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Battle between moths, bats driving acoustical evolution

According to a new study, moths use an acoustic metamaterial -- an array of ultrasonic frequency-absorbing scales -- on their wings to dampen the echolocation signal returning to hunting bats. Photo by Thomas Neil/University of Bristol
According to a new study, moths use an acoustic metamaterial -- an array of ultrasonic frequency-absorbing scales -- on their wings to dampen the echolocation signal returning to hunting bats. Photo by Thomas Neil/University of Bristol

June 9 (UPI) -- The battle between predators and prey inspires all kinds of evolutionary adaptations. Prey do what they can to avoid being eaten, while predators evolve new techniques for spotting and capturing their next meal.

When it comes to camouflage, the hunted usually seek ways to blend in visually, but not all predators rely on eyesight to track and kill their food. Bats, for example, use echolocation.

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New research -- presented Wednesday at the 180th meeting of the Acoustical Society of America -- suggests moths have evolved impressive anti-bat defenses over thousands of years.

According to lead author Thomas Neil, a researcher at the University of Bristol, and the title of Wednesday's presentation: "Moth wings are acoustic metamaterials."

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Motivated by the evolutionary pressures of predation, moth wings have been engineered to absorb ultrasonic frequencies to avoid the echolocation systems deployed by bats.

Over millennia, subtle tweaks in body and wing structure have helped moths dampen the echo return signal, giving them a better chance of avoiding hungry bats.

"The wings of a moth will produce strong echoes to a hunting bat owing to their large size," Neil said. "As such, it is important the moth cloaks the wing with sound-absorbing material, so it matches the acoustic camouflage brought about by the fur on the body.

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"The only way to create the much thinner sound absorber allowed on the wings is by developing a resonant absorber, and we discovered moth wings have evolved this approach," Neil said.

Lab tests showed as much as 70 percent of the ultrasonic waves hitting the scales of a beating moth wing are absorbed.

Perhaps more impressive, scientists found moths have evolved scales that are individually tuned to narrow frequency bands, ensuring their wings absorb the full spectrum of echolocation signals emitted by hunting bats -- it's the first example of an acoustic metamaterial in nature.

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By creating an array of resonant absorbers, the moth can capture ultrasonic wavelengths that are much thicker than their wings. A moth's sound-absorbing wings are 100 times thinner than the wavelength of a bat's echolocation pulse.

"In theory, we could take inspiration from the moths and build sound absorbing panels made from lots of differently tuned resonating paddles, with the goal of achieving sound absorption that is on par with traditional sound absorber panels but being just a fraction of the width," Neil said.

"With this approach, we would be getting close to a much more versatile and acceptable sound absorber wallpaper rather than the typically bulky absorber panels we use today," Neil said.

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