To understand complex sensory-motor behavior related to object perception by echolocating bats, precise measurements are needed for echoes that bats actually listen to during flight. Recordings of echolocation broadcasts were made from flying bats with a miniature light-weight microphone and radio transmitter (Telemike) set at the position of the bat's ears and carried during flights to a landing point on a wall. Telemike recordings confirm that flying horseshoe bats (Rhinolophus ferrumequinum nippon) adjust the frequency of their sonar broadcasts to compensate for echo Doppler shifts. Returning constant frequency echoes were maintained at the bat's reference frequency +/-83 Hz during flight, indicating that the bats compensated for frequency changes with an accuracy equivalent to that at rest. The flying bats simultaneously compensate for increases in echo amplitude as target range becomes shorter. Flying bats thus receive echoes with both stabilized frequencies and stabilized amplitudes. Although it is widely understood that Doppler-shift frequency compensation facilitates detection of fluttering insects, approaches to a landing do not involve fluttering objects. Combined frequency and amplitude compensation may instead be for optimization of successive frequency modulated echoes for target range estimation to control approach and landing.
To understand complex sensory-motor behavior underlying echolocation by bats, we need to precisely measure acoustic characteristics of echoes that the bats actually listen to during flight. However, it has been technically-difficult to develop a small and light enough microphone set at the position of the bat's ear, and information about the echoes is so far virtually lacked. Here, we have successfully monitored these returning echoes by means of an onboard telemetry microphone mounted on the bats, and we confirmed that the free-flying horseshoe bats compensated for Doppler-shifts by adjusting their call frequency, thus maintaining the echo frequency at constant. Furthermore, we found that the bats also adjusted pulse amplitude with the distance to maintain the echo intensity within the narrow range. Receiving echoes with stable amplitude could help the bats to sustain consistent analysis of successive echoes, and combined frequency and amplitude compensation may be for optimization of successive echoes for target range estimation to control approach and landing. [Supported by a grant to RCAST at Doshisha Univ. from MEXT of Japan: Special Research Grants for the Development of Characteristic Education from the Promotion and Mutual Aid Corporation for Private Schools of Japan and the Innovative Cluster Creation Project.]
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