Sonic deception: stealth moths

Forget stealth military jets. Moths were there first. But rather than dodging enemy radar, they’re playing a different deadly game: trying to hide from bat sonar.

Because moths and bats are both nocturnal, much of moth evolution has been driven by the need not to get eaten by bats.

Over millions of years, they’ve developed a number of tricks. Some have ears tuned to hear the high-frequency clicks bats emit not only to navigate but to lock in on their prey. That allows them to hear an approaching bat and dodge.

Other moths emit their own ultrasound clicks in an effort to jam the bats’ sonar. Still others have long streamers that drag behind their wings and serve as acoustic decoys. When a bat tries to grab this false target, the moth may lose the streamer, but nevertheless “lives to fly another day”, says Thomas Neil, a bioacoustics researcher at the University of Bristol, UK.

Deaf moths versus predator bats

But moths have another trick up their wings, Neil said last week at a virtual meeting of the Acoustical Society of America. Their wings can be superbly stealthed against bat sonar.

It works, he says, because they are covered in tiny scales that serve as acoustic camouflage.

Butterflies have similar scales – they are what provide the colour patterns to their wings – but because butterflies fly in the daytime, they have no need to hide from bats. In fact, Neil says, experiments in his lab have shown that the scales on butterfly wings actually increase the amount of ultrasound they reflect. Fly a butterfly at night, and its wings would be like a giant beacon for any hungry bat in the vicinity.

The scales on moth wings, he says, are about 100–200 microns long, and only 1 to 2 microns thick. That thinness is important because it’s much smaller than the wavelength of even the highest frequency echolocation sonar used by bats. That means that when hit with bat ultrasound, they don’t simply reflect it. Instead they vibrate. “[They] turn sound into kinetic energy,” Neil says.

Also important is that unlike butterfly wing scales, which are fairly uniform in size, moths’ scales are widely varied. One might vibrate best in response to one wavelength of bat sonar, while its neighbor responds better to a different wavelength. Put them together, Neil says, and they cover the entire range of noises bats are capable of producing.

When tested against imitation bat sonar, he adds, “each moth we tested reduced the strength of the echo by six decibels. “That may not sound like a lot, but it’s actually a 70 per cent reduction – more than enough to make them much harder for bats to spot.

Understanding how this works isn’t just relevant to the acoustical life-and-death battle between bats and moths. It might also be of use to humans, Neil says.

Perhaps we can take a cue from moths and find better ways of developing our own echo-reducing materials – useful for everything from improving the sound quality of concert halls and recording studios to making our homes quieter and more calming.

“We are currently working on prototypes,” Neil says.

The goal, he adds, is to come up with something like sound-absorbing wallpaper, “rather than the bulky absorber panels we use today”.

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