Sirenia

The Sirenia, 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 Sirenia currently comprise the families Dugongidae (the dugong and historically, Steller's sea cow) and Trichechidae (manatees) with a total of four 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. The Dugongidae diverged from the Trichechidae in the late Eocene or early Oligocene. 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 now extinct Steller's sea cow was the largest sirenian to have lived, and could reach lengths of 8 metres (26 feet) and weights of 8 to 10 metric tons (8.8 to 11.0 short tons). Sirenians have a large, fusiform body to reduce drag through the water. They have heavy bones that act as ballasts to counteract the buoyancy of their blubber. They have a thin layer of blubber and consequently are sensitive to temperature fluctuations, which cause 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, they hold just their nostrils above the surface, sometimes standing on their tails to do so. Sirenians 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 (polygyny), and may participate in lek mating. Sirenians are K-selected, and display parental care. The meat, oil, bones, and skins are valuable items sold in markets. Mortality is often caused by direct hunting by humans or other human-induced causes, such as habitat destruction, entanglement in fishing gear, and watercraft collisions. Steller's sea cow went extinct due to overhunting in 1768. Sirenia, commonly sirenians, are also referred to by the common name sirens, deriving from the sirens of Greek mythology. This comes from a legend about their discovery, involving lonely sailors mistaking them for mermaids. Seekoei (sea cow) is also the name for a hippopotamus in Afrikaans. 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 based on 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. Based on morphological similarities, Tethytheria, Perissodactyla, and Hyracoidea were considered to be grouped together as the Altungulata, but this has been invalidated by molecular data. The evolution of sirenians is characterized by the appearance of several traits, which are found in all sirenians (monophyly). The nostrils are large and retracted, the upper-jaw bone contacts the frontal bone, the sagittal crest is missing, the mastoid fills the supratemporal fenestra (an opening on the top of the skull), a drop-like ectotympanic (a bony ring that holds the ear drum), and pachyosteosclerotic (dense and bulky) bones. Sirenians first appeared in the fossil record in the Early Eocene and significantly diversified throughout the epoch. They inhabited rivers, estuaries, and nearshore marine waters. Sirenians, unlike other marine mammals such as cetaceans, lived in the New World. One of the earliest aquatic sirenians discovered is Prorastomus which dates back to 40 million years ago, and the first known sirenian, the quadruped Pezosiren, lived 50 million years ago. An ancient sirenian fossil of a petrosal bone was found in Tunisia, dating back to approximately the same time as Prorastomus. This is the oldest sirenian fossil to be found in Africa and supports molecular data suggesting that sirenians may have originated in Africa. Prorastomidae and Protosirenidae, the earliest sirenian families, consisted of pig-like amphibious creatures who died out at the end of the Eocene. When the Dugongidae appeared at this time, sirenians had evolved the characteristics of modern variety, including an aquatic streamlined body with flipper-like front legs with no hind limbs, and a powerful tail with horizontal caudal fins which uses an up-and-down motion to move them through the water.