Some butterfly species sport striking patterns on their wings which they use to visually camouflage themselves from predators. But the luna moth is a nocturnal creature. Scientists have suggested that the unique twisty tails of these moths help throw off predators like bats that rely on sound to hunt and navigate — a kind of acoustic camouflage. Image: Andy Reago and Chrissy McClarren/Flickr
This latest work builds on a parallel study out of Boise State University, published last year, demonstrating that big brown bats are significantly better at capturing luna moths that have lost their tails. Scientists at the University of Washington and Johns Hopkins University set out to explore the related acoustics in more detail for clues to why this might be the case. They described their results in a paper in the Journal of the Acoustical Society of America.
Image: Wu-Jung Lee/University of Washington
First, they tethered luna moths so they were limited in their range of flight. Then they aimed high-frequency sound waves designed to mimic the ultrasonic pulses used by bats, and the echoes bouncing off the moths' bodies were tracked by microphones placed about 2m from each moth. This enabled them to simulate a bat's "view" of the resulting cloud of reflected echoes.
The position of the wings plays a big role in how those chirps bounce off the moth mid-flight: There will be more reflected sound when the wings are perpendicular than when they are parallel, providing a bigger target for the bat. Because there is so much variance in the reflected echoes, the researchers hypothesise that bats might just shoot for the centre of the sound cloud when they hunt — the equivalent of taking an average.
"A moth is a very complicated object in space," lead author Wu-Jung Lee said in a statement. "It could be difficult for a bat to track each individual point of the echo cloud. It would be much easier for it to say, 'There's a ball of echoes coming back, I'm going to hit the center of it, and maybe I'll catch something.'"
That's a great strategy for luna moths who've lost their tails, since Lee and his colleagues found that in such cases, the centre of the echo cloud is a bulls-eye for the moth. But the tail adds its own reflections to the mix, from every direction because of its twisting shape, and this shifts the centre of the cloud just enough to better avoid a hungry bat. "We think that twist could be a key for how the tails function acoustically," said Lee.
Granted, their model for the bat-centric view is just an approximation. Scientists don't actually know what type of signal processing the bats are using. But it's an intriguing clue. Lee hopes to learn more in subsequent experiments about how the tail influences a luna moth's flight.