The Sirenia ( sy-REE-nee-ə), commonly referred to as sea cows or sirenians, are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The extant Sirenia comprise two distinct families:
Dugongidae (the dugong and the extinct Steller's sea cow) and Trichechidae (manatees, namely the Amazonian manatee, West Indian manatee, and West African manatee) with a total of three species. The Protosirenidae (Eocene sirenians) and Prorastomidae (terrestrial sirenians) families are extinct. Sirenians are classified in the clade Paenungulata, alongside the elephants and the hyraxes, and evolved in the Eocene 50 million years ago (mya). The Dugongidae diverged from the Trichechidae in the late Eocene or early Oligocene (30–35 mya). Sirenians grow to between 2.5 and 4 metres (8.2 and 13.1 feet) in length and 1,500 kilograms (3,300 pounds) in weight. The recently extinct Steller's sea cow was the largest known sirenian to have lived, reaching lengths of 10 metres (33 feet) and weights of 5 to 10 tonnes (5.5 to 11.0 short tons). Sirenians have a large, fusiform body which reduces drag through the water and heavy bones that act as ballast to counteract the buoyancy of their blubber. They have a thin layer of blubber and consequently are sensitive to temperature fluctuations, which cause large-scale migrations when water temperatures dip too low. Sirenians are slow-moving, typically coasting at 8 kilometres per hour (5.0 miles per hour), but they can reach 24 kilometres per hour (15 miles per hour) in short bursts. They use their strong lips to pull out seagrasses, consuming 10–15% of their body weight per day. While breathing, sirenians hold just their nostrils above the surface, sometimes standing on their tails to do so. They typically inhabit warm, shallow, coastal waters, or rivers. They are mainly herbivorous, but have been known to consume animals such as birds and jellyfish. Males typically mate with more than one female and may gather in leks to mate. Sirenians are K-selected, displaying parental care.
The meat, oil, bones, and skins of sirenians are commercially valuable. Mortality is often caused by direct hunting from humans or by other human-induced causes, such as habitat destruction, entanglement in fishing gear, and watercraft collisions. Steller's sea cow was finally driven to extinction due to overhunting in 1768.
Taxonomy
Etymology Sirenia, commonly sirenians, are also referred to by the common name sirens, deriving from the sirens of Greek mythology.
Classification Sirenians are classified within the cohort Afrotheria in the clade Paenungulata, alongside Proboscidea (elephants), Hyracoidea (hyraxes), Embrithopoda, Desmostylia, and Afroinsectiphilia. This clade was first established by George Gaylord Simpson in 1945 on the basis of anatomical evidence, such as testicondy and similar fetal development. The Paenungulata, along with the Afrotheria, are one of the most well-supported mammalian clades in molecular phylogeny. Sirenia, Proboscidae, and Desmotylia are grouped together in the clade Tethytheria. On the basis of morphological similarities, Tethytheria, Perissodactyla, and Hyracoidea were previously thought to be grouped together as the Altungulata, but this has been invalidated by molecular data.
Sirenia families † = Extinct Family Dugongidae:
Family Trichechidae:
†Family Protosirenidae:
†Family Prorastomidae:
Distribution The warm shallow waters of the equator have been the center of sirenian habitation. The northernmost living population, the Florida subspecies of the West Indian manatee (T. manatus latirostris), inhabits the coast and frequents freshwater springs, power plants, and canals in Florida to stay warm during the winter. Individuals may migrate north in the warm summer months, some up to 1,000 kilometers (about 621.37 mi) from their winter range. The Antillean subspecies (T. manatus manatus) occurs in the Caribbean, South America, and Central America and frequent drowned cays, mangroves, lagoons, and sea grass beds. The Amazonian manatee (T. inunguis) has been documented in all parts of the Amazon River Basin in South America. River channels that connect allow easy travel to other waterways where food may be plentiful. The Amazonian manatee lives only in freshwater. The West African manatee (T. senegalensis) lives in murky isolated inland mangroves and coastal flats in West Africa. It is found in waters above 18 °C (64 °F), and its range spans Senegal to Angola. The dugong (Dugong dugong), the closest living relative of Steller's sea cow, lives in the Indo-West Pacific Ocean in more than 40 different countries. They are coastal animals supported by wide protected sea grass meadows. Steller's sea cow was discovered in 1741 around islands in the Bering Sea and was specialized for cold subarctic temperatures. It ranged from Alaska through the Amchitka and Aleutian Islands, and even to Japan. Steller's sea cow was reported to have congregated in shallow, sandy areas along coastline and mouths of rivers and creeks to feed on kelp.
Evolution
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