SIRENIAN

(Left) Juvenile and (right) adult female manatees (Trichechus manatus). any member of the order Sirenia, a group of large aquatic mammals that have become rare or extinct as a result of exploitation by man for meat and oil. The largest, Steller's sea cow (Hydrodamalis gigas), which reached lengths of about eight metres (about 26 feet), was eaten out of existence by hungry seal hunters within a few decades of its discovery in 1741 in the Bering Sea. The remaining forms, to which the common name sea cow is also sometimes applied, are the dugong (Dugong dugon) and manatees (three species of Trichechus; see photograph), which, if their stocks were allowed to rebuild themselves, could again become of economic importance. Being the only large aquatic herbivores, other than some turtles, they could provide meat from the vast expanses of marine and freshwater vegetation, at present quite unused by man, and so bring another marginal area into production. The dugong seems to have had a wider distribution in the past, but in recent times it has been restricted to the warm coastal waters of the Indo-Pacific region. Throughout most of its range it is now much depleted in numbers but exists from the Solomon Islands in the east to the head of the Red Sea in the west and from the Philippine Islands and the Persian Gulf in the north to Brisbane, Perth, and Mozambique in the south. It is entirely marine and rarely even enters estuaries. Manatees, however, seem more adaptable and inhabit the coastal, estuarine, and riverine waters of the tropical and subtropical Atlantic region: T. senegalensis from West Africa; T. manatus, with two subspecies, from the Caribbean; and T. inunguis, landlocked in the Amazon Basin. Dugongs and manatees are both usually seen up to lengths of three to four metres (10 to 13 feet), but larger specimens, up to six metres (20 feet), have been mentioned in tales of early travellers. (Left) Juvenile and (right) adult female manatees (Trichechus manatus). a member of the mammalian order Sirenia, which consists of 2 living and more than 20 extinct genera of large aquatic animals. The four surviving species have become rare owing to their exploitation for meat and oil by humans. The largest sirenian, Steller's sea cow (Hydrodamalis gigas), grew to a length of 8 m (24 feet) but was hunted out of existence within decades of its discovery in 1741. The two remaining genera, also occasionally called sea cows, are the dugong (Dugong dugon) and the manatee (three species of Trichechus; see photograph). Both usually attain lengths of up to 4 m, but there have been sightings of animals nearly twice that size. Sirenians are completely herbivorous and are confined to the shallow waters of coastal areas where higher aquatic plant life is abundant. Dugongs are strictly marine mammals and are found mainly in the Indo-Pacific area. Manatees also live in the sea but can thrive in estuarine or riverine waters as well, and if put on nearby land they can often wriggle their way back into their native element. They are found in both the tropical and subtropical Atlantic as well as the Amazon Basin and the Caribbean. Fossil records show that the order Sirenia of the superorder Subungulata, which also includes elephants, dates to the Eocene Epoch (57.8 to 36.6 million years ago). Having no hind limbs, sirenians propel their torpedo-shaped bodies through the water with their flat tails. The dugong has a downturned snout and a forked tail, while the manatee has a straight snout and a rounded tail. The skin of both animals is very thick and tough with little hair. There is not much blubber under the skin, but a great deal of fat is found in the body. The eyes are lidless, small, and round; ear openings exist, but they are tiny and hard to detect. The bones of these mammals are very dense, making them heavy for their size. They feed through the use of powerful muscular lip action, stripping vegetation from plants and crushing it with the aid of horny pads at the front of each jaw. The dugong's teeth consist of six cusped molars, four of which gradually fall out as the animal matures, leaving the adult with two. In the male a modified incisor grows to become a tusk. Manatees have 20 to 30 molars in each jaw that gradually move forward with age. Manatees are also unusual in that they have six neck vertebrae instead of the seven found in all other mammals. Sirenians live in groups for the most part. Because of their natural habitat and their extreme shyness, little is known of their breeding, growth, or other physiological processes. Gestation is thought to be about five months. Single births are most common, but occasionally twins are born to dugongs. Both males and females are the same size at any age. Mammary glands are in the pectoral area, and it is thought that the suckling of a young sirenian held upright by a flipper may have been the inspiration for the myth of the mermaid. Sirenians breathe by making frequent trips to the water's surface and have been known to spend up to 16 1/2 minutes underwater between breaths. Manatees may be seen, usually lying with their backs up, basking in the sunshine. They are very sensitive to sound, wary and suspicious by nature, and will instantly submerge if the slightest unfamiliar noise disturbs them. Additional reading G.M. Allen, Extinct and Vanishing Mammals of the Western Hemisphere, with the Marine Species of All the Oceans, Spec. Publs. Am. Comm. Int. Wildl. Prot., no. 11 (1942), provides information on the causes of extinction and the basis for conservation of sirenians; C. Bertram, In Search of Mermaids: The Manatees of Guiana (1963), is a readable popular account of the biology of manatees; and C.K. and G.C.I. Bertram, The Sirenia as Aquatic Meat-Producing Herbivores, Symp. Zool. Soc. (London), no. 21, pp. 385391 (1968), contains an extensive bibliography of the literature on the Sirenia. G.C.L. Bertram C.K. Bertram