In case you didn't already think that bats' ability to navigate with their ears instead of their eyes was cool enough, get this: Mexican free-tailed bats can actually use biological sonar to jam the signals of other bats and steal their food.
The findings, published in the journal Science, reveal a heretofore unknown sonic weapon in the arsenal of these impressive echolocating animals.
Bats move through the world by sending out high-pitched calls and judging distances to the objects around them based on the sound that bounces back. As they get closer and closer to their prey — say, a tasty moth fluttering through the air — they speed up those calls to home in on their target.
It's the same concept that underlies radar and sonar technology, which harness radio and sound waves, respectively, to locate distant objects. But what scientists didn't know is that bats don't just sense these signals; they can jam them, too.
"It's the same solution that sonar and radar engineers came to for jamming sonar and radar for military purposes," said study co-author William Conner, an animal behaviorist at Wake Forest University. "And the cool thing about it is, bats came up with this idea about 65 million years earlier than the engineers."
The one problem with using sound to find your food is that it announces to all the other bats nearby that you're on the verge of nabbing a tasty meal. And in the bat-eat-moth world of Mexican free-tailed bats, which can live in colonies of perhaps a million individuals, competition for food can be fierce.
If they hear another bat about to move in on a morsel, Mexican free-tailed bats will send out a well-timed counter-call. This jamming signal, which has the wavering, up-down quality of a police siren, matches the frequency range of the other bat's food-finding signal. The jamming bat strikes when the other bat is at its most vulnerable, just as it's about to nab its target moth. Confused, the bat loses track of its prey. Then the bat doing the jamming can swoop in and steal the meal.
"It's a form of competition; make the other guy miss, and when he misses, you then go in and clean up and take the insect," Conner said.
Of course, the thwarted bat can also then jam the thieving bat's signal just before it nabs the moth. So dueling jamming signals go back and forth for several rounds, a test of wills until one bat calls it quits, Conner said.
The researchers had been studying how moths escaped capture (using a jamming technique of their own, it turns out) when they noticed the strange bat calls — "a new sound," Conner said.
"Hearing it for the first time, noting when it was produced, that made us think that it could be a jamming signal," Conner said.
Bats send out calls for two general reasons: echolocation, and social communication. Using their ultrasound vocalizations as a jamming call was unheard of (so to speak).
To study the bats, the researchers went to field sites at the Southwestern Research Station in Arizona and a high school parking lot in Animas, N.M. But tracking them was no easy task. Since Mexican free-tails fly fairly high, the scientists had to get a 6-meter-tall scaffolding tower, where they mounted an infrared camera. Using the camera and two arrays of high frequency microphones, they found that most of the time, bats missed their prey when another bat was jamming them.
The scientists then hung a moth on an ultra-thin fishing line as bait and played the jamming signal as bats swooped in for the kill. They found that the sound had to be played at just the right moment and at the right frequency, just as the bat was homing in on the moth. If they played the jamming call too early, it didn't work.
There's much still left to study in how the bat performs this remarkable feat, Conner said, including what effect the jamming signal has on the jammed bat's brain. In the meantime, he said, it would be interesting to see if other animals that use sonar, such as dolphins and other toothed whales, can jam each other's frequencies too.