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Whales are a group of large, exclusively aquatic placental mammals. They are broadly classed as predators, but their food ranges from plankton to large fish. Along with the porpoises and dolphins, the whales form the order Cetacea.

The blue whale is the largest living animal, and the largest mammal ever to have lived on Earth. It is a filter-feeder, using its baleen to strain plankton out of the seawater and may reach 100 feet long and 150 tons.

Many species of whales were hunted nearly to extinction, for their meat, fat (used to make lamp oil),oil and ambergris, a perfume ingredient. International treaties now sharply restrict whaling, but Japan and Norway still engage in some commercial whaling, and small aboriginal groups in the United States and Pacific island nations hunt whales on a small scale.

Whales have lungs and breathe air; they evolved from land mammals which returned to the sea. The precise evolution of whales is still somewhat obscure, but recent discoveries of 47-million-year-old fossils in Balochistan, Pakistan suggest that their closest relatives on land are hippos. Previous DNA studies have also suggested a hippo-whale link.