"We think that this tradeoff between detecting a object and determining its location is fundamental to any process that involves tracking an object whether done by a bat, a dog or a human, and whether accomplished through hearing, smell or sight," said co-author Cynthia Moss, a University of Maryland professor of psychology, who directs interdisciplinary bat echolocation research in the university's Auditory Neuroethology Lab.
Bat sonar (echolocation) obeys the same physical laws as the sonar on a submarine: bats 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? The researchers 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 the 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.
The researchers suggest other animals reflect similar approaches to the detection versus tracking tradeoff. "People without sight use echolocation, in some cases also generating sound by tongue clicking," said Moss.
One such person, she noted, is Daniel Kish, Executive Director of World Access for the Blind. Totally blind from birth, Kish uses tongue clicking for sonar that allows him to "see" his environment at a very high level, even allowing him to safely ride a bike in a city street. Kish is "the first totally blind, national certified Orientation&Mobility Specialist," according to the website for World Access for the Blind.
"There are no measurements of the directionality of the sound beams used by blind echolocators like Kish," Moss said. "But looking at head movements in echo locating blind individuals, it seems that some may show a similar strategy [to that of fruit bats]."
Citation: Yovel et al., 'Optimal Localization by Pointing Off Axis', Science, February 2010, 327(5966), 701 - 704; doi: 10.1126/science.1183310