The gray whale, (Eschrichtius robustus), is a baleen whale that migrates between feeding and breeding grounds yearly. It reaches a length of about 15.2 m (50 ft), a weight of 36 tonnes (35 long tons; 40 short tons), and lives 50–70 years. The common name of the whale comes from the gray patches and white mottling on its dark skin.[4] Gray whales were once called devil fish because of their fighting behavior when hunted.[5] The gray whale is the sole living species in the genus Eschrichtius, which in turn is the sole living genus in the family Eschrichtiidae. This mammal descended from filter-feeding whales that developed at the beginning of the Oligocene, over 30 million years ago.