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A Mexican free-tailed bat emits a special ultrasonic call to block the echolocation of another bat in this composite image; the wavy lines depict the jammer's vocalization.

A Mexican free-tailed bat emits a special ultrasonic call to block the echolocation of another bat in this composite image; the wavy lines depict the jammer's vocalization.

Nickolay Hristov

Holy blocked bat signal! Bats jam each other's calls

Just before nabbing an insect, a bat emits a rapid series of ultrasonic calls whose echoes back pinpoint the prey’s exact location. Scientists call these sounds “the feeding buzz,” and they’re known to attract other bats presumably in search of a meal. When another bat arrives, it can jam the hunter’s buzz, according to a new study, much like someone blocking a radio signal. That causes the original bat to miss its meal, allowing its competitor to swoop in to grab the insect instead.

“It’s a thrilling finding,” says Mirjam Knörnschild, a behavioral ecologist and bat vocalization expert at the University of Ulm in Germany, who was not involved in the study. “Sonar interference has always been an exciting possible explanation … for the fact that certain bat species are highly vocal, and this elegantly designed study is a convincing demonstration.”

Aaron Corcoran, a biologist at Wake Forest University in Winston-Salem, North Carolina, first detected the jamming call while recording bat-moth interactions in Arizona. Other researchers had previously discovered that Mexican free-tailed bats (Tadarida brasiliensis) make at least 15 types of social calls and even adjust their ultrasonic vocalizations to avoid interfering with those of others. To find out how the bats were using this call, the scientists recorded their competitive bouts on video and with an array of ultrasonic microphones at field sites in Arizona and New Mexico. They matched up the calls to the bats’ flight paths so that they could see at what point hunters made the feeding buzz and competitors emitted a blocking signal. From this 3D reconstruction, Corcoran and William Conner, a biologist also at Wake Forest University, realized that the bats were more competitive than cooperative, and readily wielded their highly effective and disruptive jamming call. “They use it at the moment of truth, when the hunter is zeroing in on its prey,” Conner says.

The jamming call works by overlapping the competitor’s final frenzied feeding buzz, creating sound waves that confuse the processing done by the bat’s auditory neurons and ability to target the prey’s position with its ultrasonic clicks, a process known as echolocation. Still, a jammed bat can turn the tables on its competitor and use the same method to interfere with its hunt. “They whiz back and forth, back and forth, fighting over the prey, until one finally gives up,” Conner says.

In Arizona, the scientists documented 145 bat attacks on insects; 85.9% of these attempts failed when another bat emitted the jamming call. Without a competitor to interfere, the bats’ success rate jumped to 30%. The researchers also tested bats with controlled playback experiments. They tethered a moth to a monofilament line about 5 meters above the ground, then played a recorded jamming call just as a hunting bat targeted the insect with its feeding buzz. The bats’ capture success rate dropped by 73.5% compared with when they hunted without interference from the recording, the team reports online today in Science.

Five years ago, Corcoran and Conner showed that tiger moths can jam the hunting sonar of brown bats. But this is the first time that this type of competitive interference among individuals of the same species has been discovered in animals, they say.

Other researchers had previously heard similar calls from different bat species, including male big brown bats (Eptesicus fuscus), and suggested that the sound might be used in cooperative foraging, or to claim food. “It’s clearly not for cooperating,” says Cynthia Moss, a neuroscientist at Johns Hopkins University in Baltimore, Maryland, whose team proposed the food-claiming explanation—and is sticking to it. “There is evidence for both claiming and jamming,” she says, noting that her group “didn’t see the timing of the call the way” that Corcoran and Conner did, consistently overlapping that rapid, terminal feeding buzz. “While this paper is very provocative, interesting, and important, it is not the last word.”

Still, the very idea that bats have not only sonar but also a jamming signal is “cool,” Conner says. “We think engineers are pretty clever when they use a signal to jam sonar and radar. But bats came up with this idea 65 million years earlier.” Now, he and Corcoran wonder if other echolocating species such as dolphins engage in competitive jamming bouts as well.