September 20, 2013
In a species of tiger moth native to the Arizona desert, scientists have discovered a new weapon in the endless evolutionary arms race between predator and prey. New research shows that the moths, Bertholdia trigona, have the ability to detect and jam bats’ biological sonar—the technique that allows bats to “see” through echolocation. The moths’ remarkable ability, which as far as scientists know is unique in the animal kingdom, allows the insect to evade hungry bats and fly away.
Evidence of this ability was first uncovered in 2009, by a group led by Aaron Corcoran, a wildlife biologist who was then a PhD student at Wake Forest University. “It started with a question has been out there for a while, since the 1960s—why do some moths produce clicking sounds when bats attack them?” Corcoran explains.
Scientists knew that most species of tiger moths that emitted ultrasonic clicking sounds did so to signal their toxicity to bats—similar to how, for example, poison dart frogs are brightly colored so that predators can easily associate their striking hues with toxic substances and learn to look elsewhere for food. This particular species, though, emitted about ten times as much sound as most moths, indicating that it might be serving a different purpose entirely.
To learn more, he and colleagues collected trigona moths, put them in a mesh cage, attached them to ultra-thin filaments to keep track of their survival, and introduced brown bats. “If the sounds are for warning purposes, it’s well-documented that the bats have to learn to associate the clicks with toxic prey over time,” he says. “So if that were the case, at first, they’d ignore the clicks and capture the moth, but eventually they’d learn that it’s toxic, and avoid it.”
But that wasn’t what happened. The bats didn’t have to learn to avoid the moths—rather, Corcoran says, “they couldn’t catch them right from the beginning.” The reason for this, they determined, was that the moths were using the clicks to jam the bats’ sonar.
A bat’s sonar works like this: Normally—because they hunt at night and their eyesight is so poorly developed—bats send out ultrasonic noises and analyze the path they take as they bounce back to “see” their environment. But when approached by the bats, the moths produced their own ultrasonic clicking sounds at a rate of 4,500 times per second, blanketing the surrounding environment and cloaking themselves from sonar detection.
“This effectively blurs the acoustic image the bat has of the moth,” Corcoran says. “It knows there’s a moth out there, but can’t quite figure out where it is.”
But the experiment left a remaining question: How did the moths know when to activate their anti-bat signal? The team’s latest work, published this summer in PLOS ONE, shows that the trigona moths are equipped with a built-in sonar detection system.
As the bats approach, they increase the frequency of their calls to paint a more detailed picture of their prey. Corcoran’s team hypothesized that the moths listen to this frequency, along with the raw volume of the bats’ calls, to determine when they’re in danger of attack.
To test this idea, he attached tiny microphones to moths to record the exact sounds they heard when attacked by bats. He also stationed microphones a few feet away. The mics near the moths heard a slightly different sound profile of approaching bats. Then, he played each of these sounds to an entirely different group of moths to see their responses.
The moths that heard the recordings only began emitting their own ultrasonic noises when the researchers played the sounds heard by the moths actually in peril—and not the sounds that would be heard by moths a few feet away from the one in danger. By analyzing the two acoustic variables (volume and frequency), the moths could effectively differentiate between the two.
The moths click “only when they can confidently determine that they’re getting attacked,” Corcoran says. This makes sense, because the ability to figure out exactly when they’re in danger is particularly crucial for this species of tiger moths—unlike other, toxic species, these ones taste good to bats.
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