Tiger beetles may weaponize ultrasound against bats

Sounding like a toxic moth might keep some beetles safe from hungry bats.

When certain tiger beetles hear an echolocating bat draw near, they respond with extremely high-pitched clicks. This acoustic countermeasure is a dead ringer for the noises toxic moths make to signal their nasty taste to bats, researchers report May 15 in Biology Letters. Such sound-based mimicry may be widespread among groups of night-flying insects, the scientists say. 

At night, bats and bugs are locked in sonic warfare. At least seven major insect groups have ears sensitive to bat echolocation pitches, and many often flee in response. Some moths have sound-absorbent wings and fuzz that impart stealth against bat sonar (SN: 11/14/18). Others use their genitals to make ultrasonic trills — above the range of human hearing — that may startle bats or jam their sonar (SN: 7/3/13).

Previous research suggested some tiger beetles — a family of fast-running, often strikingly colored predatory beetles with strong jaws — also make high-pitched clicks as a response to human-made imitations of bat ultrasound. So Harlan Gough, a conservation entomologist now at the U.S. Fish and Wildlife Service in Burbank, Wash., and his colleagues set out to answer why.

The researchers collected 19 tiger beetle species from southern Arizona and brought them into the lab. They tethered the insects to a metal rod and prompted them to fly. The team then filmed and recorded audio to see how the beetles responded to playback of a bat clicking sequence that immediately precedes an attack. Right away, seven of these species — all nocturnal fliers — pulled their hard, case-like forewings into the path of their beating hindwings. The resulting collisions made high-pitched clicking noises.

Gough and his colleagues thought that perhaps the clicks warned bats of the beetles’ unpalatability and toxicity, since the insects produce defensive chemicals and are often brightly colored as a warning to would-be aggressors. But in the lab, big brown bats (Eptesicus fuscus) devoured 90 of the 94 beetles the scientists offered. “It’s pretty clear that tiger beetles are not chemically defended against bats,” Gough says, though the chemicals might deter insect foes.

Instead, the researchers think the tiger beetles are mimicking the “stay away” clicks of foul-tasting tiger moths. In an acoustic analysis, the ultrasonic frequency, click length and other characteristics of the tiger beetles’ clicks closely resembled those of the tiger moths that live alongside them in Arizona.

While more research is needed to confirm the mimicry hypothesis, Gough says, the tiger beetles appear to be the first known insects besides moths to use anti-bat ultrasound. The phenomenon may be widespread in this nocturnal “acoustic world,” he says, with many insect orders mimicking each other. “We just have so much more to know about what’s going on in the night sky.”

Ted Stankowich, an evolutionary ecologist at California State University Long Beach, says that most research on animal warning communication targets visual signals, but the new findings show the need to consider potential warning signals that are based on sound or smell. In some species, these may be undetectable to human senses.

Gough thinks it would be fascinating to see how widespread the ultrasonic clicking is among the world’s roughly 3,000 species of tiger beetles. Doing so may allow researchers to compare the timing of the evolutionary origins of these acoustic defenses with the evolution of the first echolocating bats tens of millions of years ago.