Every summer evening 1.5 million bats emerge from underneath the Congress Avenue Bridge in Austin, Texas, on a quest for their favorite meals of mosquitoes and other insects. To track their tiny flying prey, the bats emit high-pitched sounds that deflect from an insect back to bat’s large ears. The information from this process of echolocation tells the flying mammals the precise path of their fast-moving food.
But how does any single bat in a swarm of thousands know that the ping it registers is not some other bat’s echo? Navigating this potential interference, known as sonar jamming, is not only the province of bats, however. Dolphins and other animals that rely on echolocation must also find ways around the maze of sound waves ricocheting around them.
People want to figure out how bats and dolphins do it because these animals are what Laura Kloepper, an assistant professor of biology at Saint Mary’s College in Indiana, calls “‘bio-inspiration’—helping us to find technological solutions to problems in our everyday lives.” The built-in biology of an echolocating bat holds secrets that would help human researchers develop better “active sensing” devices that mimic what bats do.
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Originally, people used active sensing to defend coastal waters via submarines or to sound the ocean depths. But now, Kloepper says, we turn to it increasingly for more commonplace needs like robotic vacuuming or self-driving cars. The rub, she says, is that “bats and dolphins are still a mystery to us.” The more researchers can find out about how they echolocate, the more we can reap technological advances. Among these potential developments is having sensors that can distinguish interfering echoes from those that matter.
A Harris’s hawk equipped with a camera and mic. Credit: Laura Kloepper
Kloepper made clear the power of bats’ active sensing abilities in a presentation at the 176th meeting of the Acoustical Society of America that compared bats against the formidable echolocating capabilities of dolphins. As a bat researcher, Kloepper described herself as emphatically being on “Team Bat,” although she relied on dolphins to make her point. Her work, she says, is the first such study of how dolphins navigate around sonar jamming.
In her talk, Kloepper told of how her research group bombarded a pair of dolphins with synthetic dolphin-like clicks to try to confound them as they chose between two options. Her team challenged the marine mammals by pulling tricks such as adjusting the angle of speakers emitting the sounds to confuse them as they homed in on their targets of choice. What the humans learned was dolphins use two possible strategies to block out the nonsense noise. They either change the frequency of their calls—to a higher or lower pitch—or they change the timing. Either tweak gives a dolphin a personal call, a signature it can recognize on the echo.
Kloepper has, of course, also studied bats. For this research she uses what she calls a “biological drone,” a trained Harris’s hawk named Belle. Adorned with a tiny camera and microphone, Belle wings into the midst of bat swarms and records their many calls for the sake of science. Kloepper says the reason she uses hawks, besides the fact “hawks are supercool,” is that sending a regular drone with spinning propellers into a dense bat swarms “wouldn’t be ideal.” Her team does use drones during night swarms, when the bats fly at greater distances from one another and are less likely to collide with the machine.
A dolphin’s click is about a 20th the duration of a bat’s call. This difference, Kloepper says, leaves bats better able to make subtle and layered tweaks to their calls. Whereas a dolphin might change tempo or pitch, a bat has a slightly more nuanced repertoire to deal with the jamming. “Dolphins make impulsive signals that sound like clicks—sort of if you snapped your fingers together,” Kloepper says—whereas bat calls are more like human whistling. “Sure, we could slightly change some of the characteristics of our fingers snapping,” she notes, “but you could make your whistle go up or down in pitch or even jump between pitches, and you can control how long your whistle is.” Bats show a similar level of fine control over their echolocation, she says.
The result is bats not only can detect and track moving prey but also can sense the textures of different objects, says Erin Gillam, an associate professor of biological sciences at North Dakota State University who was not involved in the dolphin work. She also sides with Team Bat because the animals “are the coolest,” adding, “Dolphins are capable of some flexibility, but not nearly as much as bats.”
Despite an apparent bias from Team Bat, “my results really don’t prove the superiority of bats,” Kloepper says. It’s just that “bats have much longer calls and are known to echolocate in massive groups, which is why I'm arguing that when it comes to avoiding jamming, bats win.”
But the mystery of how bats pull off this jamming evasion remains. Researchers’ next step in solving this challenge is to zero in on individual bats and their calls. Kloepper foresees “new electronic hardware to go on our drone and hawk that will allow us to really home in on which bat is making which call when it’s in the middle of this massive group.”
The dolphins are not left out either. Kloepper plans to expand on her work by challenging the chatty marine mammals with even more interference to see if they can still echolocate.