Many bats have extremely poor eyesight, yet put them in a room with a tiny insect and they will quickly seek it out and gobble it up. When foraging bats have to rely on echolocation to determine the location of their next tasty treat. They produce beams made up of pulses of ultrasound waves and analyse the returning echoes to guide them. Although routine for these bats, it is nonetheless a remarkable feat and one that we haven't mastered, as Shizuko Hiryu from Doshisha University, Japan, points out: ‘So far, acoustic sensing technology developed by humans using ultrasound cannot detect and track small moving targets like bats do, because the [returning] echoes are very small.’ So how do they do it? Hiryu and some of his students set out to discover the bat's secret to detecting prey (p. 1210).
The team placed Japanese horseshoe bats, Rhinolophus ferrumequinium nippon, in a flight chamber and tempted them into flight by dangling an appetising moth from the ceiling at the other end of the room. As the bats zoomed in on their prey, the team picked up their calls using small ultrasonic microphones located throughout out the chamber, and also filmed their aerial trajectory.
Upon release, the bats scanned their new environment by issuing regular pulses lasting 30–40 ms, but as soon as they picked up the tiny echoes from the moth, the pulses lengthened to 70–80 ms and the bats began their pursuit. When the bats were within 1 m, the moth understandably took evasive action, moving considerably in the hope of escaping the hungry bat. The team found that this prompted the bats to respond in two ways; firstly, they produced more closely spaced, shorter (10 ms) pulses and secondly, they broadened the width of their ultrasonic beam by, on average, ±14 deg horizontally and ±17 deg vertically to cover a larger area. In 97% of cases this beam-width broadening was sufficient to cope with the increased movement of the moth and insured that the insect was never out of the range of the bat's ultrasonic call. Furthermore, in cases where the moths seemingly gave up hope and failed to move at all, the preying bats likewise failed to widen their calls.
At first, Hiryu thought that the appearance of beam expansion was due to an artefact in their microphone array, but after careful re-evaluation the team are sure that these bats are definitely widening their beams as they approach their prey. However, it remains a mystery how these bats adjust their beams. The ability to vary beam width has previously been seen in bats that emit calls where ultrasound waves vary in frequency throughout the pulse, called frequency modulation (FM). As sound waves with lower frequencies spread further, these bats just lower the maximum frequency reached during their pulse to widen their beam. Japanese horseshoe bats, however, emit their pulses in a slightly different way, incorporating two FM elements interspersed by a section of waves with a constant frequency (CF). When the team analysed changes in the FM elements of the horseshoe bat's calls they didn't see a large enough drop in maximum frequency to explain the beam-width expansion. CF–FM bats must therefore use a different way to modulate their beams. As horseshoe bats emit their calls through their nostrils, the team suggests that the orientation of their noseleaf – the tip of the nose – may control beam width.
Whilst we still don't know exactly how bats initially detect minute echoes, Hiryu's study has unveiled for the first time one of the bat's secrets for capturing moving prey.