On-board telemetry of emitted sounds from free-flying bats: compensation for velocity and distance stabilizes echo frequency and amplitude
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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.
KeywordsDoppler-shift compensation Echo-intensity compensation Rhinolophus ferrumequinum nippon CF–FM bats
We thank Prof. James A. Simmons for careful reading of the manuscript and valuable comments. We also thank T. Hagino, M. Fukuda, E. Fujioka, M. Omura and Y. Osawa for analysis and technical support during this experiment; N. Urano for assistance in capturing bats in the field. The experiments complied with the Principles of Animal Care, publication no. 86–23, revised in 1985, of the National Institutes of Health, and the procedures were approved by the animal care committee of Doshisha University. This work was partly supported by a grant to the Research Center for Advanced Science and Technology (RCAST) at Doshisha University from the Ministry of Education, Culture, Sports, Science, and Technology (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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