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 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 prevent 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.