the Harbor Branch Oceanographic Institution.

CONSERVATION STATUS

Several species of manatees are threatened, according to the World Conservation Union (IUCN). The dugong is listed as Vulnerable, facing a high risk of extinction. It is also listed as Endangered under the U.S. Endangered Species Act. Sirenia are in danger due to habitat destruction brought on by human activities such as recreational boating and fishing. Today great conservation efforts are being made around the world in hopes of keeping the dugong and manatee from the sharing the fate of Steller's sea cow.

FOR MORE INFORMATION

Books:

Harman, Amanda. Manatees and Dugongs. New York: Benchmark Books, 1997.

Glaser, Karen, and John Elliott Reynolds III. Mysterious Manatees. Gainesville, FL: University Press of Florida in association with Santa Fe, NM and Harrisburg, VA: The Center for American Places, 2003.

Powell, James. Manatees. Stillwater, MN: Voyageur Press, 2003.

Ripple, Jeff, and Doug Perrine. Manatees and Dugongs of the World. Stillwater, MN: Voyageur Press, 2002.

Silverstein, Alvin and Virginia. The Manatee. Brookfield, CT: Millbrook Press, 1995.