Using only a microphone array system, echolocation pulses and three-dimensional flight paths in the frequency-modulated bat, Pipistrellus abramus, during natural foraging, were simultaneously examined. During the search phase, the inter-pulse interval, pulse duration, and moving distance of the bat between successive emissions were relatively constant at around 89.5 ± 18.7 ms, 6.90 ± 1.31 ms, and 0.50 ± 0.20 m, respectively. The bats started to decrease these acoustical parameters within 2-3 m of the prey capture point. For every emission along a flight path, the distance between a bat and its prey capture point was calculated as both direct distance to capture (DDC), which corresponded to the target distance, and flight distance to capture (FDC) along the flight path. The DDC matched the FDC after the start of the approach phase, indicating that foraging bats followed a straight-ahead path to the target. In addition, the duration of the quasi-constant frequency component of emitted pulses was slightly extended just before the convergence of the DDC with the FDC. These findings suggest that the bats confirm the presence of target prey by extending the duration of the pulse and then select a straight-ahead approach by forecasting the movement of the prey.

Echolocation sounds of Rhinolophus ferrumequinum nippon as they approached a fluttering moth (Goniocraspidum pryeri) were investigated using an on-board telemetry microphone (Telemike). In 40% of the successful moth-capture flights, the moth exhibited distinctive evasive flight behavior, but the bat pursued the moth by following its flight path. When the distance to the moth was approximately 3-4 m, the bats increased the duration of the pulses to 65-95 ms, which is 2-3 times longer than those during landing flight (30-40 ms). The mean of 5.8 long pulses were emitted before the final buzz phase of moth capture, without strengthening the sound pressure level. The mean duration of long pulses (79.9 ± 7.9 ms) corresponded to three times the fluttering period of G. pryeri (26.5 × 3 = 79.5 ms). These findings indicate that the bats adjust the pulse duration to increase the number of temporal repetitions of fluttering information rather than to produce more intense sonar sounds to receive fine insect echoes. The bats exhibited Doppler-shift compensation for echoes returning from large static objects ahead, but not for echoes from target moths, even though the bats were focused on capturing the moths. Furthermore, the echoes of the Telemike recordings from target moths showed spectral glints of approximately 1-1.5 kHz caused by the fluttering of the moths but not amplitude glints because of the highly acoustical attenuation of ultrasound in the air, suggesting that spectral information may be more robust than amplitude information in echoes during moth capturing flight.

Analysis of the bat's reactions to relevant target echoes enables us to directly assess biosonar performance. Here, we recorded the sonar broadcast and its echoes the bat received during flight by using an on-board telemetry microphone (Telemike) mounted on the bat's back. Telemike recordings confirmed that flying bats adjust the amplitude and frequency of their sonar broadcasts to compensate for increases in echo amplitude and for Doppler-shifts. For insect capturing, the bat exhibited Doppler-shift compensation for echoes from the static target ahead, but not for echoes from the target moth even though the flying bat attended to the moth for capture. Positive and negative Doppler shifts (acoustic glints) caused by insect fluttering were observed in the constant-frequency component of observed echoes, which synchronized with wingbeat cycle of the moth. Combined frequency and amplitude compensation for the static target may be advantageous for detection of acoustic glints of target prey. We also constructed multiple-microphone arrays for tagging wild aerial-feeding insectivorous bats. Not only the location of the bat, but also direction and directivity of the bat's broadcast can be measured. This will allow us to investigate 3-D search algorithm of multiple targets by the bat. [supported by JSPS and ONR]

Bats are supposed to have effective strategies for achieving a good balance between echolocation and flight behaviors while capturing small moving insects in the field. To reveal their strategies for catching insects, we successfully reconstructed 3-D flight trajectories for the bat to forage in the field by a four-microphone array system, and conducted both acoustical and behavioral analyses for capturing behavior. Data show that the flying bats changed their flight direction flexibly, and sometime repeated capturing insects every two to three seconds. During the search phase, the bat moved 0.5-0.8 m during an interval between successive pulses (IPI) and then decreased that the moving distance during an IPI up to 0.1 m just before capturing a prey. Interestingly, we found that the bat tended to descend toward a prey from above when the approach phase started. This suggests that foraging bats may effectively utilize gravity for an easy acceleration toward the prey to concentrate on the complex echolocation for capturing moving insects. [Supported by a grant to RCAST at Doshisha Univ. from MEXT of Japan: Special Research Grants for Development of Characteristic Education from the Promotion and Mutual Aid Corporation for Private Schools Japan, Innovative Cluster Creation Project.]