Species-specific control of acoustic gaze by echolocating bats, Rhinolophus ferrumequinum nippon and Pipistrellus abramus, during flight

Abstract

Based on the characteristics of the ultrasounds they produce, echolocating bats can be categorized into two main types: broadband FM (frequency modulated) and narrowband CF (constant frequency) echolocators. In this study, we recorded the echolocation behavior of a broadband FM (Pipistrellus abramus) and a narrowband CF echolocator species (Rhinolophus ferrumequinum nippon) while they explored an unfamiliar space in a laboratory chamber. During flight, P. abramus smoothly shifted its acoustic gaze in relation to its flight direction, whereas R. ferrumequinum nippon frequently shifted its acoustic gaze from side to side. The distribution of the acoustic gazes of R. ferrumequinum nippon was twice as wide as that of P. abramus. Furthermore, R. ferrumequinum nippon produced double pulses twice as often as P. abramus. Because R. ferrumequinum nippon has a horizontal beam width (-6 dB off-axis angle) half as wide (±20.8 ± 6.0°) as that of P. abramus (±38.3 ± 6.0°), it appears to double the width of its acoustical field of view by shifting its acoustic gaze further off-axis and emitting direction-shifted double pulses. These results suggest that broadband FM and narrowband CF bats actively control their acoustic gazes in a species-specific manner based on the acoustic features of their echolocation signals.

Keywords: Acoustical field of view; Beam width; Microphone array; Pulse direction; Telemetry microphone.

Fig. 1

Sonograms of typical pulse emission sequences in P. abramus (a) and R. ferrumequinum nippon (b) during flight in the flight chamber. Sounds were recorded by the on-board microphone (Telemike) mounted on the back of each bat

Fig. 2

System for measuring echolocation pulses and flight trajectory of bats during flight. a Arrangement of the microphone array in the flight chamber. b Procedure for deriving horizontal pulse direction and beam width from microphone array recordings. The blue arrow indicates pulse direction, and the green double-headed arrow indicates the beam width of the pulse. c Acoustic gaze, defined as the angle between pulse direction and flight direction

Fig. 3

Echolocation of P. abramus and R. ferrumequinum nippon during first flights in the chamber. a, b Top views of flight trajectory (red line) and pulse directions (blue lines). c, e Changes in IPI as a function of flight time. d, f Histograms of IPI for all bats (seven P. abramus and six R. ferrumequinum nippon) during their first flight

Fig. 4

Comparison of acoustic gaze control between P. abramus and R. ferrumequinum nippon during the first flight. Top views of flight trajectories (red line) with pulse directions (blue lines) for representative P. abramus (a) and R. ferrumequinum nippon (b). c, d Angle of acoustic gaze during the flights shown in (a) and (b). e, f Distributions of acoustic gaze during flights of all bats. g, h Distributions of the amount of absolute change in acoustic gaze (∆gaze) between successive pulses in all bats

Fig. 5

Horizontal beam patterns of echolocation pulses emitted by P. abramus (a) and R. ferrumequinum nippon (b) during first flights. Data were obtained from all bats (seven P. abramus and six R. ferrumequinum). Solid line shows the pattern of pulse directivity which was fitted with a Gaussian shape. The peak frequencies used when calculating beam patterns were 49.8 ± 3.0 kHz for P. abramus and 63.8 ± 2.2 kHz for R. ferrumequinum nippon