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Sirenia (commonly referred to as sea cows) are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. Four species are living, in two families and genera. These are the dugong (one species) and manatees (three species). Sirenia also include Steller's sea cow, extinct since the 18th century, and a number of taxa known only from fossils. The order evolved during the Eocene, more than 50 million years ago. Sirenia, commonly sirenians, are also referred to by the common name sirens, deriving from the sirens of Greek mythology.〔(What are sirenians? ) Sirenian International - Manatee & Dugong Research, Education, & Conservation〕 This comes from a legend about their discovery, involving lonely sailors mistaking them for mermaids. "Sea cow" (''seekoei'') is also the name for a hippopotamus in Afrikaans. In Germanic languages, the word ''Sea'' can mean either a body of fresh or salt water, so this follows from the species inhabiting lakes in southern Africa rather than the sea itself. ==Description== Sirenians have major aquatic adaptations: the forelimbs have modified into arms used for steering, the tail has modified into a paddle used for propulsion, and the hindlimbs (legs) are but two small remnant bones floating deep in the muscle. They appear fat, but are fusiform, hydrodynamic, and highly muscular. Their skulls are highly modified for taking breaths of air at the water's surface, and dentition is greatly reduced. The skeletal bones of both the manatees and dugong are very dense, which helps to neutralize the buoyancy of their blubber. The manatee appears to have an almost unlimited ability to produce new teeth as the anterior teeth wear down. They have only two teats, located under their forelimbs, similar to elephants. The elephants are thought to be the closest living relatives of the sirenians. The lungs of sirenians are unlobed.〔〔Marine Mammal Medicine, 2001, Leslie Dierauf and Frances Gulland, CRC Press〕 In sirenians, the lungs and diaphragm extend the entire length of the vertebral column. These adaptations help sirenians control their buoyancy and maintain their horizontal position in the water. Living sirenians grow between 2.5 and 4.0 meters long and can weigh up to 1,500 kg. ''Hydrodamalis gigas'', Steller's sea cow, could reach lengths of 8 m. The three manatee species (family Trichechidae) and the dugong (family Dugongidae) are endangered species. All four are vulnerable to extinction from habitat loss and other negative impacts related to human population growth and coastal development. Steller's sea cow, extinct since 1786, was hunted to extinction by humans. Manatees and dugongs are the only marine mammals classified as herbivores. Unlike the other marine mammals (dolphins, whales, seals, sea lions, sea otters, and walruses), sirenians eat primarily sea grasses and other aquatic vegetation, and have an extremely low metabolism and poor tolerance for especially cold water (the Steller's sea cow, which inhabited the cold waters of the northern Pacific, was an exception). Sirenians have been observed eating dead animals (sea gulls), but their diets are made up primarily of vegetation. Like dolphins and whales, manatees and dugongs are completely aquatic mammals that never leave the water—not even to give birth. These animals have been observed eating grass clippings from homes adjacent to waterways, but in this rare occurrence, only the top portion of the sirenian is lifted out of the water. The combination of these factors means sirenians are restricted to warm, shallow, coastal waters, estuaries, and rivers with healthy ecosystems that support large amounts of seagrass or other vegetation. The Trichechidae species differ from the Dugongidae in the shape of their skull and tails. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sirenia」の詳細全文を読む スポンサード リンク
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