Tiger beetles mimic toxic moths to deceive predatory bats 

Bats, as primary predators of night-flying insects, have driven many prey species to evolve high-frequency hearing to detect bat echolocation. At least six insect orders, including moths, beetles, crickets, and grasshoppers, have developed such auditory systems. 

However, according to a new study, tiger beetles take this further by emitting their own ultrasonic signals when they detect bats, a behavior that puzzled scientists for 30 years.

Using ultrasonic sounds for defense

“It’s such a foreign idea to humans: these animals flying around at night trying to catch each other in essentially complete darkness, using sound as their way of communicating,” said lead author Harlan Gough. 

While conducting his doctoral research at the Florida Museum of Natural History, Gough hypothesized that tiger beetles must gain significant benefits from producing the sound, despite it potentially aiding bats in locating them.

Tiger beetles are unique among beetles in their response to bat predation through ultrasound. In contrast, about 20% of moth species share this ability, providing a reference point for understanding similar behaviors in other insects. 

“This was a really fun study because we got to peel apart the story layer by layer,” Gough said.

Tiger beetles produce ultrasound 

The researchers first confirmed that tiger beetles produce ultrasound when under bat threat. Bats emit ultrasonic pulses to navigate and locate prey, increasing the frequency of these pulses when targeting an insect. 

The researchers replicated this bat echolocation attack sequence to observe the beetles’ response. They found that 7 out of 20 studied tiger beetle species responded by moving their elytra (protective wing covers) slightly backward, causing their hind wings to clap against the elytra, producing a faint buzzing detectable by bats.

Responding to bat echolocation

“Responding to bat echolocation is a much less common ability than just being able to hear echolocation,” Gough noted. 

“Most moths aren’t singing these sounds through their mouths, like we think of bats echolocating through their mouth and nose. Tiger moths, for example, use a specialized structure on the side of the body, so you need that structure to make ultrasound as well as ears to hear the bat.”

What is the purpose of tiger beetle ultrasound signals?

Tiger beetles’ response to bat echolocation was evident, but the purpose remained unclear. Unlike some moths that jam bat sonar with rapid clicks, tiger beetles’ simple ultrasound couldn’t achieve this. 

The researchers hypothesized that the beetles might use ultrasound to signal their noxiousness, as they produce defensive chemicals like benzaldehyde and hydrogen cyanide.

Testing this theory, they fed 94 tiger beetles to big brown bats. Surprisingly, 90 were eaten entirely, two were partially consumed, and only two were rejected, indicating the beetles’ chemicals do little to deter bats. 

This experiment, led by Akito Kawahara, director of the museum’s McGuire Center for Lepidoptera and Biodiversity, was the first to test whether tiger beetles were genuinely noxious to bats.

“Even if you identify a chemical, that doesn’t mean it’s a defense against a particular predator,” Kawahara explained. “You don’t actually know until you do the experiment with the predator.”

Mimicking the sounds of toxic tiger moths 

The results showed that tiger beetles don’t use ultrasound to warn bats of their noxiousness. Instead, the researchers considered that tiger beetles might mimic the ultrasonic signals of genuinely noxious tiger moths to deceive bats. 

Comparing recordings of tiger beetle ultrasound with those of tiger moths, they found a clear overlap, supporting the mimicry hypothesis.

This mimicry is specific to nocturnal tiger beetles. The 12 diurnal tiger beetle species included in the study did not respond to bat echolocation, indicating that these beetles have likely lost the ability to fear bat echolocation.

“If you get one of those tiger beetles that goes to sleep at night and play bat echolocation to it, it makes no response at all,” Gough said. “And they seem to be able to pretty quickly lose the ability to be afraid of bat echolocation.”

Ultrasonic mimicry in other species

The researchers believe that more examples of ultrasonic mimicry likely exist, given the understudied nature of nocturnal insect acoustics. 

“I think it’s happening all over the world,” Kawahara said. “With my colleague, Jesse Barber, we have been studying this together for many years. We think it’s not just tiger beetles and moths. It appears to be happening with all kinds of different nocturnal insects, and we just don’t know simply because we haven’t been testing in this manner.”

These delicate ecological interactions are threatened by human impacts like noise and light pollution, which can disrupt acoustic mimicry. “If we want to understand these processes, we need to do it now,” Kawahara said. “There are amazing processes taking place in our backyards that we can’t see. But by making our world louder, brighter and changing the temperature, these balances can break.”

The study is published in the journal Biology Letters.

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