The magnificent humpback whale (Megaptera novaeangliae) is renowned for its impressive leaping displays and for the mysterious 'singing' of solitary males. The robust body is blue-black in colour, with pale or white undersides. The flippers may also be white and are the largest appendage of any animal; reaching up to five metres in length. On the underside of the mouth are 12 to 36 throat grooves, which can expand when filtering water during feeding. Humpbacks have characteristically knobbly heads, covered in many raised lumps ('tubercles') and barnacles. There are two blowholes on the back and the spout of water can appear very bushy. The spreading tail flukes have a distinct indentation in the middle; as the whale undertakes a deep dive it usually arches its back (hence the common name) so that the tail flukes are raised above the water and clearly visible. The pattern on the underside of the flukes is unique and can be used to photo-identify and track individuals. The humpback whale completes the longest annual migration of any mammal, travelling from polar regions to the tropics and back each year. The humpback whale is named for the distinctive 'hump' formed by its back when about to take a long dive. Humpback whales sometimes hunt cooperatively, exhaling underwater to form a 'bubble net' trapping prey within a wall of air. The humpback whales flippers are the largest appendage of any animal at up to 5m in length.