Zoology: Nature's Counter-Sonar

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Modern bomber-plane crews know just what to do when their receivers pick up the pings of an enemy radar. They transmit pings of their own designed to confuse an oncoming fighter or trick an attacking missile into veering toward empty air. Such sophisticated electronic countermeasures may be the latest thing in aerial warfare, say Entomologists Dorothy C. Dunning and Kenneth D. Roeder of Tufts University, but the idea is not at all new to non-human flyers. For millions of years, shifty moths have been using similar sound-pulsing stunts to protect them selves from marauding bats.

Bats hunt night-flying moths by echolocation, uttering rapid chirps of ultra sonic sound and flying toward echoes that bounce back from their prey. It is a simple and effective system, but Dr. Roeder proved several years ago that noctuid moths can hear the search sonar of a cruising bat and take evasive action. To save their lives, they fold their wings and dive to the ground or shift suddenly into a zigzag course (TIME, June 9, 1961).

Now, with Dr. Dunning's help, Roeder has discovered much more advanced moths that send out their own sonic signals; they can make clicking sounds that are not very different from the search-sonar pulses of bats.

To learn the purpose of such moth clicks, Dunning and Roeder caged the insects in front of a loudspeaker and exposed them to batlike trains of ultrasonic pulses. At once the moths started clicking in what seemed to be an effort to confuse an oncoming bat. To test the effectiveness of the countermeasure, Dunning and Roeder built an electrically operated gun that tosses live meal worms on short trajectories. They trained captive bats to find the meal worms by echolocation, and to pick them skillfully out of the air. Then the entomologists recorded the clicks made by moths and played them over the loudspeaker just as a bat was making its swoop at a meal worm. In 85% of the tries, the hungry bat abandoned the juicy worm and dodged away. A clicking moth presumably would have escaped the bat too.

Roeder and Dunning are not quite sure why the trick works. The moth's sounds may convey the message that the sender is not good to eat, or in some way they may deceive the bat's echo-location system. Whatever the moth clicks do, they are as effective as any man-made radar jammer.