Adaptive frequency shifts of echolocation sounds in Miniopterus fuliginosus according to the frequency-modulated pattern of jamming sounds.

When flying in a group, echolocating bats have to separate their own echoes from pulses and echoes belonging to other individuals to extract only the information necessary for their own navigation. Previous studies have demonstrated that frequency-modulated (FM) bats change the terminal frequencies (TFs) of downward FM pulses under acoustic interference. However, it is not yet clear which acoustic characteristics of the jamming signals induce the TF shift according to the degree of acoustic interference. In this study, we examined changes in the acoustic characteristics of pulses emitted by Miniopterus fuliginosus while presenting jamming stimuli with different FM patterns to the bat flying alone. Bats significantly altered their TFs when responding to downward (dExp) and upward (uExp) exponential FM sounds as well as to a constant-frequency (CF) stimulus, by approximately 1-2 kHz (dExp: 2.1±0.9 kHz; uExp: 1.7±0.3 kHz; CF: 1.3±0.4 kHz) but not for linear FM sounds. The feature common to the spectra of these three jamming stimuli is a spectrum peak near the TF frequency, demonstrating that the bats shift the TF to avoid masking of jamming sounds on the TF frequency range. These results suggest that direct frequency masking near the TF frequency range induces the TF shift, which simultaneously decreases the similarity between their own echolocation sounds and jamming signals.