Echolocating bats need to solve the problem of signal jamming by conspecifics when they are in a group. However, while several mechanisms have been suggested, it remains unclear how bats avoid confusion between their own echoes and interfering sounds in a complex acoustic environment. Here, we fixed on-board microphones onto individual frequency-modulating bats flying in groups. We found that group members broaden the inter-individual differences in the terminal frequencies of pulses, thereby decreasing the similarity of pulses among individuals. To understand what features most affect similarity between pulses, we calculated the similarity of signals mimicking pulses. We found that the similarity between those artificial signals was decreased most by manipulation of terminal frequency. These results demonstrate that the signal jamming problem is solved by this simple strategy, which may be universally used by animals that use active sensing, such as echolocating bats and electric fish, thereby transcending species and sensory modalities.

The ability to detect behaviourally relevant sensory information is crucial for survival. Especially when active-sensing animals behave in proximity, mutual interferences may occur. The aim of this study was to examine how active-sensing animals deal with mutual interferences. Echolocation pulses and returning echoes were compared in spaces of various sizes (wide and narrow) in Rhinolophus ferrumequinum nippon flying alone or in a group of three bats. We found that in the narrow space, the group-flying bats increased the duration and bandwidth of the terminal frequency-modulated component of their vocalizations. By contrast, the frequency of the returning echoes did not differ in the presence of conspecifics. We found that their own echo frequencies were compensated within the narrow frequency ranges by Doppler shift compensation. By contrast, the estimated frequencies of the received pulses emitted by the other bats were much more broadly distributed than their echoes. Our results suggest that the bat auditory systems are sharply tuned to a narrow frequency to filter spectral interference from other bats.

It remains mystery how echolocating bats extract their own echoes from an acoustic jamming caused by vocalizations of multiple conspecifics flying together. Here, we first successfully recorded echolocation pulses of each animal during four-bat group flight by individually mounted on-board microphones, allowing us to assess vocalization in a jamming environment. We used Miniopterus fuliginosus which emit downward frequency-modulated (FM) ultrasounds. The bats experienced both single and group flights in an experimental chamber. As a result, each bat of the group significantly expanded differences in terminal frequency (TF) of pulses from the neighboring bat from 0.6 ± 0.6 (mean ± SD) kHz in single flight to 1.2 ± 0.6 kHz in group flight (Tukey’s HSD test, P < 0.05). Although the difference in TF observed in group flight was much smaller than the bandwidth of pulses (approximately 40 kHz), computation of cross-correlation providing an index of similarity of two signals demonstrated that the difference is useful to distinguish own signal from others. These results suggest that innate FM signal of bats is jamming-tolerant, and FM bats flying in group highlight differences in TF to avoid or reduce jamming from conspecifics in a closed space, as implied by previous studies.