The beluga is adapted to life in the Arctic, so has anatomical and physiological characteristics that differentiate it from other cetaceans. Amongst these are its all-white colour and the absence of a dorsal fin, which allows them to swim under ice with ease. It possesses a distinctive protuberance at the front of its head which houses an echolocation organ called the melon, which in this species is large and deformable. The beluga's body size is between that of a dolphin's and a true whale's, with males growing up to 5. 5 m (18 ft) long and weighing up to 1,600 kg (3,530 lb). This whale has a stocky body. A large percentage of its weight is blubber (subcutaneous fat), as is true of many cetaceans. Its sense of hearing is highly developed and its echolocation allows it to move about and find breathing holes under sheet ice.