Hungry Bats Jam Each Other's Sonar Signals

Mexican free-tailed bats emit specialized signals that jam the echolocation of other bats competing for insect prey. This composite image depicts two bats competing for prey and a visual rendering of the jamming call / Nickolay Hristov
Janet Fang 07 Nov 2014, 19:49

To locate food, bats make calls and listen for the echoes that return. Being able to echolocate has allowed them to become fierce aerial predators who hunt in complete darkness -- but this biological sonar is vulnerable to interference. When rival bats are on the same trail of delectable insects, a special jamming call can throw off the competition. The findings were published in Science this week. 

Aaron Corcoran and William Conner of Wake Forest University studied foraging tactics of Mexican free-tailed bats (Tadarida brasiliensis) in a series of field recordings and playback experiments. Up to a million of these bats live together in a single cave, and they emit at least 15 different types of social calls. The duo recorded audio and video of the bats at the Southwestern Research Station in Arizona and in a high school parking lot in New Mexico. They also created 3D reconstructions of their flight paths. 

To the right, the ultraviolet light tower attracts foraging bats, and bat sounds were recorded on two ultrasonic microphone arrays placed below the light. 

One of the sounds they captured, called the “feeding buzz,” is a fast call that gets faster as the bats hone in on the position of their insect prey during the final moments of pursuit. They also captured a previously unstudied call that bats only seem to make when nearby bats are doing their feeding buzz. 

Previous research showed that using different sound frequencies allows many bats to hunt in the same space -- jamming each other's signals was seemingly inadvertent. But something more devious was going on here. It was no accident: The jamming was adaptive. “The jamming signal has been designed by evolution to maximally disrupt the other bat's echolocation," Corcoran tells New Scientist. "For this type of jamming, the interfering sound needs to overlap the echoes in time and frequency." With the signal covering all the frequencies used by the other bat, there's no available frequency to shift to.

In extended interactions, the bats emitted ultrasonic signals that created sound waves to confuse the processing done by the other bat's auditory neurons. “They use it at the moment of truth, when the hunter is zeroing in on its prey,” Conner tells Science, disrupting the feeding buzz echolocation and making it impossible to determine insect position. 

"Bats have solved the puzzle of sonar jamming in one of the ways that sonar and radar engineers have used,” Conner tells Population Mechanics. “Bats just found the solution 65 million years earlier.”

In a second experiment, the team lured wild bats to moths that were suspended from an ultra-thin fishing line while different ultrasonic sounds were played from a speaker. Playbacks of the jamming call caused the foraging bats to miss their insect targets: They were 73.5 percent less successful at capturing their prey, compared with control sounds. 

"This is the first study to show that bats actively jam the echolocation of other bats,” Corcoran says in a news release, “and it increases the number of known functions of bat sounds to three: echolocation, communication, and acoustic interference.”

Images: Nickolay Hristov (top), Aaron Corcoran (middle)

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