The best way to track a moving object with a flashlight might be to aim it to one side, catching the object in the edge of the beam rather than the center. New research at the Weizmann Institute of Science reveals that bats, which 'see' with beams of sound waves, skew their beams off-center when they want to locate an object. The research, which recently appeared in Science, shows that this strategy is the most efficient for locating objects.
Dr. Nachum Ulanovsky and postdoctoral fellow Dr. Yossi Yovel of the Institute's Neurobiology Department knew that bat sonar (or echolocation) obeys the same physical laws as the sonar on a submarine: The bats (or ships) emit a sound and listen for the echo, accurately judging the type and location of objects around them by the changes in the sound waves as they're reflected back. But there's a trade-off between detection and localization. The beam is most intense in the center, returning more information, which is good for detection; but localization is better done on the slope, where the intensity drops off as the signal spreads out, making it easier to follow movement across the beam.
Are bats able to choose the best echolocation strategy? Ulanovsky and Yovel, in collaboration with Prof. Cynthia Moss and research student Ben Falk from the University of Maryland, trained bats to locate and land on a black sphere placed randomly in a completely dark room, using echolocation alone. A string of special microphones arrayed around the room's walls traced the bats' sound waves, while two infrared video cameras tracked their flight patterns.
The Egyptian fruit bats in Ulanovsky's lab produce their signals in pairs of clicks. The researchers identified a pattern: The first set of double clicks was aimed left, and then right, and the next set right, then left. As the bats closed in for a landing, they continued to throw their sound beams to alternate sides of the sphere, just where a mathematical formula for sonar sensing predicted they would be most effective. As the sphere was easily detectable, the bat's optimal strategy was one of localization. To test a situation in which detection was needed as well as localization, the scientists installed a large panel behind the sphere that echoed the sound waves back to the bats' ears. Now they had to find the sphere's echo amidst conflicting signals. This time, as the bats approached their target, they began to narrow their sweep and aim the beams more or less directly toward the sphere.
Many types of sensation, from echolocation in dolphins to sniffing in dogs to human eye movements, are based on some sort of active sensing. Ulanovsky and Yovel believe that what works for bats may well work for other animals: 'sensing on the slope' could play a role in all of these and others.
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