An order of herbivorous aquatic placental mammals, commonly known as sea cows, that includes the living manatees and dugongs and the recently exterminated Steller's sea cow. The order has an extensive fossil record dating from the early Eocene Epoch, some 50 million years ago.
The earliest known sirenians were quadrupedal and capable of locomotion on land. Fossils clearly document the evolutionary transition from these amphibious forms to the modern, fully aquatic species, which have lost the hindlimbs and transformed the forelimbs into paddlelike flippers. The living species have streamlined, fusiform bodies with short necks and horizontal tail fins like those of cetaceans, but no dorsal fins. The skin is thick and nearly hairless. The nostrils are separate, and the ears lack external pinnae.
Sirenians typically feed on aquatic angiosperms, especially seagrasses, but in ecologically marginal situations they also eat algae and even some animal material. They are normally found in tropical or subtropical marine waters, but some have become adapted to fresh water or colder latitudes. Body sizes have ranged from less than 3 m up to 9–10 m (30–33 ft). Sirenians mate and give birth in the water, bearing a single calf (occasionally twins) after about 13–14 months of gestation and then nursing it from one pair of axillary mammae. The closest relatives of sirenians among living mammals are the Proboscidea (elephants).
(sirens), an order of aquatic mammals comprising three families: Manatidae (three species of manatees), Dugon-gidae (with one species—the dugong), and Hydrodamalidae (with one extinct species—Steller’s sea cow—which was exterminated in the 18th century). Sirens are adapted to an aquatic mode of life and never emerge onto land. Their torpedolike body ends in an evenly rounded or two-lobed tail fin, which serves as the main organ of locomotion. The head is bluntly truncated, and the neck is short but movable. The forelimbs are in the form of large flippers that articulate at the ulnar and carpal joints. The rough skin is dark brown and sparsely covered with hair; there is a thick subcutaneous layer of fat. Paired nostrils are situated at the end of the muzzle. The females have two mammae, located in the chest region. The teeth and digestive organs are adapted to a diet of aquatic plants. Modern sirens have two to eight simultaneously functioning cheek teeth in each half of each jaw. Male dugongs have a pair of incisors in the upper jaw that resemble small tusks. In the course of a lifetime sirens have 30 cheek teeth. The palate and lower jaw in Steller’s sea cow were covered with horny plates. The stomach is capacious and consists of two sections. The intestine is long with a developed cecum.
Sirens, which are rare everywhere, live in small groups. A single young is born at a time, after a gestation period of five or six months in manatees and 11 months in dugongs. Since their population is diminishing, sirens require protection.
S. V. MARAKOV