Rapid frequency control of sonar sounds by the FM bat, Miniopterus fuliginosus, in response to spectral overlap

In the presence of multiple flying conspecifics, echolocating bats avoid jamming by adjusting the spectral and/or temporal features of their vocalizations. However, little is known about how bats alter their pulse acoustic characteristics to adapt to an acoustically jamming situation during flight. We investigated echolocation behavior in a bat (Miniopterus fuliginosus) during free flight under acoustic jamming conditions created by downward FM jamming sounds mimicking bat echolocation sounds. In an experimental chamber, the flying bat was exposed to FM jamming sounds with different terminal frequencies (TFs) from loudspeakers. Echolocation pulses emitted by the flying bat were recorded using a telemetry microphone (Telemike) mounted on the back of the bat. The bats immediately (within 150 ms) shifted the TFs of emitted pulses upward when FM jamming sounds were presented. Moreover, the amount of upward TF shift differed depending on the TF ranges of the jamming sounds presented. When the TF range was lower than or overlapped the bat's mean TF, the bat TF shifted significantly upward (by 1-2 kHz, Student's t-test, P < 0.05), corresponding to 3-5% of the total bandwidth of their emitted pulses. These findings indicate that bats actively avoid overlap of the narrow frequency band around the TF.