Tiger beetles produce ultrasound to avoid echolocating bats searching for food: NPR

“A lot of things fly by night,” says Harlan Gough, a wildlife biologist with the US Fish and Wildlife Service. Nightfall can set the stage for acrobatic mid-air drama – a whirlwind of bats and their prey, each trying to outdo the other in aerial pursuit and escape.

“For many of these insects, it’s life or death to get across the sky,” says Gough. Bats are expert nocturnal hunters that use echolocation to find, track and catch their prey. “When [bats are] they cruise the night sky,” he says, “sending out a pulse and listening for a response.”

These ultrasonic pulses are like an acoustic strobe light: they “illuminate” the night air with a sonic search beam that allows bats to return home for their next snack. However, insects have developed a number of strategies to avoid a bat attack.

In the latest twist in our understanding of this arms race, Gough and 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 the bat’s ultrasound. The beetles, they suggest, do this to trick their predators into thinking they are poisonous and allow them to fly away unharmed.

How moths use ultrasound against bats

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

Some species of moths produce using a special organ on their thorax 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 that sound, the bat lunges at you, but it’s eaten something similar in the past and it’s known to be really toxic.” And so the bat departs alone enough.

He says we do something similar with certain insects. “In the same way that when you grab a yellow jacket once as a kid, 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 “tail buzz” to better know the current location of its prey in order to capture it. During this buzzing, some moth species generate enough ultrasonic noise to disrupt the bat’s ability to find it.

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

Dark nights, bugs and the 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 went to sleep 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 flashlight for tiger beetles. “It was like a long, nightly Easter egg hunt where you might find one once a week,” he recalls.

Gough came face to face with rattlesnakes in his search. One night Gough heard something large shuffling in the darkness and approaching. He was terrified. “I was thinking, who the hell else was there in the middle of the night?” he says. Once it got within 15 feet he finally got a good look at the source of the commotion. It was a spear – a plant-eating pig. The two looked at each other in the moonlight before parting ways.

During those two years, Gough eventually managed to find seven species of tiger beetles. Each time he found one, he attached their outer shells to a thin stick with a bit of wax and suspended them in the air. Gough would blow a puff of air at them and send them flying. He then played an audio recording of an echolocating bat, its ultrasonic pulses speeding up as it approached.

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

A toxic imitation

When Gough heard the ultrasonic hiss from the tiger beetles, he knew it wasn’t nearly enough sound to disrupt the bat’s sonar. He wondered if the bugs were signaling to bats that they were poisonous, so he conducted an experiment in which he fed them directly to big brown bats.

“And we found that they ate all these different tiger beetles,” he says. “They dug them right down.

Gough conducted an analysis that showed that the ultrasonic pulses of tiger beetles and tiger moths (no relation) are acoustically similar. And because tiger moths they are toxic to bats, which prompted Gough’s hypothesis.

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

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

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

Ter Hofstede also wants to know how much spatial overlap exists between tiger beetles and toxic moths, because such mimicry only works if there is “a reliable correlation between the signal and the bad taste,” he says. “If there are too many cheaters in the system, the predators will not learn very efficiently.”

Most examples of this kind of mimicry are visual – the tasty kind fools the predator look like toxic 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. It’s what’s going on behind the curtain that’s really exciting.”