To escape hungry bats, these flying beetles create an ultrasonic ‘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 sky: a swirl of bats and their prey trying to outsmart each other in aerial pursuit and escape.

“For many of these insects, it’s life or death as they fly through the air,” says Gough. Bats are skilled nocturnal hunters that use echolocation to find, track and capture their prey. “When [bats are] as they cruise through the night sky,” he says, “they emit a heartbeat, listening for a response.”

These ultrasonic pulses are like an acoustic strobe light: they “illuminate” the night sky with a sonic search beam that allows bats to grab their next snack. But insects have developed a range of strategies to avoid a bat attack.

In the latest twist in our understanding of this arms race, Gough and his colleagues describe in new research published in Biological letters that tiger beetles – insects with large eyes and long legs and pincer-like jaws – produce their own ultrasound in response to a bat’s ultrasound. They suggest that the beetles 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 ultrasounds into an advantage. Many species have evolved eardrum-like structures that can detect bat echolocation, giving them escape capabilities. Sometimes they make a quick course correction to avoid the bat approaching their position. “Another strategy,” says Gough, “is for them to fold their wings and just fall to the ground.”

Using a special organ on their thorax, some species of moths produce their own ultrasound in response. One reason is to advertise to the bats that they will prepare an unpleasant meal. “With that strategy,” Gough explains, “you make that sound: the bat is coming at you, but it has eaten something similar in the past and knew it was really poisonous.” And so the bat goes quietly.

He says we do something similar with certain insects. “Just like you pick up a yellow jacket once as a child and you learn pretty quickly not to grab anything with black and yellow stripes.” It only takes one unpleasant experience for a bat or a person to generalize their avoidance behavior.

When a bat approaches an insect, it speeds up its echolocation pulses to a “terminal buzz” in order to better know the location of its prey from moment to moment and be able to capture it. During this buzz, some species of moths generate enough ultrasonic sound to hinder the bat’s ability to find it.

Gough knew that tiger beetles also produce ultrasound and wondered why – and whether they did something similar to these moths.

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 night he went to bed in his tent and set his alarm for one o’clock in the morning. Then he set out on foot, under the stars, to search the dark mountains and canyons with his headlamp for tiger beetles. “It was like an extended nighttime Easter egg hunt, where you might find one once a week,” he recalls.

Gough came face to face with rattlesnakes during his search. One night, Gough heard something large shuffling in the darkness and coming closer. He was terrified. “I wondered, ‘Who the hell else was here in the middle of the night?’” he says. Once it got within 15 feet, he finally got a good look at where the source of the commotion was coming from. It was a javelina – a pig-like herbivore. The two looked 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 tied the outer shells to a thin rod with a bit of wax and hung them in the air. Gough blew a puff of air at them, causing them to fly. He then played an audio recording of an echolocating bat, with the ultrasonic pulses speeding up as it got closer.

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

A poisonous imitation

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

“And what we found is that they were eating all these different tiger beetles,” he says. “They nibbled them straight down.”

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

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

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

Specifically, she says there’s an obvious next experiment to really determine what’s happening: “to show that if a bat attacks one of these tiger beetles in flight and they produce the sounds, the bats will avoid to eat them.”

Ter Hofstede also wants to know how much spatial overlap there is between the tiger beetles and the 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 won’t learn very effectively.”

Most examples of this type of mimicry are visual: a tasty species tricks a predator look like a poisonous species. But Harlan Gough says the tiger beetles show this also happens with sound.

“In the night sky,” he says, “there is so much that we don’t realize because we can’t see it. It’s hidden from us. The things that go on behind the curtain are really exciting.”