Dolphins do not possess vocal cords like humans and other terrestrial mammals. Land animals produce sound using membranous folds within the larynx that vibrate as air is exhaled. Since dolphins are fully aquatic, they evolved a highly specialized system that allows them to generate sound underwater without losing precious air. This unique anatomical complex allows them to produce their characteristic whistles and clicks while submerged.
The Anatomical Mechanism for Sound
Sound generation takes place within the nasal passages, located just beneath the blowhole. The primary structures responsible are a pair of soft tissue folds known as the phonic lips, sometimes called “monkey lips.” These lips are part of the dorsal bursa complex in the nasal region.
Sound is generated when the dolphin pushes air from its lungs, through the nasal passages, and past the phonic lips. The air pressure causes the lips to vibrate rapidly, generating sound pulses. Unlike breathing, this sound production does not require air to be expelled.
Instead, the air is shuttled and recycled between specialized air sacs within the head. This air-recycling mechanism allows the dolphin to produce continuous streams of sound, such as click trains, without having to surface for air or waste the air needed for respiration. Because a dolphin has two independent phonic lip complexes, it can produce two distinct sounds simultaneously, such as a whistle and a click.
Once generated, the sound pulses are transmitted forward into the water through the melon, a large, fatty organ in the dolphin’s forehead. The melon acts as an acoustic lens, focusing the sound waves into a highly directional beam. This focused beam projects sound efficiently through the water.
Types of Sound and Their Behavioral Roles
Dolphins produce a diverse range of vocalizations, broadly categorized into two main types: tonal sounds (whistles) and pulsed sounds (clicks). These two categories serve different purposes in the dolphin’s social and foraging life.
Whistles are used primarily for communication and social bonding, with frequencies ranging from 0.2 to 24 kilohertz (kHz). The most significant is the “signature whistle,” a unique frequency-modulated sound developed early in life. This signature whistle functions like a personal name, allowing individuals to identify themselves and their location to others.
A mother may whistle to her calf almost continuously after birth, imprinting the signature whistle to facilitate mother-calf reunions. Dolphins also use other whistles, moans, and trills for expressing emotion, coordinating group movements, or signaling danger.
The second major sound type, clicks, consists of high-frequency, short-duration pulses used exclusively for echolocation. These clicks are emitted in rapid sequences called “click trains” and can reach peak frequencies of up to 150 kHz. Echolocation allows the dolphin to navigate, locate prey, and survey its environment, even in dark or murky water.
The outgoing sound waves bounce off objects and return as echoes, received through specialized fatty tissues in the lower jaw and transmitted to the inner ear. By interpreting the returning echoes, the dolphin can determine an object’s size, shape, distance, speed, and internal structure. Burst-pulse sounds, rapid click trains, are a third category used during close-range social interactions, often conveying excitement or aggression.