Photograph by Dietmar Nill / NaturePL
WORDS BY JASON P. DINH
Our new series on species interconnectedness traverses through a 60-million-year-old grudge match between bats and moths—one that’s led to a rich and diverse biological tapestry.
On a biodiverse sky island elevated from the barren Arizona desert, Dr. Aaron Corcoran flicks on powerful ultraviolet lights. “You set up a light here and you get almost clouds of insects,” he said. “And then the bats very quickly figure out, that’s where the party is.”
Corcoran, a biologist at the University of Colorado Colorado Springs, just set the stage for one of nature’s oldest wars: bats versus moths. What’s to follow is a battle featuring echolocation, chemical defense, sonar jamming, stealth pursuit, and acoustic illusions, all piling on in an earsplitting, ultrasonic din—earsplitting, at least, if you have ultrasonic microphones to tune in.
“All this stuff is happening every night in everybody’s backyard,” said Dr. Nick Dowdy, a biologist at the Milwaukee Public Museum. “We just can’t hear it.”
Dr. Akito Kawahara, an entomologist at the McGuire Center for Lepidoptera & Biodiversity at the Florida Museum of Natural History, said the bat-moth battle is global. “It’s really wherever there’s moths and wherever there’s bats, and that’s almost everywhere in the world.”
Bats hunt using echolocation. They emit intense clicks that are too high-pitched for the human ear to hear. These bounce off of targets and back towards the bat’s ears. Remarkably, they are born with an innate sense of the speed of sound, which means that they can estimate a target’s distance based on the time it takes for an echo to return. That allows them to home in on prey even on the darkest of nights.
For bats, moths are convenient prey because they fly and are active at night. Meanwhile, for moths, the worst possible outcome is to get eaten before they can mate. That intense evolutionary pressure for bats to eat and for moths to fly under the radar have joined the two groups at the hip.
Most experts agree that the battle began when bats arose roughly 60 to 65 million years ago, just after the dinosaurs went extinct. Since then, each animal’s evolutionary fate would depend on the other. When one group gained an edge, the other countered to stay abreast. Their age-old conflict, albeit gruesome and lethal, would paint a vibrant slate of remarkable adaptations—mystifying natural feats rising out of the ashes of combat.
The first step, though, was simple. “The key adaptation on the moth side is the ability to hear and respond to the bats,” Corcoran said.
Bat-moth showdowns occur in two phases—one at a distance, and one in close combat—and hearing is essential in the first. With sensitive enough hearing organs, a moth can detect an echolocation click from far away and escape before a bat has the time and capacity to perceive its own echo.
By the time bats arose, moths had already been around for nearly 240 million years, and they had already evolved hearing organs nine separate times. Today, Kawahara estimates that half of moth species have hearing organs, and they can be situated anywhere from their abdomens to the base of their wings.
Scientists are still figuring out how ancient moths used hearing before bats—maybe for mating, or perhaps to hear predators like rodents or lizards. But what’s clear is that since then, those “ears” have been co-opted and specialized to tune into bat sonar.
The greater wax moth (Galleria mellonella), for example, holds the world record for the highest hearing frequency range at 300 kHz, allowing them access to even the highest pitched bat clicks. Other moths, like the large yellow underwing (Noctua pronuba), have pedestrian hearing under normal circumstances, but when they get walloped by intense bat clicks, their ears actively tune upward to better detect higher frequencies. Some owlet moths (family Noctuidae) even have ears that are tuned to the frequencies used by their local bat community.
Evolution can only work with what it’s given, and many moths simply don’t have ears. Some of these earless species countered bat echolocation using a different strategy: acoustic camouflage. The Suraka silk moth (Antherina suraka), for example, covers its body and wings with sound-absorbing scales. Like acoustic panels in a recording studio, that invisibility cloak dampens returning echoes and acts as a stealth coating.
Although known examples are less common, bats have evolved to thwart their meddling prey, too. For example, at least one species called the barbastelle has evolved a stealth pursuit strategy. They produce echolocation clicks that are 100 times quieter than that of other similar bat species. That allows them to get within striking distance. And if they get close enough, the second phase of battle begins: close combat.
Their age-old conflict, albeit gruesome and lethal, would paint a vibrant slate of remarkable adaptations—mystifying natural feats rising out of the ashes of combat.
Large and maneuverable bats have the edge within arm’s length, but moths have taken great strides to avoid predation. Some moths, like tiger moths (family Arctiidae) have taken a simple-yet-effective strategy: Taste gross, and advertise it.
Tiger moths can either produce foul-tasting substances or sequester them from plants they eat as caterpillars. “It’s very, very disgusting, so [bats] just throw them up,” Kawahara said.
They can pair those toxins with an ultrasonic warning click—the acoustic analog to conspicuous coloration in poison dart frogs or monarch butterflies. And, like a kid reeling after a bite of bitter chocolate, it doesn’t take long for bats to learn that clicking moths are a no-go.
The strategy is so effective that 20% of moths produce anti-bat sounds, evolving at least 10 separate times. Moths even mimic each other’s sounds. When two toxic moths mimic each other, their warning calls further ingrain a bat’s avoidance response. Meanwhile, nontoxic moths can mimic toxic ones and freeload off their neighbors noxious taste. This chain of imitation, experts have argued, could be the most diverse mimicry ring on Earth.
