Darkness evokes fear of hidden dangers and of menacing, supernatural forces. But our limited senses make it easy to miss a real nocturnal drama taking place above our heads: a battle between bats and moths. For insectivorous bats, it’s a battle to eat. For moths, the battle is to avoid being eaten.
Since the 1940s, scientists have known that bats navigate obstacles through echolocation — a term coined by animal behavior researcher Donald Griffin, whose early studies with neuroscientist Robert Galambos took advantage of a new device that allowed eavesdropping on sounds above the human hearing range. His groundbreaking experiments — setting obstacle courses for bats and monitoring the creatures while they flew — revealed what some scientists of the time could scarcely believe: Bats navigate the world by emitting sounds at far higher frequencies than people can hear, and map their environment through the bounce-back of sound.
In the decades since, researchers have amassed myriad more details about the echoic understanding bats have of their environment, and how they use echolocation not only to navigate but also to hunt. “They experience the world in such a different way than we do,” says evolutionary biologist Juliette Rubin, a PhD student at the University of Florida. The moths that bats chase, for their part, have evolved a broad array of biological tricks to avoid becoming treats for the bats.
From warning cries and jamming bat signals to creating false targets and sound-absorbing cloaking devices, here are some of the ways that moths have fought back through the 65-million-year arms race between them and their furry, flying predators.
How moths trick bats with clicks
By the 1960s, scientists had realized that some moths could produce ultrasonic clicking sounds, seemingly in response to hearing bat signals. Noise-making moths were using tiny blisters of cuticle called tymbal organs on their thoraxes: When the moths contract their muscles, these ridged organs buckle, producing clicks similar to how an empty soda can clicks when you squeeze it. At first, the function of moth clicks was unclear. Were they used to startle bats, to interfere with echolocation, or to warn about an unpleasant taste? Many of the clicking moths were tiger moths, a group including many that taste nasty to bats because their bodies are filled with noxious chemicals acquired from their food.
To find out, animal behaviorist William Conner of Wake Forest University and his team visualized bat-tiger moth interactions with high-speed infrared videography. Inside a special sound-absorbent testing area called a flight room, they dangled click-producing tiger moths on thin lines, then allowed free-flying eastern red bats and big brown bats to swoop in and check out the moths. In a series of trials, Conner and then-graduate student Jesse Barber, now at Boise State University, found that bats quickly learned to associate moth-clicking with a bad taste, and thereafter avoid eating them.
Not only that: Other, perfectly tasty moth species, the scientists found, had evolved to avoid bat predation by mimicking the bad-tasting tiger moth’s “don’t eat me” clicks. A devious disguise.
Some species of tiger moths have different acoustic defenses. Physiological ecologist Aaron Corcoran, who did graduate studies with Conner and now runs a bat lab at the University of Colorado at Colorado Springs, discovered that certain tiger moths, upon hearing bat echolocation, could turn on a jamming signal. As the bat closes in, moths begin producing 4,500 clicks per second, throwing off bat ranging. With bats unable to discern target distance, moths could get away.
That many moths use sound for bat deterrence and befuddlement is now well-established. Says Corcoran, who coauthored an article with Conner on bat-moth coevolution in the 2012 Annual Review of Entomology: “We used to think that it was just a couple of groups of moths that made sounds in response to bats.” Not anymore.
Moth wings as decoys
Since 2012, “we’ve learned a lot,” says Akito Kawahara, Lepidoptera curator at the Florida Museum of Natural History and University of Florida, in Gainesville. One new vein of research sprang from the knowledge that not all moths can hear. Perhaps, scientists thought, these moths had other defenses against bats up their sleeves — ones to do with features of their body architecture.
Kawahara, for his part, decided to look at moth wing shape after admiring striking moths in the wild. “In the field in Borneo, this really amazing moth came to the light trap that we had out in the jungle,” he recalls. One undergraduate asked why the moth had such long tails, and “we didn’t actually know.” A scan of the scientific literature turned up nothing. So they set out to investigate.
Using high-speed infrared videography to test how long-tailed luna moths (Actias luna) fared in a flight room with big brown bats (Eptesicus fuscus), Barber, Kawahara and others found a strong clue. In over half of the trials, the spinning hindwing tails of tethered luna moths lured echolocating bat attacks away from their vulnerable bodies, toward non-essential appendages. Moths with intact tails had a 47 percent survival advantage over those with tails removed. The tails are twisted, so when the moth flies, they flutter. Kawahara suggests that they act as a decoy, masquerading as a little moth: a false acoustic target.
Wing shape and its influence on bat defense has been explored in other moths too. In silk moths, long tails have evolved independently multiple times, on several continents. To test the hunting behavior of 16 big brown bats, Rubin’s team used flight rooms with three species of tethered live silk moths with tails experimentally shortened or lengthened. “We found that as hindwing tails were elongated, the bats were increasingly being drawn towards those tail ends,” Rubin says.
The wing-tail ends have a cupped shape that, the scientists think, might send back strong echoes and cause the bat to preferentially attack there, allowing the moth to survive. The Kawahara and Barber labs are working on imaging those echoes to actively explore the hypothesis.
Sensory biologist Marc Holderied at the University of Bristol, UK, has found similar false acoustic targets in moth forewings. Using an apparatus called an acoustic tomograph, which builds a 3D image of the acoustic properties of a structure by examining how sound echoes from different angles, he tested how sound waves bounced off the wings of 10 moth species. Looking at the wing structure of silk moths more broadly, he found that half of the 72 species examined had ripples and folds in wingtip membranes that produced strong echoes.
For big moths, highly reflective structures in wing tips mean that when a bat strikes that strong echo-target, it may flip the moth or cause wing damage, but probably misses the moth body. The modern military uses similar acoustic decoys to defend against radar guided ground-to-air rockets and torpedoes, says Holderied, to try to thwart a direct hit when under attack. Moths had a multimillion-year head start on this defensive trick.
Fuzzy bodies, carpeted wings
Ever wondered why some moth bodies are so fuzzy? That’s a neat trick too. Analogous to visual camouflage, hairlike scales on a moth thorax are for stealth acoustic camouflage, like the fibrous sound absorbers found in homes, offices and concert halls.
Ideally, as a moth, you’d need to have the equivalent acoustic protection on a wing as on your body, “but you can’t fly if you put 2 millimeters of fur on the upper end of your wing,” Holderied says. That creates a conundrum. So evolution has delivered moth wings layered all over with tiny, light, thin scales of different shapes and sizes. “It’s beautiful,” says Holderied. Tuned to absorbing a multitude of different frequencies, moth wings cloak the bounce-back of sound with what Holderied calls a “perfect carpet.”
“There are lots of new surprises,” Conner says of all the recent revelations about moth defensive strategies. And in this hide-and-seek in the night skies, it looks like there are plenty more to come.