Dolphins on the Hunt

June 19, 2003 — Dolphins make clever changes to their sonar signals as they home in on their prey, according to a study published Thursday in the journal Nature.

Operating in the dark, bats, submarines and dolphins have to rely on sound rather than vision, sending out high-frequency pings or "clicks" to locate their targets.

But they all face a similar problem: as they get closer to the target, the echo comes back faster but also becomes progressively louder.

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This is because more energy is reflected back than before, rather than gets lost in the surrounding water or air. And if the echo gets too loud, it could become deafening.

Bats resolve this problem by decreasing the sensitivity of their hearing once they have emitted the signal, and sub sonar operators do the same by turning down the "gain" control on their receivers. That way, if the target is close by, the echo is heard at the same volume as if it were far away.

What dolphins do, though, has always been a puzzle, for the muscles in their middle ear are stiffer and denser than those of bats and apparently incapable of doing the selective sensitivity trick.

Instead of turning down their sonar receivers, dolphins adjust the volume on their transmitters, said University of Hawaii marine biologists Whitlow Au and Kelly Benoit-Bird.

The pair set up four hydrophone arrays, each of them in the shape of a three-pointed star, with a microphone at each tip and one in the center. They then recorded the sonar sounds of four species of Pacific dolphins.

As the dolphins came closer to their prey, the animals automatically adjusted their signals, keeping them at the same amplitude so as not to be deafened by the echo.

There was a strict mathematical correlation — the amplitude fell by six decibels every time the distance was halved.

The number of signals — "clicks" generated by air flow through the dolphin's nasal system — also rose as the animals closed in on their target, thus providing an accurate picture of fast-moving prey at the vital moment of interception.

"The dolphin sonar system has arrived at a gain control system that is different from bats and technological sonars, but which is just as effective," the visibly impressed authors said.