Bats, then, aren’t simply a needle guiding each individual moth’s evolutionary thread; they are also the hand that braids those strands together.
Noxious clicks need not be long or complex. Just a chirp here or a buzz there is sufficient. But by accelerating the pace of their clicking, some species like the tiger moth Bertholdia trigona can take on a new function: sonar jamming.
In a classic experiment, Corcoran showed that sonar jamming is incredibly effective. He and his colleagues built a feeding chamber where bats hunted tethered, sonar-jamming moths. Even when moths were restrained, bats could only make contact 20% of the time. After they experimentally silenced the moths, though, bats made contact 100% of the time. “Just being off by a couple of centimeters could be enough for the moth to survive,” Corcoran said.
Hawkmoths and at least five other subfamilies of moths have independently evolved sonar jamming—in fact, just within hawkmoths, it’s evolved four different times. Unlike tiger moths, which generate sound by buckling a thin membrane on their bodies called a tymbal, hawkmoths generate anti-bat sound by scraping their genitals against their abdomens.
Scientists still aren’t certain how sonar jamming works, but Dowdy and his collaborators are working on a theory. To successfully jam a bat’s sonar, moths need to produce clicks that arrive 2 milliseconds or less before an echo does. That slightly early arrival tricks the bat into thinking the target is closer than it really is, and producing more clicks per second increases the odds that one will land in that critical window.
What is clear from the patterns of anti-bat sound production is that the evolutionary linkage didn’t happen just once. It happened over and over, around the world and across the moth evolutionary tree. “All moths everywhere were exposed to similar selective pressures at the same time.” Dowdy said. “[Anti-bat sounds] evolved 10, 15, 20 times, which is really amazing if you think about it.”
Each bat-moth juncture was a starting point of a creative project with infinite possibilities. Sometimes, as in the case of sound production, they converged on similar solutions that are effective and achievable. But other times, as in the case with earless, sound-absorbing moths, they frayed into the unknown and generated new biological possibilities. Perhaps there’s no better example of this than luna moths.
“We know so little. We oftentimes as human beings tend to focus on things that we can see or perceive directly, and we have to remember that the world is not necessarily designed for us.”
Luna moths (family Saturniidae) are tricksters. They rely on illusion and distraction to escape a bat’s deadly clutch.
Trailing behind a luna moth’s wings are winding, cupped tails. And recently, Kawahara, his colleague and bat guru Dr. Jesse Barber, and his former graduate student Dr. Juliette Rubin, discovered that the cupped tails produce a sensory illusion—one that’s evolved four independent times and tricks bats into attacking nonessential parts of their body.
That sleight of hand is surprisingly effective. In controlled feeding experiments, Bats captured tailed moths on just 35% of attempts. After researchers experimentally removed the tails, that percentage more than doubled to 81%.
Like sonar jamming, the researchers are still figuring out how it works, but the prevailing theory is that the tail functions as a decoy. “From an echoic standpoint, it appears like a small moth,” Kawahara said.
The tails pinwheel behind a moth’s body when they are in flight. That draws a bat’s attention and pursuit towards the moth’s tail end, which often gets damaged or lost entirely during an interaction but allows the rest of the moth to escape alive.
Missing a chunk of your wing might seem worrisome, but it doesn’t hamper flight, and because luna moths live just a week or two, it isn’t of paramount importance. “Getting hit [on the tail] by a bat once or twice is probably okay,” Kawahara said. “By the time it gets hit twice, it will probably have served its purpose, which for these male moths, is mating.”
One expert once called the bat-moth interaction a magic well that never runs dry: The more scientists learn, the more surprising discoveries come to light. Maybe that’s because their sensory world lies entirely outside the human experience, Kawahara said. “We know so little. We oftentimes as human beings tend to focus on things that we can see or perceive directly, and we have to remember that the world is not necessarily designed for us.”
It might not be the only magic well out there, either. There are millions of flying, nocturnal insects under predation pressure from bats. And as moths have shown so vividly, lethal conflict can be the engine that drives wonderous evolutionary innovation.
Crickets, katydids, scarab beetles, and praying mantises all have bat-sensitive ultrasonic hearing, for example. If, like moths, their evolutionary trajectories have tethered onto their echolocating predators, then they might be outfitted with countless other anti-bat trappings. “We have barely scratched the surface,” Corcoran said.
Leaping into an unknowable sensory realm drives Corcoran’s curiosity. He starts many of his research talks with Thomas Nagel’s famous philosophical question: What is it like to be a bat? “We can’t ultimately know,” he said. ”But I like to say that it sure is fun to try.”
Certainly, a moth can’t understand what it’s like to be a bat, either. But when the stakes are life or death and two species are so deeply intertwined, evolution can concoct remarkable workarounds. For moths, the endless years of lethal combat have weaved a rich and diverse biological tapestry laced with acoustic hijinx and illusion—all meant to hack into the mind of their nemeses, to know the unknowable, and to disrupt what it’s like to be a bat.
