Dolphin sonar: using their heads to click.

To locate food and elude predators, many species of dolphins emit rapid sequences of clicks in tightly focused beams of high-pitched sound. Echoes of these sonar-like bursts provide dolphins with a rich source of information about their environment. However, precisely where and how these sounds are generated has long eluded researchers.

Now, a computer model of sound propagation in a dolphin's head lends credence to the recent conjecture that the animal's clicks emanate from a small packet of tissue near the top of its head, close to its blowhole. Researchers had previously suggested that these sounds originate farther down in the head, near a set of nasal air sacs or even in the larynx.

Reporting in the November JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA, physics graduate student James L. Aroyan of the University of California, Santa Cruz, and his co-workers also note that a dolphin's skull-supported air sacs appear to act together as an acoustical mirror, focusing sound from this source into a highly directed beam that emerges from the dolphin's forehead.

The initial suggestion that clicks originate in the rather small, inconspicuous knobs of fatty tissue near a dolphin's blowhole came from marine biologist Ted W. Cranford, now at the Naval Ocean Systems Center Hawaii Laboratory in Kailua. Looking at X-ray scans of dolphin heads, Cranford discovered that every species he examined contained similar structures.

These flaps of tissue may act somewhat like the vibrating lips of a trumpet player, Cranford contends. In a dolphin, "when air is pushed past the lips and the lips flab together, there's a little quivering of the fatty structure," he says. "That little pulse is what is transmitted out into the water as the echolocation signal."

To produce his simplified, two-dimensional computer model, Aroyan derived the basic geometry of a dolphin's head from one of Cranford's X-ray images, added data concerning tissue density and speed of sound at various points, and computed the paths followed by high-frequency sound waves as they traveled outward from their source. Only when the sound source was put at the spot where Cranford had found the fatty structures did the simulation show a directed beam matching experimental measurements of emanations from a dolphin's head.

"I think we've actually pinpointed the right location, the right structures," Cranford says. "It's a matter of coming up with the proof, and that's going to take a while."

Meanwhile, Aroyan is trying to refine his results by developing a three-dimensional computer model of a dolphin's head.