A hungry bat screeches out ultrasonic waves and listens as they echo off surrounding objects. One of those echoes sounds an awful lot like a tasty moth, so it swoops in for the kill--but grabs only air. Thwarted again by the tiger moth Bertholdia trigona. New research explains the clever defense; the moth emits ultrasonic clicks that throw off bats' sonarlike echolocation, like jamming a radio signal. It's the first time this type of acoustic interference has been demonstrated in the natural world.
For about 40 years, researchers have been intrigued by the clicking tiger moth, which ranges from Central America to Colorado. Lots of moths click by vibrating membranes on their abdomens, but B. trigona is an order of magnitude louder. "You can hold them up to your ear and hear them," says the study's co-author, biologist William Conner of Wake Forest University in Winston-Salem, North Carolina.
Researchers have noticed that clicking moths were eaten less often than their quieter cousins, but how the rapid, high-pitched zzt-zzt-zzt wards off bats has been a mystery. Three possible explanations have emerged. One is that the clicks startle the bats. If that were the case, though, you'd expect bats to learn to ignore the sound, Conner says. Another hypothesis is that the clicks serve as a warning, letting bats know the moth is distasteful. That's thought to be the case with some toxic moths, such as the related dogbane tiger moth, Cycnia tenera; and other nontoxic moths might mimic the technique. Finally, the moths may somehow jam the bats' echolocation, because the clicks occur in the same frequency range as the ultrasound used by the bats.
Conner and his grad students devised a test to find out which hypothesis was right. Over 9 nights, they presented brown bats with a series of moths tethered to the ceiling with super-thin fishing line. Some of the moths were B. trigona with their clicking apparatus intact, others had it removed, and still others were various silent or quieter-clicking species. Infrared cameras captured the action.
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The bats made contact with nonclicking moths 400% more often than they did with clicking B. trigona. Right from the start, the bats went after the clicking moths, discounting the startling hypothesis, and kept going for them night after night, making the warning hypothesis unlikely. That leaves jamming as the most likely scenario, Conner says. Furthermore, in about one-third of the attacks, the bats seemed to hesitate in their approach, suggesting they were baffled.
Conner, whose team reports its findings in tomorrow's issue of Science, says how the clicks interfere with bats' echolocation is still up for debate. His working hypothesis is that they imitate echoes, creating "multiple acoustic images in space," which "throws off the ranging software in the brain of the bat."
Mitchell Masters, an evolutionary biologist at Ohio State University in Columbus, says that the authors "have made the most convincing case to date that jamming is, indeed, part of the equation," but he'd like to see further experiments flesh out just how bats' approach behavior is affected.
Behavioral ecologist, Gareth Jones of the University of Bristol in the United Kingdom is intrigued by the fact that only loudly clicking B. trigona seems to disrupt echolocation, whereas quieter-clicking moths don't. That may suggest a dual-clicking strategy for moths, he says. Loud clickers jam bats' acoustic signals, whereas quiet clickers try to mimic the sounds of unpalatable moths.