Despite decades of research on the sounds made by dolphins, the precise source of these sounds remains unknown. Now scientists using computed tomographs (CT scans) have homed in on the source of the high-pitched clicks dolphins use in echolocation.
A biologist and an acoustical physicist from the University of California, Santa Cruz teamed up to create a two-dimensional computer model of a dolphin skull, hollow air sacs surrounding the internal parts of the blowhole and the fatty forehead structure called a melon. Physicist James L. Aroyan, marine biologist Ted W. Cranford and their colleagues used a supercomputer to simulate the effect these structures—one by one and in combination—produced on a simulated dolphin click emanating from three spots within the dolphin’s head.
The scientists could move the source of the sound, and record the pattern of sound emanating from the whole head. The skull by itself focused the sound to some extent upward and forward. Adding the air sacs, which include a tissue-air interface particularly effective at reflecting sound, focused the resulting click even more toward the front. The skull and air sacs appear to act as an acoustical mirror, reflecting the sound in much the same way the curved mirror in a flashlight reflects light into a condensed, directional beam. The melon, finally, fine-focused the sound into a more intense beam aimed forward and slightly upward from the axis of the dolphin’s beak. This angle corresponds well with those measured in live clicking dolphins.
By moving the source of sound around, Aroyan and Cranford found the most likely source of dolphin clicks, the monkey lips. Cranford had previously peered down a clicking dolphin’s blowhole with an endoscope—like those used to investigate human throats. Those observations, combined with CT scans of a dolphin that died of natural causes, allowed Cranford to propose a pair of fingernail-like structures inside the blowhole—the monkey lips—as the source of sound. When Aroyan simulated several other locations for the source of sound, the focus and directionality of the resulting sound beam worsened.
To produce a sound, a dolphin exhales high-pressure air through its nasal passages to its blowhole. On the way, the air must pass between the monkey lips, like air blown hard between two pieces of grass, or an oboist’s breath between the two reeds of his instrument. The sound echoes off the skull and air sacs and passes through the melon into the water as a tightly focused beam of sound energy.