Sirenians (manatees and dugongs) are often called sea cows since they are the only totally aquatic herbivorous mammals. While whales and dolphins are closely related to cows, manatees and dugongs are subungulates, more closely related to elephants, hyraxes and aardvarks. They are thought to have diverged from a common ancestor back in the Eocene, some 50-60 million years ago, when one group began feeding on aquatic vegetation in the coastal shallows of Africa and southern Asia. Over thousands of years, the forelimbs of these creatures evolved into flippers, the body became streamlined, the hindlegs regressed and a flattened tail developed.
Numerous sirenian species developed by the Miocene (about 20-10 million years ago) but only one dugong species and four species of manatee survive today; dugongs are found in the tropical waters of east Africa, India, Indonesia and Australia, while manatees inhabit west Africa, the Caribbean, the Amazon Basin and tropical coasts of the Americas. Favoring shallow coastal bays, estuaries and warm, near-shore waters, manatees feed in both marine and freshwater areas; dugongs are limited to saltwater environments. Manatees can be found along Florida's coasts throughout the year and migrate as far north as the Carolinas in summer; occasional wanderers turn up in southern New England but, since they lack the blubber insulation of seals, whales and walruses, manatees cannot survive in cold water regions.
It is estimated that about 2500 manatees inhabit the U.S. Under constant threat from pollution, habitat loss and boat collisions, manatee populations are also kept in check by their low reproductive rate, averaging a single calf every 2-5 years (twins are rare). Calves remain with their mother for 2 years, initially nursing from one of two nipples behind her front flippers (female elephants have a nipple behind each foreleg). Feeding for much of the day, manatees can submerge for 20 minutes and, though they tend to move slowly, are capable of speeds up to 20 mph. Since they feed entirely on aquatic vegetation, a diet that causes excessive wear on tooth enamel, manatees replace their molars by horizontal progression, a process typical of many terrestrial herbivores (dugongs do not have this capability).
Despite their many threats, which include predation by sharks and crocodiles, manatees may live up to 60 years and can reach 13 feet in length. Dugongs are a bit smaller and have a fluked (dolphin-like) tail; the tail of manatees is paddle-shaped. Both have a prehensile upper lip, a feature that evolved into a trunk in elephants, and both have thick, nearly hairless skin, another trait shared by their terrestrial cousins.