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An Acoustic Arms Race

Bats and other animals use sound as a hunting tool—but their prey has also evolved ways to thwart detection

William E. Conner

Evolutionary Combat

Radar and sonar engineers are in an ever-escalating competition. Each improvement in target detection generates efforts to design a countermeasure to make detection more difficult. Most recently engineers have made efforts to reduce the radar cross-section (RCS), a measure of the detectability of a target, to minuscule proportions. We know the result as stealth aircraft. One seldom attends an air show or large sporting event without being treated to a flyover by an impressively strange-shaped B-2 stealth bomber. Its RCS has been decreased by hiding bulky engines and control surfaces inside the wing, emphasizing angles that deflect reflections away from radar receivers, and using composites and paints that absorb or otherwise impede radar reflections. It is rumored that stealth bombers have an RCS as small as a postage stamp.

Not to be outdone, insects can play the stealth game, too. Some prey insects may use stealthlike mechanisms to dampen their echo signature to bats. Jinyao Zeng, Shuyi Zhang and their colleagues at East China Normal University in Shanghai have suggested that the scales on moth wings may decrease the amplitude of the echoes they return to bats by absorbing the bats’ echolocation cries. This feature would give the moth a small but significant advantage in avoiding detection. The scales of the nocturnal moths more than double the absorption factor of the wings for sounds at frequencies between 40 and 60 kilohertz, resulting in a decrease of echo intensity of up to 2 decibels over scaleless wings—and making it more difficult for the bat to detect the moth at a distance. Control butterfly wings, which also have scales, did not show this effect. Although the mechanism of sound absorption remains to be determined, moth scales frequently have spaces between them and are typically covered with micropores and lacunae (oblong spaces) reminiscent of manmade sound-absorbing materials. It seems likely that researchers have just scratched the surface in this regard, and many more stealthlike examples will be found that foil echolocating predators.

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