Navy engineers aren't the only ones who can jam sonar. Scientists have discovered a species of tiger moth that thwarts hungry bats by emitting extra-loud clicks to block the bats' ability to echolocate.

Researchers have long known that some species of moths send out clicks in response to bat sonar, but until now, no one has been able to prove that the clicks actually interfere with echolocation. "The idea of a jamming mechanism has been thrown around for 50 years, but nobody has really put a moth and a bat together in a flight room to see what happens," said ecology graduate student Aaron Corcoran of Wake Forest University, co-author of the study published Thursday in Science.

Corcoran and his colleagues pitted a particularly noisy species of tiger moth, the Bertholdia trigona, against big brown bats trained to hunt in a flight room. As long as the moths were able to click, the bats couldn't catch them, even though the moths were tethered on a string.

But when the scientists pierced a small hole in the moths' sound-producing structures, called tymbals, the silenced moths quickly became lunch.

"It's the first good, solid case of this going on," said insect behavior expert James Fullard of the University of Toronto at Mississauga, who was not involved in the study. "For this bat and this moth, it looks pretty convincing that jamming is what's going on."

Not all clicking moths can jam sonar, Fullard said, and that's part of what makes this discovery so exciting. Previous research revealed that two other varieties of tiger moth make clicks that are too quiet to interfere with bat echolocation. Instead, he said, these moths likely use the clicks as a warning: Because most moths that click back at bats are poisonous, scientists think the noise may communicate, "Don't eat me, I taste bad."

But B. trigona isn't poisonous, and the Wake Forest researchers experimented with young bats that had no prior exposure to clicking moths, so they hadn't already learned to equate clicking with a bad taste. Nor did it seem like the bats were just startled by the clicking moths. Even after multiple attempts on multiple nights, the bats still couldn't catch the intact B. trigona.

"Mammals habituate to startle rather quickly," Corcoran said. "We went through seven days of trials, but the bats never habituated. They were put off by the clicks right away and throughout the whole experiment."

The researchers haven't yet proven how the moth's sonar-jamming mechanism works, but they have two leading hypotheses: The moth's clicks may act as false echoes, essentially making the bat "see" double, or they may interrupt the bat's own echoes, making its prey appear closer than it is.

Unlike other moths, B. trigona appears to be particularly suited for jamming sonar because it can make up to 4,500 clicks per second. Near-constant noise is important because it prevents a bat from hearing the echoes of its own sonar clicks.

"If the timing is just right, if a click arrives in the two millisecond window shortly before the arrival of a real echo, it's going to throw off the ranging software of the bat," said echolocation expert Bill Conner, who led the project. "That’s why this animal, we think, evolved sounds that cover all of acoustic time. If you listen to the recordings, the moths produce clicks all of the time, and that greatly increases the probability that some clicks will fall into that precise time window."

The group first spotted the noisy B. trigona in a cloud forest in Ecuador, but they were particularly excited to discover the moth as far north as Arizona. To search for evidence of sonar jamming outside the lab, the researchers have now set up a field station in the Chiricahua National Monument of southeast Arizona, where 18 species of bats interact with more than 30 kinds of tiger moths, including B. trigona.

"There will always be some researchers who will say, 'Well you’ve proven that you can jam sonar in the laboratory, but does it really happen in the field?'" Conner said. "That’s the reason for the follow-up."

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Image: Nickolay Hristov

Video: Science/AAAS

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