Embarking on twilight foraging excursions, bats have only their ears to guide them through cluttered environments. Uttering high-pitched cries and listening for their reflections, bats construct a sonic view of their world. But no two echolocation calls are the same. Marc Holderied, from the University of Erlangen, Germany, explains that each call is optimised for a different circumstance. Cries ranging over a narrow frequency-range are better suited for long-range echolocation, while cries swooping over a wide frequency-range function best for short-range detection. But no one knew whether bats tailored their calls to improve their navigational precision. Holderied and his colleague Otto von Helversen decided to record the positions and signal structures of echolocating bats to see if the bats fine-tuned their calls in response to the obstacles they encounter(p. 1816).
Carefully rigging up eight microphones close to the bat's path, Holderied recorded 22 individuals' echolocating cries as the tiny aviators departed their farmhouse colony and followed a hedge to their nearby hunting grounds. But Holderied also wanted to relate the bats' calls to their surroundings, so he surveyed the bats' guiding hedge with a theodolite and laser to reconstruct the hedge's structure.
Teaming up with Gareth Jones at the University of Bristol, UK, Holderied calculated the position of each bat relative to the hedge when it squeaked,and the cry's duration to see whether the animal's call length changed as they tracked the hedge. But when he analysed each call's duration, the animal's proximity to the hedge seemed to have no effect. The bats weren't moderating their call length to prevent the outgoing call interfering with the incoming reflection, as he had thought.
However, when Holderied analysed each call's bandwidth as the bats progressed along the hedge, there was a strong correlation; when the bats neared the hedge, they increased the bandwidth of their cries. Holderied explains that increasing the bandwidth allows the bats to estimate the distance to near-by objects more accurately.
Curious to know if the bats modulated their cries in a systematic way as they followed the hedge's contours, the team decided to calculate each cry's`distance of focus'. Holderied explains that the distance of focus is the distance where the squeaking bat can accurately estimate the position of obstacles; there are always errors in the flying bat's distance measurements if the obstacle is nearer to, or further away, than the distance of focus. Calculating each cry's distance of focus from the squeak's bandwidth and duration, and plotting it as a hemisphere centred on and in front of the bat,Holderied realised that the bats continually adjusted their calls so that the distance of focus skirted the hedge's foliage as they progressed along its length. The bats were continually adapting their calls to follow the hedge with maximum precision.
Holderied suspects that echolocating bats are able to select calls with distances of focus that roughly match the location of an object as it looms,freeing the bats from complex echolocation calculations and improving their in-flight precision.