The structures responsible for sound production differ between toothed whales and baleen whales, but exactly how they work remains poorly understood.
In baleen whales, sounds are believed to originate in folds in the larynx, structures akin to the vocal cords of terrestrial mammals. The vibrations are then transmitted through the ventral grooves before being finally emitted into the water. The larynx of male humpback whales, who sing during the reproduction period, is wider than that of females. Conversely, female bowhead whales sing to attract males and their larynx is proportionately larger than the latter’s.
In toothed whales, air that enters the nasal tract triggers the movement of thick membranes called phonic lips, causing the surrounding tissues to vibrate and creating the sound. The vibration then passes through the skull to reach the melon, a fatty sound box in the forehead, which modulates and focuses the sound beam in the water. After passing through the phonic lips, the air is released through the blowhole or sent back to the nasal apparatus in order to be reused to emit a new sound.
As sounds are the result of a movement of air, the challenge for the whales is to produce them without losing their breath. After a respiratory cycle, part of the inhaled air thus remains in the nasal passage and this recycled air is thought to move back and forth in the respiratory cavity, allowing sound production to continue.