Sonar Sabotage

Picture yourself standing quietly in a woodland clearing. It is twilight. The air silent and still. You catch a glimpse of something above you. A tiny dark shape zips past. Then another. In minutes the air above you is filled with movement. Bats. The virtuosos of the night sky, emerging from their roosts and executing incredible manoeuvres as they snatch insects on the wing a few metres above your head.

Bats achieve this mastery by echolocation, a biological sonar, bouncing ultrasonic calls around and listening for the rebounding echoes. It allows them to operate in total darkness, expertly navigating the night sky. They can pinpoint an insect in flight, accurately track it, and finally swoop in for the kill, all in light conditions that would see a human stumbling and groping fresh air, near blind.

We have long known about bat’s nightly hunting forays and the wonders of echolocation. But having this secret weapon doesn’t make them unstoppable. As is the case in the natural world, effective predation techniques usually are faced with a response from the prey. A classic arms race. The victims of bats have developed a few tricks of their own. Some moths flutter like butterflies, their flight pattern itself erratic, unpredictable and hard to track. Some have flashy displays, aiming to startle bats as they close in. Others have superb ultrasonic hearing, allowing them to make an early escape from any rapidly approaching danger, while some rely on their ability to outmanoeuvre winged predators.

Tiger moths (Arctiidae) have developed a wonderful and unique defensive strategy: acoustic countermeasures. When threatened, a tiger moth has the ability to fire off sounds of their own. These clicks act as a jamming signal, confusing chasing bats by overlapping sounds and affecting their ability to determine distances and pinpoint the moth.

Researchers Aaron Corcoran and William Conner, from the Wake Forest University, provided this evidence in a 2009 study, recording that tiger moths employing the strategy survived the attacks of bats time and time again. Even when the tiger moths found themselves tethered to a tree stump, unable to escape, their clicks alone were still enough to make the bats frequently miss their final strike.

While this research was taking place, something unusual happened that would shape the team’s future studies: in a stroke of serendipity, Corcoran recorded a new, mysterious bat call. “The unique acoustic structure of the sound, combined with the fact that I only ever recorded it when another bat was making the sounds it used to capture insects made me immediately wonder if it was a jamming signal,” says Corcoran. “It took me the next five years to answer that question.”

Mexican free-tailed bats (Tadarida brasiliensis) are abundant throughout the Americas. They form huge groups, up to an estimated 1.5 million in number. A group of this size can take hours each evening before its last members and stragglers finally leave the cave in which they roost. They fill the skies with their bodies, beating wings, and importantly, their calls. In such a huge swarm, echolocation can become a tricky proposition. The species is thought to have at least 15 separate social calls to communicate to their airborne neighbours, vocalisations ranging from collision avoidance to warning off a patch of territory. Corcoran and Conner discovered a new call to add to the free-tailed bats’ impressive vocal repertoire, and this new call appears to be for strictly nefarious purposes.

Free-tailed bats emerge en masse from their roost
Free-tailed bats emerge en masse from their roost. Credit: USFWS / Ann Froschauer

In swarms of over a million tiny bodies, competition for food is exceptionally fierce. There are so many mouths to feed, and only so many insects to go around. The competition intensifies, rivals find themselves vying for the same moth or flying insect. The situation get heated. And then, as one bat takes aim, its call changing to a rapid-fire buzz to pinpoint the prey for a final strike, it gets hit by something else, something unexpected. A flurry of sounds from another direction, buffeting the bat’s own final killer buzz, disorienting it and sending it veering away, off course. The bat misses its target. The saboteur and origin of the interfering sounds, another Mexican free-tailed bat, does not. It swoops in to snatch the prey and claim its prize.

Corcoran and Conner have named this vocalisation sinFM. It is a rapid raising and lowering of the pitch of their call, repeated a dozen times in bursts of less than a tenth of a second. Was sinFM merely another form of communication, perhaps a hands-off warning to a nearby bat competing for the same food? The team had a theory it was not, and was in a way similar to the jamming they had witnessed tiger moths so effectively use. To properly test if the call was doing what they suspected, in 2014 Corcoran and Conner set up experiments at sites in New Mexico and Arizona, US. The first part of the testing involved the use of an array of ultrasonic microphones and highly-sensitive cameras. Using the kit, they were able to record the foraging behaviour of Mexican free-tailed bats and their competitive interactions, and reconstruct flight paths by using their emitted sounds.

They found that when Mexican free-tailed bats were hit with sonar interference from another bat, they almost always missed their targets. It was a fascinating finding, and one that was further backed up by the second experimental phase.

This time, an unfortunate selection of moths with no form of acoustic defence once again found themselves tethered. They were suspended from ultra-thin fishing line, hanging helpless from a street light. An irresistible lure for wild bats to detect and home in on. For the bats, there was a significant obstacle, however. The scientists were poised, armed with different ultrasonic sounds and a speaker with which to play them in the vicinity.

A simple playback of regular buzzes resulted in a doomed moth and a successful capture on around 70% of trials. When sinFM was unleashed, though, everything changed. Time and time again, a hunting bat hit with the disruptive interference of sinFM missed their dangling, helpless prey. Their final success rate when sinFM was played plummeted to below 20%, a startling drop.

Perhaps even more fascinating is the timing involved: sinFM was clearly incredibly effective, but it had important limitations. As mentioned previously, when nearing their target, the call of a bat changes. It becomes a rapid-fire buzz, a last second burst that pinpoints the exact location of their prey in the moments before plucking it from the air. During that buzz, and only then, is sinFM effective. The interference only hampers the bat’s hunting when overlapping with that final, vitally-important buzz. Saboteurs not only have a stunning tactic to employ when the competition is fierce and the going gets tough, it is clear they also need to have impeccable timing to jam their rival’s sonar and swoop in to claim the prize.

“There were several details that surprised me along the way,” says Corcoran. “For example, I was shocked to discover that bats frequently reverse their roles in encounters. That is, the bat getting jammed will often turn the tables and jam the jammer.​”

While, so far, these impressive acts of sonar sabotage have only been recorded in Mexican free-tailed bats, the findings potentially open up a world of subtle or not-so-subtle same-species saboteurs. Other bat species may use the same devious strategy.  Even other animals that use echolocation, such as dolphins, may have the potential to do the same. Though perhaps with the remarkable feats of coordination and teamwork seen in dolphins, sabotaging a pod member’s feeding efforts is unlikely.

“This research changes our understanding of the possible ways animals compete with each other for food,” Corcoran concludes.

As if the high-speed manoeuvres and deadly strikes based on echolocation were not impressive enough alone, consider this: next time you find yourself catching glimpses of bats briefly silhouetted in a darkening sky, mere metres above your head there may be dramatic aerial contests and extraordinary acoustic warfare as bats perform stunning feats of sonar sabotage.

Sonar Sabotage was originally published in the December 2014 issue of Biosphere magazine. Subscribe to it here for the latest research from the natural world and stunning photography:


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