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Moths Muffle Bat Sonar with Sound-Absorbing Wings

To shield themselves from bat echolocation, moths don an acoustic cloak of invisibility—using the sound-absorbing scales on their wings.

ByKatherine J. WuNOVA NextNOVA Next
Bart Wursten flickr Bunaea alcinoe.jpg

The Cabbage tree emperor moth (Bunaea alcinoe) decorates its wings with tiny, sound-absorbing scales. By blocking echolocation, these scales lower the chances of the moth being pinpointed and eaten by a bat. Photo Credit: Bart Wursten, flickr

It’s not easy being deaf in the dark—especially when your greatest enemy is a master of sound.

Such is the twilight plight of the humble cabbage tree emperor moth (Bunaea alcinoe): It’s all these umber underdogs can do to avoid becoming a hungry bat’s dinner. Bat sonar is so sensitive that these predators attain an impressive intimacy with their surroundings—one they manipulate to easily locate and devour the minute, wriggling bodies of their prey. But even moths that can’t hear their stalkers’ telltale screech aren’t as powerless as you might expect.

Today, in the journal PNAS, researchers report that the wings of cabbage tree emperor moths are flecked with tiny scales capable of absorbing the sounds of echolocating bats, dampening reverberations that might give away their location. The evolution of this stealth-mode strategy may have helped moths evade detection by their nocturnal predators.

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Often dismissed as drab butterfly alternates or light-loving nuisances, moths might just be one of the most underappreciated members of the insect world. Like their butterfly relatives, moths are essential pollinators and sources of food, supporting diverse ecosystems worldwide. Even in the face of danger, the modest moth is mighty: In recent years, researchers have uncovered a dizzying array of anti-bat defenses in these little insects. Because bats rely heavily on sonar to navigate the world around them, much of the moth’s modus operandi is auditory. Some moths will produce their own ultrasonic clicks to jam bats’ sonar or vocally advertise their toxicity, while others have adopted a strategy of constant vigilance by evolving the insect equivalent of ears, which are sensitive to the audio of approaching bats.

However, one group of oddballs exhibits none of the aforementioned tricks of the trade—and it was these miscellaneous moths that piqued the interest of Marc Holderied, a sensory ecologist at the University of Bristol in the United Kingdom.

Previous research had hinted that these mysterious moths, while effectively “deaf” to bats, were literally flying under the radar: They absorbed the sounds of sonar using the exquisite architecture of tiny, leaf-shaped scales that decorate their wings. These scales might act like the auditory equivalent of an invisibility cloak, allowing moths to pass through bat territory neither seen nor heard. But no one had yet figured out exactly how these little scales were accomplishing such an acoustic feat.

Cape Serotine Bat (Eptesicus capensis) bernard dupont flickr.jpg

The Cape serotine (Neoromicia capensis) is a sub-Saharan species of bat that feasts on insects like moths, using echolocation to home in on its prey's location. Photo Credit: Bernard Dupont, flickr

To puzzle out the mechanics of the muffling, Holderied and his colleagues, led by acoustician Zhiyuan Shen, plucked individual scales from cabbage tree emperor moths and constructed 3D models of their structure to see how they would behave in the presence of sound waves.

When bat sonar hits a typical object, some of the sound waves will ricochet off the surface and boomerang batward. This outcome is ideal for the bat, which can harvest the information contained in the echo and home in on its prey. All objects, however, have what’s called a resonant frequency, or a natural frequency at which they vibrate. (This is the phenomenon behind the common trope of opera singers shattering wine glasses—the sound hits the glass at its resonant frequency, causing the poor vessel to vibrate itself to pieces.) If incoming sonar meets its target at its resonant frequency, the energy of the sound wave can be converted into motion. This curbs the sound waves’ ability to reverberate and muddies the bat’s estimation of the vibrating object’s whereabouts.

For this strategy to be viable for a moth in a bat’s crosshairs, the resonant frequency of its scales would need to overlap with the frequency of bat sonar, typically between 20 to 150 kilohertz. When the researchers analyzed their model scale, they found one resonant frequency. And then another. And then another. To Holderied’s amazement, an individual insect scale was capable of vibrating at not one, but three specific frequencies—28.4, 65.2, and 153.1 kilohertz—depending on how exactly the scale trembled in space.

“They were perfectly spaced to cover the bat echolocation range,” Holderied explains. “At that moment, we know this wasn’t a coincidence: They’re there for a reason.”

As a point of comparison, the researchers repeated their analysis with a modified scale resembling that of a butterfly, a close relative of the moth. Like moths, butterflies adorn their wings with scales—but because most butterflies are most active during the day, as a group, they probably have much less in the way of anti-bat defenses, Shen explains.

Like the B. alcinoe model scale, the butterfly-like scale exhibited three resonant frequencies—but this time, they fell less cleanly within the relevant range of bat sonar, clocking in at 88.4, 150.9, and 406.0 kilohertz. It seemed that, under pressure from their bat predators, moths had embarked on this specific evolutionary trajectory alone.

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A single scale from the wing of a cabbage tree emperor moth (Bunaea alcinoe). Typical scales are less than 0.25 millimeters wide. Photo Credit: Marc W. Holderied and Zhiyuan Shen, University of Bristol

But bats are not to be outdone. They wield a high degree of control over the frequency of their own sonar, meaning there’s a chance they could pinpoint gaps in the moth’s acoustic armor. However, not all moth scales are created equal, and every wing is dotted with units of different shapes and sizes, Holderied explains. With such diversity, B. alcinoe wings may stifle an even broader range of frequencies. It’s not yet clear exactly how much these moths compromise bat echolocation in a more natural context—but in the arms race between predator and prey, any amount of sound absorption could tip the scales in favor of Team Moth.

Accordingly, one of the clearest next steps is to return to the field and see if the findings hold true for moths on the run from actual bats, says Juliette Rubin, a sensory ecologist at Boise State University who also studies anti-bat strategies in moths, but did not contribute to the newest finding. For instance, it will be important to test if sound-absorbing moths actually fall prey to bats less often than their not-so-covert cousins—or if shaving scales off B. alcinoe makes them more vulnerable to the hunt. After all, subtle sculpting of scales doesn’t guarantee that B. alcinoe can actually flummox an experienced, living predator. But if these wings are truly as functional as they seem, moths may just fly in the face of bats’ acoustic aptitude.

“If the scales are really functioning to absorb sonar, this shows you the incredible adaptations that moths have made to bats,” says Akito Kawahara, a sensory ecologist at the Florida Museum of Natural History who did not participate in the research. “It really highlights the importance of bats in the nocturnal landscape and how much impact they have on insects.”

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The researchers are also currently investigating whether these scaly structures are present on other moths—especially those known to engage in other anti-bat antics. Doubling up on defenses could pay some serious dividends, but these adaptations often come with a cost. For instance, Holderied explains, soundproofing scales may have made moth wings heavier, potentially impeding flight.

On the other hand, tinkering with the microstructure of a tiny scale is nothing compared to some of the evolutionary triumphs of some other moths—like growing a set of ears, explains biologist Jessica Fox, who studies insect flight at Case Western Reserve University but was not involved in the new research.

This is not to undersell the exquisite nature of these little scales. While humans continue to painstakingly construct more lightweight, effective resonant absorbers to soundproof buildings and conceal military operations, moths have executed a feat of engineering that’s eluded us for years. Evolution, it seems, operates on even the smallest of scales in the war waged between moth and bat.

“Natural evolution and human engineering are converging on the same point,” Shen adds. “But nature is better than engineering.”