Sea cows share an Eocene cousinship with elephants
Sirenia—dugongs and manatees—are fully aquatic herbivores of warm coasts and rivers, cousins of elephants and hyraxes within Paenungulata. Evolving about fifty million years ago, they now number a dugong plus three manatee species after Steller's sea cow was hunted out by 1768.
Sea cows belong with elephants and hyraxes in the clade Paenungulata, a grouping George Gaylord Simpson proposed in 1945 from anatomy and that molecular studies now strongly support. The oldest known sirenian, Pezosiren, walked on four legs about 50 million years ago, and Prorastomus, among the first aquatic forms, dates to about 40 million. Manatees branched off from early dugongids some 30 to 35 million years ago, and all sirenians share dense, heavy bones, large retracted nostrils and a skull with no sagittal crest.
Their bodies are built for slow grazing. Sirenians reach 2.5 to 4 metres and up to 1,500 kilograms, with ballast-like bones offsetting the buoyancy of their blubber. They cruise at about 8 kilometres per hour, sprint to 24 for short bursts, and tear up seagrass with muscular lips, eating 10 to 15 percent of their weight each day. A dugong's tail is notched like a dolphin's and its flippers lack nails, while a manatee's tail is a rounded paddle. Thin blubber makes them sensitive to cold, so Florida manatees crowd into springs, canals and power-plant outflows in winter and may range 1,000 kilometres north in summer.
Each species keeps its own waters. The Amazonian manatee lives only in fresh water across the Amazon Basin; the West African manatee stays in murky mangroves and flats from Senegal to Angola where water tops 18 °C; and the dugong grazes seagrass meadows off more than 40 countries in the Indo-West Pacific.
Steller's sea cow, found in the Bering Sea in 1741, grew to 10 metres and fed on kelp in cold subarctic shallows, but hunters wiped it out by 1768. Hunting, fishing nets, boat strikes and habitat loss still kill the survivors.
Source: Sirenia
