To escape hungry bats, these flying beetles create an ultrasound ‘illusion’

“A lot of things fly at night,” says Harlan Gough, a wildlife biologist with the U.S. Fish and Wildlife Service. Nightfall can set the stage for a high-stakes acrobatic drama in the air: a whirlwind of bats and their prey each trying to outwit the other in aerial pursuit and flight.

“It’s a matter of life and death for many of these insects to cross the sky,” says Gough. Bats are skilled nocturnal hunters that use echolocation to find, track and catch their prey. “When (bats) navigate the night sky,” he says, “they send out a pulse and wait for a response.”

These ultrasonic pulses are like an acoustic strobe light: they “light up” the night air with a sonic search beam that allows bats to focus on their next snack. But insects have developed a whole series of strategies to avoid an attack by a bat.

In the latest turning point in our understanding of this arms race, Gough and his colleagues describe in new research published in Biology letters that tiger beetles – insects with big eyes, long legs and claw-like jaws – produce their own ultrasounds in response to those of a bat. The beetles, they suggest, do this to fool their predators into thinking they are poisonous, allowing them to fly away unharmed.

How Butterflies Use Ultrasound Against Bats

Many species of moths have found ways to turn bat ultrasound into an advantage. Many species have evolved eardrum-like structures that can detect bats’ echolocation, providing them with escape options. Sometimes they make a rapid correction of their trajectory to escape the bat that is closing in on their position. “Another strategy,” says Gough, “is for them to fold their wings and fall to the ground.”

Using a special organ on their thorax, some species of butterflies produce their own ultrasound in response. One reason is to let bats know that they will be preparing an unpleasant meal. “With this strategy,” says Gough, “you make this noise, the bat swoops at you, but it’s already eaten something similar and knows it’s really toxic.” And so the bat leaves itself pretty well alone.

He says we do something similar with some insects. “In the same way that you pick up a yellow jacket once when you’re a kid and learn pretty quickly not to take anything with black and yellow stripes.” It only takes one unpleasant experience for a bat or person to generalize their avoidance behavior.

When a bat approaches an insect, it speeds up its echolocation pulses to form a “terminal buzz” to better know the instantaneous location of its prey and be able to catch it. During this buzzing, some types of moths generate enough ultrasonic noise to prevent the bat from finding it.

Gough knew that tiger beetles also produced ultrasound and wondered why – and if they did something similar to these butterflies.

Dark nights, beetles and the occasional scare

To study the beetles, Gough spent two summers as a graduate student at the University of Florida, camping in southeastern Arizona. Every evening, he went to bed in his tent and set his alarm for one in the morning. He then set off on foot, under the stars, looking for tiger beetles in the mountains and dark canyons with his headlamp. “It was like a long nightly Easter egg hunt where, maybe once a week, you found one,” he recalls.

Gough came face to face with rattlesnakes in his search. One night, Gough heard something large moving in the darkness and getting closer. He was terrified. “I was wondering, ‘Who else was here in the middle of the night?'” he said. Once he got within 15 feet, he finally got a good look at the source of the commotion. It was a javelina – a pig-like plant eater. The two men looked at each other in the moonlight before going their separate ways.

Over the course of these two summers, Gough ultimately managed to find seven species of tiger beetles. Each time he found one, he attached their outer shell to a thin stem with a little wax and suspended them in the air. Gough blew a breath of air at them, making them fly. It then played an audio recording of a bat echolocating, its ultrasonic pulses speeding up as it got closer.

“When you get to that feeding buzz,” says Gough, “and that beetle knows that the bat is right on its tail, it responds. And what you hear are these little clicks. Those clicks are emitted by the flapping wing. It is therefore a very clear response to the bat’s echolocation.

A toxic imitation

When Gough heard the tiger beetles’ ultrasound, he knew it wasn’t loud enough to jam a bat’s sonar. He wondered if the beetles signaled to the bats that they were poisonous. So he conducted an experiment in which he directly fed them big brown bats.

“And what we found was that they were eating all these different tiger beetles,” he says. “They ate them in one go.”

Gough performed an analysis that showed that the ultrasonic pulses of tiger beetles and moths (no relation) are acoustically similar. And because tiger butterflies are toxic to bats, which left Gough with a guess.

“It is likely,” he says, “that these tiger beetles produce the (ultra) sound that resembles that of other similar moths.” That is, he thinks these beetles imitate bad-tasting moths to trick bats into not eating them either, even though they would make a perfectly tasty meal.

“I’m pretty convinced by their data,” says Hannah Ter Hofstede, a biologist at the University of Windsor who was not involved in the research. “Of course I think they could do more and they say they can do more.”

Specifically, she says there’s an obvious next experiment to really understand what’s going on – “to show that if a bat attacks one of these tiger beetles in flight and it makes the sounds, the bats will avoid eating them.”

Ter Hofstede also wants to know to what extent there is spatial overlap between tiger beetles and poisonous butterflies, because such mimicry only works if there is “a reliable correlation between the signal and the bad taste,” she says. “If there are too many cheaters in the system, predators won’t learn very effectively.”

Most examples of this type of mimicry are visual: a tasty species tricks a predator into resembling a toxic species. But Harlan Gough says tiger beetles show it happens with sound, too.

“In the night sky,” he says, “there are so many things that we don’t realize because we can’t see them – it’s hidden from us. What happens behind the curtain is really exciting.”