Three-dimensional sonar beam control of FM echolocating bats during natural foraging revealed by large scale microphone array system

Abstract

In this study, 3-D flight paths and directivity pattern of the sounds emitted by Pipistrellus abramus during natural foraging were measured by a large scale microphone array system. The results show that the bats approached prey with covering the direction of them within their sonar beam. The means of horizontal and vertical beam widths were 49 deg and 46 deg, respectively. Just before capturing prey, the bats decreased the terminal frequency (TF) of the pulse. Simultaneously, the beam widths were expanded to 64 deg (horizontal) and 57 deg (vertical). We assumed a circular piston model to estimate how much the beam width was changed by decreasing the frequency of emitted pulse. It was found that the observed expansion of the beam width was smaller than those of theoretical estimations. This suggests that the bats decrease the TF of pulse for compensating their beam width narrowed by taking a large bite for the prey. We also measured echolocation calls and flight behavior of Myotis macrodactylus during natural foraging. M. macrodactylus uses FM echolocation pulse which is similar to P. abramus, but they forage for prey above the water surface. We compare echolocation strategies between two FM bats with different foraging habitat.