Echolocating animals like bats broadcast calls, then listen for echoes bouncing off objects to build up a picture of their surroundings. But how do bats distinguish between echoes bouncing back from distractions like tree branches and those revealing the location of a tasty morsel (p. 394)?
Big brown bat cries contain two main harmonics called FM1 and FM2, which both sweep down from high to low frequency, explains Mary Bates of Brown University, USA. If a target such as a moth is straight ahead and nearby, the bat hears equally strong echoes of FM1 and FM2; but if an object is far away or off to the side, the echo of FM2 becomes softer than that of FM1. As a result, the bat's ear responds later to the echo of FM2. ‘We know that the neurons in the bat's brain respond later to quieter sounds than louder ones,’ says Bates. Working with James Simmons, she reasoned that bats might use this auditory mechanism to ‘tune out’ clutter such as tree branches, since these are more likely to be far off or on the periphery.
To demonstrate that this mechanism exists, Bates and Simmons needed to show that bats can no longer tell how far away objects are when there is a delay between FM1 and FM2. First, they needed a way to determine how the bats perceive echo delay (which reveals how far away something is), so they devised a setup that allowed the bats' behaviour to tell them what the bats perceived. Bates caught big brown bats in Rhode Island and trained them to sit on a Y-shaped platform, call out, listen for echoes from both arms of the platform, then walk to the echo they considered to be closest.
Having trained the bats, Bates was ready to assume control and test how well they detect delay when their FM1 and FM2 harmonics are misaligned. ‘We were able to create phantom echoes by picking up the bats' calls, manipulating them electronically to introduce a delay between FM1 and FM2, and then playing them back to the bats,’ she explains. When Bates played back the bats' calls without any delays between FM1 and FM2, they correctly judged which echo was closer 90% of the time. But sure enough, when Bates introduced a 300 μs delay between the harmonics, the bats' accuracy dropped to 75%, revealing that they were not able to detect distances as well as before. And when she tested the bats with incremental differences in the introduced delay, Bates was pleased to find that the bats continued to get it wrong, and even made mistakes 25% of the time when echoes were misaligned by only 2.6 μs. These results suggest that some auditory mechanism detects even tiny delays between harmonics and then defocuses the resulting images, Bates concludes.
‘We have shown that bats have a perceptual mechanism for rejecting echoes from clutter that is off to the side or some distance away in order to focus on more important targets directly in front of them,’ says Bates. She likens the bats' ability to ignore misaligned echoes to our peripheral vision; just as we can vaguely distinguish objects on our periphery but not see them in high resolution, big brown bats don't perceive far-off clutter as accurately as a juicy moth right in front of their noses.