(sìrē'nēən) or sea cow, name for a large aquatic mammal of the order Sirenia. Living sirenians are the dugong and the manatee, both found in warm, shallow waters in sheltered regions, where they feed on seaweeds and sea grasses. Sirenians are the only marine mammals, outside of the whale order, that spend their entire lives in the water, and they are the only marine mammals that feed exclusively on vegetation. Their heavy, thickset bodies are fishlike in form, the tail ending in a horizontally flattened fin. There are no hind legs, and the forelegs are modified into weak flippers. The gray skin is largely hairless, but there are short bristles (vibrissae) concentrated around the fleshy lips and distributed more sparsely on the body that aid the sense of touch; the upper lip is cleft into two lobes, used for gathering food. The female has a pair of mammary glands on the chest, and holds the pup in her flippers while nursing. It has been speculated that the manatees, which nurse on the water's surface, are the source of mermaid legends.
Manatees (genus Trichechus) are found on both sides of the tropical Atlantic Ocean. They are sluggish, largely nocturnal bottom feeders and consume up to 100 lb (45 kg) of vegetation daily. They must surface for air every 15 or 20 min. They are usually 7 to 12 ft (2.1–3.6 m) long and weigh about 500 lb (225 kg), although males sometimes grow much larger. Their paddlelike tail fin is nearly circular. Both parents care for the young, one holding it while the other dives for food. The West Indian manatee, Trichechus manatus, lives in offshore waters, bays, estuaries, and river mouths in Florida, Central America, and the West Indies. It is protected by law in Florida. The Amazonian manatee, T. inunguis, is found in rivers of NE South America, ascending the Amazon as far as Ecuador. Both are listed as endangered species. The West African manatee, T. senegalensis, lives in the coastal rivers and lagoons of W Africa.
The dugong, Dugong dugon, is found in offshore waters of the Red Sea, Indian Ocean, and W South Pacific Ocean. More strictly marine than the manatee, it seldom enters rivers. Its tail fin is crescent-shaped, and the male has long, tusklike incisor teeth. The male may reach a length of 12 ft (3.6 m) and weigh over 600 lb (270 kg). The dugongs' numbers have been depleted by hunting for its flesh, hides, tusks, and oil. The extinct Steller's sea cow was a large northern species that formerly inhabited the Bering Straits. It reached a length of 30 ft (9 m), weighed up to 4 tons (2,400 kg), and was insulated by very thick blubber. When it was discovered by Bering in 1741, the population was very small; within 30 years it was exterminated by hunting.
Although superficially they resemble whales, sirenians have evolved independently. They are descended from the group of ancient land mammals that also gave rise to the elephants. The living sirenians are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Sirenia, families Trichechidae and Dugongidae.