Sirenia

Sirenia are an order of aquatic herbivorous mammals, including the dugong, the manatee, and the extinct Steller's sea cow. The term sea cow is also popularly used for the dugong and the manatee; they attain a length of 8 to 15 feet and superficially resemble the walrus. The dugong has a bilobed tail fin; that of the manatee is broad and rounded. The dugong inhabits the Red Sea, the Indian Ocean, and the coastal waters north of Australia. Manatees are found in the warm waters of the Atlantic Ocean off the American and African coasts. Both dugongs and manatees may also be found far from the ocean in inland rivers and lakes. Steller's sea cows are now extinct but once inhabited the Bering Sea.

Steller's Sea Cow

A long-extinct, herbivorous, aquatic mammal related to the manatee and dugong. The only record of these animals is from Georg Wilhelm Steller, a German physician and naturalist. He discovered the enormous sea cow in 1741 when he and a Russian exploring party were shipwrecked on the Commander Islands in the Bering Sea. By the time Steller discovered these sea cows, their range had become limited to waters near these islands. In prehistoric times, the sea cow's habitat ranged from the rim of the North Pacific near Japan to Baja California in Mexico.

These massive, placid creatures inhabited shallow, sandy areas along the shore. Steller wrote that they particularly liked "the mouths of the gullies and brooks, the rushing fresh water of which always attracts them in herds." Showing no fear of humans, the sea cows allowed themselves to be touched from the shore.

Steller's sea cows reached lengths of at least 24 feet and may have weighed up to 10 tons. A Steller's sea cow skeleton in the Leningrad Zoological Museum measures just over 23 feet. Their heads were small relative to their rotund bodies, and their tiny eyes, Steller wrote, were no larger than sheep's eyes. Their short forelimbs ended in bristled, hooflike flippers. Their skin was dark brown, rough as tree bark, and sometimes streaked or spotted with white. The skin was also remarkably tough and elastic; a scientist in Hamburg, Germany, rehydrated a piece of preserved sea cow hide and remarked on its surprising resemblance to the thick rubber of car tires. This durable skin protected the giant mammal as it swam near rocks and sharp-edged ice floes.

Steller's sea cows fed exclusively on kelp growing around the Commander Islands. Lacking teeth, they ground the seaweed between two bony chewing plates. To satisfy their large appetites, they spent most of their time feeding with their heads down and their backs exposed above the water surface.

Steller guessed that most calves were born in the autumn, although he believed that some reproduction occurred year-round. He observed that mating took place in the spring. He also noted that these gentle animals protected their young, always putting them in the middle when swimming in groups. Steller further noted that when a sea cow was harpooned, others would immediately gather around it, and some would even try to overturn the hunter's dinghy. Sea cows would also lie on their side next to the wounded animal and would also try to pull the harpoon out of the body-sometimes succeeding.

At the time that Steller discovered this species, there were an estimated 1500 to 2000 sea cows. Steller and the rest of his exploring party were eventually rescued and returned to Russia laden with valuable otter, seal, and Arctic fox furs. Word spread to hunters of the fortune to be made in fur trade from the Commander Islands. From 1743 to 1763 hundreds of hunters arrived in the Bering Sea. They found this slow-moving sea cow an easy target for meat, blubber, and hide. By 1768, 27 years after it was discovered, the Steller's sea cow had become extinct.

Manatee

A common name for each of three species of a large water mammal, popularly called a sea cow because it grazes on marine grasses and other water plants. The Amazonian manatee ranges throughout the Amazon River basin in South America; the West African manatee occurs in rivers and coastal waters of tropical West Africa; and the West Indian, or Caribbean, manatee is found in rivers and coastal waters from the southeastern United States and the Gulf of Mexico to the Caribbean Sea and northeastern Brazil.

An adult manatee has a rounded body, usually colored light to dark gray or black, that tapers to a horizontally flattened, rounded tail. It is 8 to 14 feet long, depending on the species, and weighs 440 to 1300 pounds. The small head includes a straight snout and a cleft upper lip with bristly hairs. Its nostrils, set on the upper surface of the snout, are closed tightly by valves when the animal is under water. The paddlelike forelimbs are set close to the head; no external hind limbs exist. Massive, heavy bones and long, narrow lungs, which extend through the entire body cavity, give the manatee evenly distributed buoyancy. A manatee cow gives birth about a year after mating; usually a single, pink calf is born.

Manatees live in small family groups, although they occasionally travel in herds of 15 to 20. They feed in both freshwater and salt water, grazing for six to eight hours a day. An adult consumes an amount equal to between 5 and 10 percent of its body weight daily. Manatees have a slower metabolism than other mammals of similar size, which sharply reduces their energy requirements. This is especially important for Amazonian manatees when waters recede after floods, confining some for extended periods in shallow waters away from plants on river banks.

Manatees have few natural enemies because of their size. However, their population has been reduced significantly by heavy hunting for hides, meat, and blubber oil, and they are frequently injured or killed in collisions with boats. Efforts to protect manatees in Africa and South America sometimes conflict with the traditional subsistence hunting of these animals. A practical consideration that supports conservation efforts is the fact that manatees help clear plant-clogged river channels used for irrigation and transportation. The World Conservation Union lists each species of manatee as vulnerable (a term that indicates the animal is facing a high risk of extinction in the wild in ten or more years).

Dugong

A large marine mammal that inhabits the tropical waters of the Indian and western Pacific oceans. It is commonly called a sea cow. The dugong's massive body is about 8 to 9 feet long and tapers to a forked, horizontally flattened tail. The forelimbs are rounded flippers; no hindlimbs remain. Large, muscular lips (the upper one cleft) are used for tearing off water plants. Molars and hard pads at the front of the jaws grind the food; the male also has two small, tusklike incisors. Feeding mainly at night, the dugong commonly travels in pairs or in small groups. After a gestation of about 13 to 14 months, the young, usually a single offspring, is born in the water and suckles at the mother's pectoral teats.

This harmless sea animal has long been hunted for its meat, blubber, oil, and hide, and today the world population of dugongs is believed to be reduced to less than 40,000. They are now classified as endangered. Dugongs were sometimes viewed by early voyagers as mermaids and were identified with the Sirens of mythology.

Scientific classification

Manatees make up the family Trichechidae in the order Sirenia. The Amazonian manatee is classified as Trichechus inunguis, the West African manatee as Trichechus senegalensis, and West Indian manatee as Trichechus manatus. The dugong makes up the family Pugongidae in the order Sirenia. It is classified as Dugong dugon. The Steller's sea cow belonged to the family Trichechidae of the order Sirenia.