Moths use ultrasonic back-chat to fend off bat attacks

Most species of bats are nocturnal hunters and use high-frequency sounds to locate their prey in the darkness. Many moths are also nocturnal foragers and are favorite food items for hungry bats. However, recent research shows that moths are far from defenseless targets of successful bat echolocation. Some emit their own ultrasonic signals that may startle a bat into giving up the chase, convince the bat that the moth is toxic, or may even surround the moth in a shroud of sonar-jamming static.

Until now, anti-bat ultrasound production has been considered fairly rare in moths. It was recorded in tiger moths, hawk moths and a single species of geometrid moth. However, the results of a decade-long moth survey that involved collecting and studying thousands of moths in Ecuador, French Guiana, Mozambique and Malaysian Borneo have now revealed that such auditory defense mechanisms are far more common than previously thought. In fact, the night can be filled with so much bat chat and moth back-chat, that it would be quite overwhelming if we could hear any of it. 

“It’s not just tiger moths and hawk moths that are doing this. There are tons of moths that create ultrasonic sounds, and we hardly know anything about them,” said study senior author Akito Kawahara, a curator at the Florida Museum of Natural History’s McGuire Center for Lepidoptera & Biodiversity.

The findings of the study, published in the Proceedings of the National Academy of Sciences, identified no less than three new types of sound-producing organs in moths, and added eight new subfamilies and potentially thousands of species to the list of moths that use ultrasonic signals. The authors estimate that around 20 percent of moth species use ultrasonic signals to defend themselves against becoming bat fodder.

The ultrasonic emissions are so successful at thwarting bat attacks that they have evolved independently in moths many times over. In each instance, moths have modified a different part of the body to become a finely tuned ultrasonic instrument.

“Tiger moths have structures called tymbals, which buckle inward and outward,” Kawahara said. “Some of them [the moth species] use structures on their wings, some use their abdomens, while others use modified genitals!”

The authors describe several of the newly identified clickers and scrapers that moths use to produce bat-repellant ultrasounds. This includes a species of calpine moth that rubs together overlapping abdominal scales, similar to the way crickets make their characteristic chirping sounds. Another species, in a group called the snout moths, makes noise by using a structure resembling a guitar pick between its wings, which is strummed against when the moth is in flight.

The researchers undertook a final two-week project in Ecuador, where they recorded the alarm calls of every moth they could catch. Afterwards, they analyzed these recordings with the help of a theoretical physicist and a machine learning algorithm that scrutinized each note, looking for similarities.

The results showed that moth species don’t all make up their own ultrasonic melodies; instead, they tend to copy the calls of other species, especially those that are toxic or bad-tasting to bats. Some moths are not good to eat and they warn bats not to waste their time, by making ultrasonic calls when they detect a bat. Bats soon associate these warning calls with noxious-tasting moths and do not bother to catch them. The researchers propose that a surprising number of non-toxic moth species have developed acoustic mimicry and protect themselves by calling just like the toxic species do. 

Study lead author Jesse Barber is a biology professor at Boise State University. He said that more work is needed to uncover the exact nature of these sounds, but he suspects the trailblazing moths at the center of these acoustic mimicry rings are likely noxious, while the copycats at the fringes are merely false advertisers.

“Moths and butterflies are collectively one of the most diverse groups on the planet, containing one of every 10 named animals. If these results pan out, it will likely be the largest set of mimicry complexes on Earth,” noted Professor Barber.

Anti-bat signaling isn’t just restricted to moths. Some tiger beetles can also create defensive clicks at bats by beating their wings against their protective casings. Other insect groups, such as katydids, crickets and mantids can hear incoming bats, and Barber notes the possibility that some may have the yet-unknown ability to communicate with their pursuers. But with roughly 40 percent of insect species currently at risk of extinction, at a rate that eclipses the ability to researchers to discover and name them, he warns of the very real possibility that this ultrasonic symphony may fall silent before we’ve had a chance to hear it or learn what it means.

“These mimicry complexes are likely not just limited to moths,” said Professor Barber. “The whole tapestry of nocturnal insect life is probably involved, but the chance to understand the natural world is going away. So many lineages are going extinct that we’re likely in the last golden age of biology. We can still understand how life unfolded, if we do it now.”

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By Alison Bosman, Earth.com Staff Writer