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Bats, as a top predator of nocturnal flying insects, create selective pressures that have led many of their prey to develop something of an early warning system: ears uniquely tuned for bat high-frequency echolocation. To date, scientists have found at least six orders of insects — including moths, beetles, crickets and grasshoppers — that have evolved ears capable of detecting ultrasound.

But tiger beetles take things one step further. When they hear a bat nearby, they respond with their own ultrasonic signal, and for the past 30 years, no one has known why.

“This is such a foreign idea to humans: these animals are flying around at night, trying to catch each other in essentially total darkness, using sound as a way to communicate,” said Harlan Goff, lead author of a new study published in the journal Biology Letters which finally solves the mystery. While doing his PhD at the Florida Museum of Natural History, he reasoned that tiger beetles should benefit greatly from making the sound, as it would also help bats find them.

Tiger beetles are the only group of beetles known to scientists that appear to produce ultrasound in response to predation by bats. However, approximately 20% of moth species are known to have this ability and provide a useful reference for understanding the behavior of other insects. “It was a really fun study because we got to take the story apart layer by layer,” Goff said.

The researchers began by confirming that tiger beetles produce ultrasound in response to predation by bats. As bats fly through the night sky, they periodically send out ultrasonic pulses that give them snapshots of their surroundings. When a bat detects potential prey, they start to click more often, allowing them to stick to their targets.

It also created a distinctive bat echolocation attack sequence that the researchers played for the tiger beetles to see how they would respond. When the beetle flies, its hard shell opens to reveal two rear wings that generate lift. The elytra that formerly covered the wings are protective and do not aid in flight. They usually stay put and don’t get in the way.

The researchers spent two summers in the deserts of southern Arizona and collected 20 different species of tiger beetles for study. Of these, seven responded to series of bat attacks by slightly rotating their elytra towards their backs. This caused the beating hindwings to strike the trailing edges of the elytra as if the two pairs of wings were clapping. To human ears this sounds like a faint hum, but a bat would pick up the higher frequencies and hear the beetle loud and clear.

“Responding to echolocation in bats is a much rarer ability than just being able to hear an echolocation,” Goff said. “Most moths don’t sing these sounds through their mouths like we think bats echolocate through their mouths and noses. Tiger moths, for example, use a specialized structure on the side of the body, so you need that structure to make ultrasound as well as ears to hear the bat.”

Tiger beetles certainly responded to the sound of a bat attack with ultrasound. But why?

Some moths can drown out the sonar of bats by making several clicks in close, rapid succession. However, the researchers quickly ruled out this possibility for tiger beetles, as they produce ultrasound that is too simple for such a feat.

Instead, they suspected that tiger beetles, which produce benzaldehyde and hydrogen cyanide as defense chemicals, use ultrasound to warn bats that they are harmful — as many moths do.

“These defense compounds have been shown to be effective against some insect predators,” Goff said. “Some tiger beetles, when you hold them in your hand, you can actually smell some of these compounds that they produce.”

They tested their theory by giving 94 tiger beetles to big brown bats, which feed on a wide range of insects but show a strong preference for beetles. To their surprise, 90 were completely eaten, while two were only partially consumed and only two were rejected, indicating that the beetles’ defense chemicals do little to dissuade the big brown bats.

According to Akito Kawahara, director of the McGuire Museum’s Center for Lepidoptera and Biodiversity, this is the first time scientists have tested whether tiger beetles are actually harmful to bats.

“Even if you identify a chemical, that doesn’t mean it’s a defense against a particular predator,” Kawahara said. “You don’t really know until you do the predator experiment.”

It turns out that tiger beetles do not use ultrasound to warn bats of their danger. But there was one last possibility. Some moths produce anti-bat ultrasound, even though they are tasty. Scientists believe these moths attempt to trick bats by acoustically mimicking the ultrasonic signals of truly harmful moth species.

Could tiger beetles be doing something similar? The researchers compared ultrasound recordings of tiger beetles collected earlier in the study with recordings of tiger moths already in their database. When analyzing the ultrasound signals, they found a clear overlap and an answer to their question.

Tiger beetles, which have no chemical defenses against bats, produce ultrasound to mimic tiger moths, which are harmful to bats.

But this behavior is limited to tiger beetles that fly at night. Some of the 2,000 species of tiger beetles are active exclusively during the day, using their vision to track and hunt smaller insects, and lack the selective pressure of bat predation. Proof of this is the 12 species of diurnal tiger beetles that the scientists included in the study.

“If you take one of those tiger beetles that go to sleep at night and play a bat echolocation at it, it doesn’t respond at all,” Goff said. “And it seems that they can lose the ability to fear bat echolocation quite quickly.”

The researchers suspect that there may be even more undiscovered examples of ultrasonic mimicry, given how poorly studied the acoustics of the night sky are.

“I think it’s happening all over the world,” Kawahara said. “My colleague, Jesse Barber, and I have been studying this together for many years. We think it’s not just tiger beetles and moths. It seems to happen to all kinds of different nocturnal insects, and we just don’t know, simply because we haven’t tested that way.”

These delicate ecological interactions are also at risk of being disrupted soon. Acoustic mimicry needs a quiet environment to work, but human impacts like noise and light pollution are already changing how the night sky looks and sounds.

“If we want to understand these processes, we have to do it now,” Kawahara said. “There are incredible processes going on in our backyards that we cannot see. But by making our world stronger, brighter and changing the temperature, these balances can be upset.”

Juliet Rubin, a former student at the University of Florida, and Jesse Barber of Boise State University also co-authored the study.