DOLPHINS, well known for their intelligence, are even cleverer than previously supposed. Marine biologists have discovered that when dolphins use sonar to track down prey, the creatures vary the volume of the acoustic clicks they broadcast according to their distance from a target. This helps them judge the size of the target as well as avoid uncomfortably loud echoes.
With any electromagnetic or acoustic signal, the further away a target is, the fainter the echo. Radars deal with this by making the receiver more sensitive the longer a signal takes to bounce back. Bats’ ears do essentially the same thing.
Dolphins have developed a different approach. As they swim towards a target they decrease the volume of the clicks they make, so that the echoes that return from a school of fish stay at the same volume even as the dolphin gets closer, according to Whitlow Au and Kelly Benoit-Bird at the University of Hawaii in Kailua-Kona (Nature, vol 423, p 861).
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To gather the data, the team hung four hydrophones from their boat. Small differences in the timing of clicks received by the hydrophones allowed them to determine when a dolphin was echolocating directly at the hydrophones, and to calculate the animal’s distance.
The volume of the clicks decreased by 6 decibels every time the distance from the target was cut in half. For the volume of the echo to stay the same when bouncing off a solid target, the click volume should have become quieter at twice that rate. But schools of fish scatter sound, so the volume of the echoes from them decreases about twice as fast as echoes from a single object. Dolphins make just the right adjustments to home in on schools.
Modifying the click volume might do two things. It could prevent echoes from nearby objects from being overly loud. It might also help dolphins judge target size. They already know the distance to the target from how long it takes a signal to bounce back. By adjusting the volume of clicks according to distance, they will also be able to tell that a louder echo means a bigger target, no matter what the target’s distance.