Tiger beetles produce ultrasound to avoid echolocating foraging bats: NPR

“A lot of things fly at night,” says Harlan Gough, a wildlife biologist with the US Fish and Wildlife Service. Twilight can set the stage for high-stakes acrobatic drama in the air — a swirl of bats and their prey, each trying to outrun the other in an aerial chase and escape.

“It’s life or death for many of these insects to cross the sky,” says Gough. Bats are skilled nocturnal hunters that use echolocation to find, track and capture their prey. “When [bats are] cruising the night sky,” he says, “sending out a pulse, listening for a response.”

These ultrasonic pulses are like an acoustic strobe light: they “illuminate” the night air with sonic search air that allows bats to home in on their next meal. But insects have developed a number of strategies to avoid bat attacks.

The latest twist in our understanding of this arms race is described by Gough and his colleagues in new research published in Biological letters that tiger beetles—big-eyed, long-legged insects with pincer-like jaws—produce their own ultrasound in response to bat ultrasound. The beetles, they suggest, do this to trick their predators into thinking they are poisonous, allowing them to fly away unharmed.

How moths use ultrasound against bats

Numerous species of moths have found ways to turn bat ultrasound to their advantage. Many species have developed eardrum-like structures that can detect bat echolocation, giving them the ability to escape. Sometimes they make a quick course correction to avoid a club approaching their position. “Another strategy,” says Gough, “is that they will fold their wings and just drop to the ground.”

Using a special organ on the chest, some species of moths produce their own own ultrasound as an answer. One reason is advertising to bats that they will make an unpleasant meal. “With that strategy,” Gough explains, “you make that sound, the bat lunges at you, but it’s eaten something similar in the past and it’s known to be really poisonous.” And so the bat remains alone enough.

He says we do something similar with certain insects. “Just like you once grabbed a yellow jacket when you were a kid and you learn very quickly not to grab anything with black and yellow stripes.” Just one unpleasant experience is enough for a bat or a person to generalize their avoidance behavior.

When a bat approaches an insect, it speeds up its echolocation pulses into a “terminal buzz” to better know the moment-by-moment location of its prey so it can capture it. During this buzzing, some species of moths make enough ultrasonic noise to interfere with the bat’s ability to find it.

Gough knew that tiger beetles also produce ultrasound and wondered why—and if they were doing something similar to these moths.

Dark nights, bugs and occasional fear

To study the beetles, Gough spent two summers as a graduate student at the University of Florida camping in southeastern Arizona. Every night he would go to sleep in his tent and set the alarm for one in the morning. Then he would set off on foot, under the stars, to scour the dark mountains and canyons with his headlamp for tiger beetles. “It was like a long nighttime Easter egg hunt where you might find one once a week,” he recalls.

In his search, Gough found himself face to face with a rattlesnake. One night Gough heard something large rustling in the darkness and approaching. He was terrified. “I was wondering, ‘Who else was here in the middle of the night?'” he says. When he got within 15 feet, he finally got a good look at the source of the commotion. It was a javelina—a pig-like herbivore. The two gazed at each other in the moonlight before parting ways.

During those two summers, Gough eventually managed to find seven species of tiger beetles. Each time he found one, he would tie their outer shells to a thin rod with a bit of wax and hang them in the air. Gough would blow a puff of air at them, prompting them to fly. He would then play an audio recording of an echolocating bat, its ultrasonic pulses speeding up as it approached.

“When you get that feeding buzz,” Gough explains, “and that bug knows the bat is right on its tail, they respond. And what you hear are these little clicks. Those clicks are made by the flapping wing. So a very clear response to bat echolocation.”

A poisonous imitator

When Gough heard the splash of tiger beetle ultrasound, he knew it wasn’t nearly enough sound to interfere with the bat’s sonar. He wondered if the beetles might be signaling to bats that they were poisonous, so he conducted an experiment in which he fed them directly to big brown bats.

“What we found was that they were eating all these different tiger beetles,” he says. “They just chewed them up.”

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

“It is likely,” he says, “that these tiger beetles produce [ultra]it sounds like it sounds like other similar moths.” That is, he believes these beetles mimic bad-tasting moths to trick bats into not eating them—even though they would make a perfectly tasty meal.

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

Specifically, she says there’s an obvious next experiment that will really determine what’s going on — “to show that if a bat attacks one of these tiger insects in flight and they make sounds, the bats will avoid eating them.”

Ter Hofstede also wants to know how much spatial overlap there is between tiger beetles and poisonous moths 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, the predators will not learn very effectively.”

Most examples of this type of mimicry are visual – a tasty type of predator deception looks like poisonous species. But Harlan Gough says tiger beetles show that this happens with sound, too.

“There’s so much in the night sky,” he says, “that we don’t realize because we can’t see it—it’s hidden from us. The things that happen behind the curtain are really exciting.”