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How bats make short work of flying toward prey

TheAllIneed.com/ NC&T/UM

What is known is that bats "see" by sending out beams of sound into the environment and listening for the echoes that are returned from objects like bugs and trees. The bat uses these echoes to maneuver past the trees and to catch bugs.

What has not been known, however, is if bats convert what they hear into what they do in the same way that humans convert vision into action. The bat's use of sound to capture insects takes place so quickly, it's not possible to observe its behavior in real time, or with standard equipment.

Now, with a unique combination of high-speed infrared cameras and ultrasonic microphones, University of Maryland professor Cynthia Moss and doctoral student Kaushik Ghose have been able to see exactly how a bat moves in response to sound. They found that there is a strong and predictable connection between where a bat "looks" with its sonar beam - its acoustic gaze -- and how it flies.

In their study, which appears in the February 8 issue of the Journal of Neuroscience, Moss and Ghose report that a bat directs its beams of sound ahead of its flight, like a flashlight. If it directs its beam to the left, about a tenth of a second later it turns to the left. By adjusting how sharply it follows its beam, the bat is able to adapt between "looking around" in a search mode and "homing in" in attack mode.

"The findings add a significant piece of information to understanding how bats function," says Moss, professor of psychology and director of the university's Neuroscience and Cognitive Science graduate program. "Previous work on humans and other visually guided animals have suggested that we 'look' where we go. But it was not known if there was a similar relationship between the bat's acoustic gaze and its flight motion.

"The discovery that the bat adjusts how sharply it turns in flight to follow its sonar beam is also remarkable. Such an adjustable linkage between vision and locomotion hasn't been found yet in sighted animals."

Infrared photo of bat in flight in University of Maryland lab. (Photo: Stephen Dear )
Scientists know there is a relationship between where humans and other sighted animals look and how they move to a goal. For example, when you walk through a shopping mall toward your favorite store, you may be looking at the store windows to your right and left, but you continue to walk straight ahead. You aren't necessarily moving in the same direction as your gaze. But when you see the store you want to enter, you turn toward it, looking and walking in the same direction.

"We haven't been sure of the same thing about bats, because in the dark, they rely primarily on a sophisticated sonar system - called echolocation - which is quite different from the way we sense the environment," says Ghose, who will receive his Ph.D. this spring. "To test the bat's echolocation system, we removed sources of light, mimicking a very dark night, when bats would rely completely on sonar to forage for food."

"Because the entire process of detecting, tracking and capturing prey takes place in a split second and in the dark, we can't simply observe this amazing behavior with our eyes," says Ghose.

Technology was the key to the team's insight into the bat's blazingly fast skills. Moss's is the only lab in the world with a bat flight-room equipped with high-speed infrared cameras. The cameras allowed the researchers to record the bat's behavior, then slow it to analyze the moves frame by frame, like a baseball coach studying a play.

The microphone array, designed by Ghose for his doctoral thesis, revealed the beams of sound the bat put out as it flew and captured insects. Arranged around the flight room, the microphones recorded the directional patterns of sound emitted by the bat, which show where the bat is "looking."

"It required a level of analysis and processing of data that no one else has used," Moss says.

The slow motion video and audio showed that when the bat is flying along, just scanning its surroundings for prey, it makes more gentle turns toward the direction of its gaze -- like window-shopping in the mall.

"When the bat is just searching," Ghose says, "it puts out about 10 calls per second. In this stage, the bat could be directing its sonar beam far off to the side, but it turns only leisurely toward that direction."

But when the bat has located a bug with its sound beam, it begins to call more rapidly, putting out 100-150 calls a second. In this state, the bat follows its beam much more precisely, making abrupt turns to follow the direction of its acoustic gaze.

"The bat switches to attack mode when it senses the bug," says Ghose. "It uses rapid maneuvers to align its flight path and to intersect with the unfortunate insect. This is like the stage of window-shopping when we see a shiny expensive thing in the window, and we head straight for it."

Moss and Ghose used tethered insects in this phase of the study, to show the bat's flight responses independent of what its prey might do. In the next phase of their research, they are looking at how bats deal with free-flying insects that move and duck to evade the bat.

The study was funded by grants from the National Science Foundation, National Institutes of Health and the University of Maryland.

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