On warm, still summer evenings in Gainesville, Florida, a motley crowd gathers in front of two large, strange wooden structures, apparently nothing more than roofs on tall stilts. Affixed to the front of each roof is a black wooden silhouette of a bat. An elderly couple support each other as they walk towards the benches overlooking the odd houses. A young boy enacts a zombie drama for his ever-patient parents as they wait for the sun to set. Teenage girls run to join the group just as darkness starts to fall, hopping onto the fences that stop us from getting too close to the houses. To an uninitiated observer, the growing throng must seem thoroughly mystifying.
But after the sun sets, you see what all the fuss is about: the spectacle of hundreds of thousands of Mexican free-tailed bats swarming out of their daytime shelters and flying off into the night. As they first trickle and then tumble endlessly out of their houses, you can’t help but be impressed by their sheer numbers. You also can’t help smelling their pungent guano, and hearing their busy clicking chatter. But their most impressive sensory ability — echolocation — is one we humans can’t even perceive.
Insect-eating bats have evolved a range of sophisticated mechanisms by which they ‘see’ with sound. By emitting high-frequency sounds and then listening for the reflection off objects they want to land on or insects they want to eat, bats can create a fairly precise mental image of the world around them. It’s a bit like walking around at night with a torch, except that the torch is inside your eyes.
As the bats swoop out of the bat-houses, expertly avoiding trees and hawks and scores of people standing below while picking tiny insects out of the air, I start to wonder about how noisy this environment might be in the frequencies beyond my hearing. Surely, with hundreds of thousands of bats trying to navigate through it, even the sky can seem crowded. Actually, the situation is even worse than I imagined. Beyond simply dodging other travellers in the aerial equivalent of a train station platform, these Mexican free-tailed bats must cope with intentional sabotage from their swarm-mates.
The sabotage occurs precisely at the moment before a bat captures an insect, a time when its signal is most vulnerable to being jammed. To pinpoint the location of its prey, a bat emits a series of calls in rapid succession, known as a ‘feeding buzz.’ Other bats are especially alert to the feeding buzzes of their competitors, and will emit a signal that overlaps almost completely, in both timing and frequency. This interception confuses the bat enough that it almost always misses its prey, leaving the other bats a chance to catch the insect instead.
Even more interestingly, this sort of interference doesn’t come only from other bats — bats can sometimes suffer the indignity of jamming at the hands of their insect prey as well. In particular, the gloriously patterned red-and-yellow tiger moth is known to emit an ultrasonic clicking in response to the signals of certain bats. Again, the counter-clicking seems to confuse the bat’s ability to locate the insect, much like shining a light in your eyes just as you’re about to reach for your spoon and fork may well prevent you from eating. Clicking at just the right time can save the tiger moth’s life.
In the forests not far from the bat-houses lives a formidable insect that depends on its sense of hearing as much as the bats and moths do. For the Megarhyssa wasp, however, hearing allows it not to feed or avoid being fed on, but to reproduce successfully. Female Megarhyssa wasps lay their eggs on the larvae of wood-boring insects. As their name suggests, wood-boring insect larvae live inside wood. Looking at the Megarhyssa wasp, I struggled to imagine how she could locate a wood-boring insect larva through many centimetres of wood, let alone find a way in which to deposit her eggs onto it.
And yet she does. By sensing the subtle vibrations of a wood-boring beetle larva as it chews through wood, these wasps can pick out larvae that are at exactly the depth to which their ovipositors will extend. She then positions herself carefully above the larvae, and lifts her abdomen comically high in the air to be able to position her ovipositor, the tubes through which she lays her eggs, perfectly perpendicular to the wood’s surface. By exuding an enzyme that dissolves through wood, the wasp’s delicate ovipositor painstakingly makes its way through solid wood. Watching a Megarhyssa wasp lay her eggs is as enthralling as watching the enormous aggregation of bats flying into the night, but I wish I could hear what they hear too.
( Ambika Kamath studies organismic evolutionary biology at Harvard University)