Tiger beetles produce ultrasound to avoid echolocating foraging bats: NPR

Harlan Goff holds a recently collected tiger beetle on a leash.

Lawrence Reeves


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Lawrence Reeves


Harlan Goff holds a recently collected tiger beetle on a leash.

Lawrence Reeves

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

“It’s life or death for a lot of these insects to get through the sky,” says Goff. Bats are skilled nocturnal hunters that use echolocation to locate, track, and capture their prey. “When [bats are] criss-crossing the night sky,” he says, “they send out a pulse, listening for a response.”

These ultrasonic pulses are like acoustic lightning: they “illuminate” the night air with a sound search beam that allows the bats to come home to their next breakfast. But insects have developed a range of strategies to avoid a bat attack.

The latest twist in our understanding of this arms race, Gow and his colleagues describe in new research published in Biology Letters that tiger beetles—big-eyed, long-legged insects with pincer-like jaws—produce their own ultrasound in response to a bat’s ultrasound. They hypothesize that the beetles do this to trick their predators into thinking they are toxic, allowing them to fly away unharmed.

How moths use ultrasound against bats

Many species of moths have found ways to turn bat ultrasounds to their advantage. Many species have evolved drum-like structures that can detect the echolocation of bats, giving them opportunities to escape. Sometimes they make a quick course correction to avoid a bat approaching their position. “Another strategy,” Goff says, “is they’ll fold their wings and just drop to the ground.”

Using a special organ on their thorax, some species of moths produce their own own ultrasound in response. One reason is to advertise to bats that they will make an unpleasant meal. “With this strategy,” Gough explains, “you make this sound, the bat pounces on you, but it’s eaten something like this in the past and it’s known to be really toxic.” So the bat is left well enough alone.

He says we do something similar with certain insects. “The same way you grab a yellow jacket once when you’re a kid 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 human to generalize their avoidance behavior .

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

Goff knew that tiger beetles also produce ultrasound, and he wondered why—and if they did something similar to these moths.

Dark nights, beetles and the occasional scare

To study the beetles, Goff spent two summers as an undergraduate at the University of Florida camping in southeastern Arizona. Every night he went to bed in his tent and set his alarm for one in the morning. Then he set out on foot under the stars to search the dark mountains and canyons with his spear for tiger beetles. “It was like a long drawn-out all-night Easter egg hunt where maybe once a week you’d find one,” he recalls.

Gough came face to face with rattlesnakes in his quest. One night Gough heard something large stirring in the darkness and approaching. He was terrible. “I was like, ‘Who the hell was here in the middle of the night?'” he says. After getting within 15 feet, he finally got a good look at the source of the commotion. It was a spear—a pig-like herbivore. The two looked at each other in the moonlight before parting ways.

During those two summers, Goff eventually managed to discover seven species of tiger beetles. Whenever he found one, he tied their outer shells to a thin stick with a little wax and suspended them in the air. Goff blew air at them, making them fly. It would then play an audio recording of an echolocating bat whose ultrasonic pulses sped up as it approached.

A tiger beetle flies while tethered in a University of Florida laboratory.

Harlan Goff


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Harlan Goff


A tiger beetle flies while tethered in a University of Florida laboratory.

Harlan Goff

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

A toxic mimic

When Gow heard the ultrasonic noise from the tiger beetles, he realized that the sound was not enough to drown out the bat’s sonar. He wondered if the beetles might be signaling to the bats that they were toxic, so he conducted an experiment in which he directly fed them big brown bats.

“And what we found is that they were eating all these different tiger beetles,” he says. “They chewed them right up.”

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

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

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

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 they make the sounds that the bats will avoid eating them.”

Ter Hofstede also wants to know how much spatial overlap there is between tiger beetles and toxic 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 appearance tricks a predator looks like toxic species. But Harlan Goff 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.”

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