most mammalian species produce sound by passing air from the lungs across the larynx, vibrating the vocal folds. sound then enters the supralaryngeal vocal tract, which can be adjusted to produce various changes in sound output, providing refinement of vocalizations. although morphological differences between species affect sound production, neural control is thought to be more essential factor in producing the variations within human speech and song compared to those of other mammals. cetacean vocalizations are an exception to this general mechanism. toothed whales (odontocetes) pass air through a system of air sacs and muscular phonic lips, which vibrate to produce audible vocalizations, thus serving the function of vocal folds in other mammals. sound vibrations are conveyed to an organ in the head called the melon, which can be changed in shape to control and direct vocalizations. unlike in humans and other mammals, toothed whales are able to recycle air used in vocal production, allowing whales to sing without releasing air. some cetaceans, such as humpback whales, sing continuously for hours.