Moths Produce Ultrasonic Defensive Sounds to Fend Off Bat Predators

Scientists from Boise State University and elsewhere have tested 252 genera from most families of large-bodied moths. Their results show that ultrasound-producing moths are far more widespread than previously thought, adding three new sound-producing organs, eight new subfamilies and potentially thousands of species to the roster.

A molecular phylogeny of Lepidoptera indicating antipredator ultrasound production across the order. Image credit: Barber et al., doi: 10.1073/pnas.2117485119.

Bats pierce the shadows with ultrasonic pulses that enable them to construct an auditory map of their surroundings, which is bad news for moths, one of their favorite foods.

However, not all moths are defenseless prey. Some emit ultrasonic signals of their own that startle bats into breaking off pursuit.

Many moths that contain bitter toxins avoid capture altogether by producing distinct ultrasounds that alert bats to their foul taste. Others conceal themselves in a shroud of sonar-jamming static that makes them hard to find with bat echolocation.

While effective, these types of auditory defense mechanisms in moths are considered relatively rare, known only in tiger moths, hawk moths and a single species of geometrid moth.

“It’s not just tiger moths and hawk moths that are doing this,” said Dr. Akito Kawahara, a researcher at the Florida Museum of Natural History.

“There are tons of moths that create ultrasonic sounds, and we hardly know anything about them.”

Anti-bat ultrasound-producing structures in Lepidoptera.  Image credit: Barber et al., doi:10.1073/pnas.2117485119.

Anti-bat ultrasound-producing structures in Lepidoptera. Image credit: Barber et al., doi: 10.1073/pnas.2117485119.

dr Kawahara and colleagues were interested in understanding how these sounds might converge between moth species.

In the same way that non-toxic butterflies mimic the colors and wing patterns of less savory species, moths that lack the benefit of built-in toxins can copy the pitch and timbre of genuinely unappetizing relatives.

After collecting and studying thousands of moths for over a decade in Ecuador, French Guiana, Mozambique and Malaysian Borneo, the researchers spent a final two weeks 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.

They found that moth species don’t act like individual composers, each with their own unique calling card and distinct style. Instead, a small number of moths derive their own scores, which other moths seem to replicate in complex acoustic mimicry rings.

“More work is needed to uncover the exact nature of these sounds, but we suspect the trailblazing moths at the center of these rings are likely noxious, while the copycats at the fringes are merely false advertisers,” said Professor Jesse Barber, a researcher at Boise State University.

“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.”

These ultrasonic warning systems seem so useful for evading bats that they’ve evolved independently in moths on multiple separate occasions.

In each case, moths transformed a different part of their bodies into finely tuned organic instruments.

“Tiger moths have structures called tymbals, which buckle inward and outward,” Dr. Kawahara said.

“Some of them use structures on their wings, some use their abdomens, while others use modified genitals!”

Several of the various clickers and scrapers moths use as a bat repellant are described in the study for the first time.

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.

“These mimicry complexes are likely not just limited to moths,” Dr. Kawahara said.

“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.”

The teams work was published June 15, 2022 in the Proceedings of the National Academy of Sciences.

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Jesse R Barber et al. 2022. Anti-bat ultrasound production in moths is globally and phylogenetically widespread. PNAS 119 (25): e2117485119; doi: 10.1073/pnas.2117485119

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