In a North Carolina laboratory, a live moth was clamped tight in a box with a microphone and made to panic. Through the panic, its powers of prediction were probed.
The moth, a species of tiger moth called Bertholdia trigona, isn’t psychic. Instead, the moths’ hearing is the key. It’s one of two weapons it uses to stymy a deadly predator: the big brown bat. The panicking, if moths can truly be said to panic, may even tell us something more broadly about how, and when, animals respond to the threat of predators.
Tiger moths have a peculiar response to the presence of bats, and B.trigona especially. When they hear them approaching, using ears just behind their wings, the moths start making ultrasonic clicks. The clicks—the reason for the microphone in the box— are made by an organ on their sides called a tymbal. For some species of tiger moth, the clicks act as a warning, a message alerting bats that they taste icky or are poisonous: best find something else to munch on. Others species click to pretend they’re the same horrid tasting fair, when in fact—in bat terms—they may be delicious. It’s a cheat, but one which can keep them alive. But that’s not why B.trigona clicks.
B.trigona’s clicks are far more sophisticated than a warning. As a bat readies itself to snatch B.trigona from the air, it’s hit by the moth’s secret weapon: a burst of ultrasonic clicks, coming at around 4,500 per second. The clicks are just the right frequency to screw up the bats ability to judge their target’s distance, blurring its ‘acoustic image’. This is the only known example of an animal that actively jams its attacker’s sonar.
It’s impressive. But at what point does B.trigona decide it’s time to deploy its countermeasures? How does any animal decide just when it’s time to respond to the threat of a predator? To run, to fight, or to jam sonar?
When danger reared its ugly head, he bravely turned his tail and fled….
The first step is detection. As the tiger moth goes about its business, doing moth stuff—like creating streetlight orbiting moth solar systems or flying needlessly into the open mouths of unsuspecting cyclists*—it listens for the shrieks of bats. But these moths don’t high-tail it the second a bat is nearby; instead they appear to do a kind of risk assessment.
Generally, if any animal is too skittish and flees at the first sign of a predator, it may live another day, but at a cost. If you spend all your time running away, not only do you expend valuable energy needlessly, you also have less time to eat, graze, mate and forage. This trade-off ultimately affects the animal’s fitness to survive and reproduce more than its peers.
The distance at which a prey species decides a threat is too high and scarpers is called the Flight Initiation Distance, or FID. Think of approaching a bird or squirrel in the park. At first they stay put—with an eye on you—as you approach (maybe you want to get a better look or have velvet dreams of telling stories about that time you petting a wild squirrel…). But then, you’re too close for comfort, they whizz off in a cloud of acorns or feathers. The threat you represent has grown too high to bear, you’ve exceeded their FID, and so they get the hell out of there.
But distance isn’t necessarily the only factor that determines when something runs away. For any animal there may be a number of factors that may feed in to the FID; how close is refuge? How experienced is the animal? How fast is that predator moving? It may not be a straight line between increasing threat and distance that causes flight, or some defensive reaction. Aaron Corcoran and colleagues at the Wake Forest University, Department of Biology, in North Carolina had a hunch that an animal may choose to flee, or react, based on the stage of the attack.
The risk to a prey animal would rise sharply between detection and when a predator commits to an attack, and the prey will likely respond accordingly. For example, a predator that spotted you but continues walking at right angles to you in the distance is far less of a threat than one at the same distance galloping towards you, eyes fixed and salivating.
So to find out how their hypothesis holds up, Corcoran and colleagues gently clamped a moth into a sound chamber, with a microphone, and panicked it with simulated bat sounds.
Banshees in the night
Bats are masters of the dark hunt. They do it they use a special talent: echolocation. They shriek like a banshee in the night, the shrieks coming in pulses. And they can be loud—up to 120 dB—loud enough to damage human hearing, were they not too high a frequency for us to hear. To protect their own hearing, muscles contract in the bats middle ear as it calls, separating and muting bony amplifiers—the hammer, anvil and stirrup. The muscles relax again as they listen for the returning sound; by measuring how long the echo takes to return, they can home in on their prey’s distance and direction.
The hunt moves in stages. Shrieks come 1-12 times a second as a bat scans its environment. It spots a potential victim and gradually increases the rate of calls, gathering more information, assessing the target. It begins an approach, directing its sonar at the target, calls coming faster still, rapidly updating its ‘acoustic image’. It will then ‘lock on’ to the target, working out the distance more precisely. There’s a final ID of the target, and the decision to commit or break off the attack is made. If it commits, it switches to ‘high-def’, ramping up calls to 160 times a second: the ‘terminal buzz.’ The final flight path adjustments are made, and the target snatched out of thin air.
Hunting in this way makes bats a formidable foe for any night flying insect. But while echolocation lets bats hunt in the dark, for those with the ability to hear its shrieks they may as well be announcing their intentions with… well… a megaphone.
Moth in a box
Corcoran, and colleagues, decided to use B.Trigona to find out at what stage of a bat’s attack the moths click; a kind of moth FID. Once the moths were safely clamped still, they played them clips of ultrasonic sound, mimicking bats—like the big brown bat—that hunt them. Each clip was a series of 2ms pulses followed by a short gap, repeated for ten seconds. Different clips had different size gaps between pulses, some closer together, some further apart, spoofing the different stages of a bats searching, approaching, and terminal buzz. The clips also gradually got louder as they were played, mimicking an approaching bat.
Sound pulses of a speed and intensity of a bat searching caused little reaction from the moths, even when they got louder. They didn’t think they’d been spotted. But when pulses mimicking a bat targeting or on the attack run were played, the moth rapidly increased its rate of clicking. The moths were working out when and if they’d been targeted, or were under attack, and responding accordingly. The bats’ very hunting strategy gave the game away, and only when a bat had ‘locked on’ did the moth react with its sonar jamming.
A tethered moth in the lab, strange though it is, is one thing, but is it representative of moths in the wild?
To find out, the team headed into the field. They used ultrasonic microphones to record the shrieks of the bats and the clicks of the moths, as well as high speed infrared cameras, letting them film in the dark. The cameras were set up at a variety of different angles so they could later reconstruct the 3D flight patterns of bats and moths.
As in the lab, they found that shortly after a bat found and targeted a moth, the number of clicks shot up. But there was more. When a bat selects a target, it directs its sonar towards it; this increases the sound intensity on the target. When moths heard targeting and attacking pulses, but no increase in intensity, they could tell someone else was the target, so didn’t bother with countermeasures. They knew a real from a ‘fake’ threat, and only responded when they were threatened.
The moths really could predict a bats intention. This suggests that when a prey species can detect changes in a predators attack stage, and even to whom the attack is directed, the actual distance may not be the deciding factor for taking evasive action.
B.trigona uses an impressive set of strategies to live another day. By predicting a bats intentions and jamming its sonar, coupled with some emergency moth acrobatics (no pun intended…), like spiralling crazily towards the ground, around 93% of bat attacks end in failure and B.trigona fluttering on happily, in search of the next cyclist’s mouth….
*OK, so yes, maybe I have unresolved issues with moths from my cycling days…
P.S Here’s a video of a big brown bat hunting a B.trigona, and losing.
Corcoran, A., Wagner, R., & Conner, W. (2013). Optimal Predator Risk Assessment by the Sonar-Jamming Arctiine Moth Bertholdia trigona PLoS ONE, 8 (5) DOI: 10.1371/journal.pone.0063609
Big Brown Bat with scary teefs by Matt Reinbold.
Bertholdia trigona from Wikipedia
Clown Face Tiger Moth by Andreas Kay
Scaredy Cat squirrel by Porsupah Ree