It is adapted to life in the Arctic, so has anatomical and physiological characteristics that differentiate it from other cetaceans. Amongst these are its unmistakable all-white colour and the absence of a dorsal fin. 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 plastic (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 (3,500 lb).