The argument above your head

It is called Bertholdia trigona. When a bat closes on it, the moth produces a burst of ultrasound so dense it is less a sound than a wall, spread across frequencies from about 25 to 89 kilohertz. The bat, mid-attack, misses.

For a while nobody could say why it missed. There were three reasonable explanations, and the distance between them is the distance between a curiosity and one of the odder facts in animal biology. The clicks might simply startle the bat. They might be a warning label, the acoustic version of a monarch's orange, announcing that this one tastes bad. Or the moth might be jamming the sonar outright, corrupting the signal the bat is using to find it.

In 2009, Aaron Corcoran, Jesse Barber and William Conner put the question in a room. They flew big brown bats, Eptesicus fuscus, against the moths night after night and watched what happened as the bats gained experience. A startle wears off, and these did not. A warning is a claim about flavor, and it fails the moment you show that the predator will happily eat the animal, which is exactly what the bats did whenever the moth could not click. What was left was jamming. The paper ran in Science that July under a title with nothing extra in it: "Tiger Moth Jams Bat Sonar."

Three years later the same work went outside, into open air, with wild bats and no room. They compared moths that could click against moths whose sound-producing organs had been silenced. The bats caught the silenced moths 10.4 times more often, roughly seven attempts in ten ended in a capture. Against a clicking moth, fewer than one in ten. And when a moth combined the clicking with a dive, the bats got none at all. Twenty-four attempts, zero captures.

The buzz

To understand what is being jammed, you need to know what a hunting bat actually sounds like.

A bat searching open air calls at a steady, unhurried rate. When it detects something, the rate climbs. In the last fraction of a second before contact it produces what researchers call the terminal buzz: calls arriving so fast that the returning echoes fuse into something like a continuous read on where the insect is and where it is going next. Well over a hundred calls per second, produced by muscles in the larynx working at the same rate.

That buzz is the bat's whole advantage. It is also, for those few hundred milliseconds, the only thing between the bat and an empty mouth. Which is precisely the window the moth aims at.

The human ear gives out somewhere around twenty kilohertz, and rather earlier than that for most adults. The search calls, the buzz, the moth's four thousand five hundred clicks: all of it sits above that line.

How common this is

Bertholdia trigona is a southwestern species. You are not going to hear it in a Pennsylvania backyard, partly because it is not there and mostly because you could not hear it if it were.

But the behavior is not a one-off. In 2022, a team led by Jesse Barber, the same Barber, sampled moths worldwide and found that roughly one in five produces anti-bat ultrasound of some kind. Sonar jamming specifically has evolved at least six separate times, in lineages that are not closely related, which is the signature of a solution good enough that evolution keeps arriving at it independently. Beyond the ones that make noise, more than half of the world's roughly 140,000 moth species have ears sensitive to ultrasound, tuned to the frequencies a bat hunts with.

Moths, as a group, are not passive. They are one half of an arms race with a very old start date.

August

Bats in North America are born from May into early July, and the young are flying within a few weeks. First flights are recorded from the middle of July. So on an evening this month, the air over a field or a pond or a lit parking lot holds the summer's adults plus an entire generation out for roughly the first time, learning to do this.

They are hunting moths. Some of those moths are answering back. None of it reaches you.You could stand underneath the whole thing and call it a quiet evening.

Go out anyway, when the light goes. You will see the bats, because their flight path stays legible even when their voices are not. Those sudden turns are not random. Each one is a decision made on information arriving faster than you can follow.