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Compensation behaviors in echolocating bats measured by a telemetry microphone during flight

Some echolocating bat species are well known to compensate for the Doppler shifts in the echoes by changing their pulse frequencies so that the echo frequencies remain constant at which the bats can hear best. This behavior is termed "Doppler-shift compensation" and is an important behavioral adaptation for echolocating bats. In this study, we present the evidence for another compensation mechanism of echolocating bats, in which pulse intensity is adjusted in relation to the distance to a target resulting in maintenance of a constant intensity of the echo. Our custom-made telemetry microphone (Telemike) mounted on the back of the bat allowed us to observe not only the emitted pulses, but also the returning echoes which the flying bat actually listened to. The signal characteristics of pulse-echo pairs were analyzed combining with a high-speed video camera system as a bat flying toward the wall on which a landing mesh was attached in the flight chamber. We found that pulse intensity in bats intending to land exhibited a marked decrease by approximately 30 dB within 2 m of distance from the target wall, and the mean reduction rate was 6.5 dB per halving of distance. In contrast, the intensity of echoes returning from the target wall at the head of the flying bat indicated a nearly constant intensity while approaching the target wall. These findings provide direct evidence that bats are supposed to adjust pulse intensity to compensate for changes in echo intensity to maintain a constant intensity within the range necessary for optimal signal processing (echo-intensity compensation).