Adaptive SONAR sounds by echolocating bats

Like dolphins, bats are known to possess highly developed SONAR systems in air. Echolocating bats can be divided into two groups: the CF-FM and FM bats depending on frequency structure of their pulses. In this study, we used one of Japanese FM bat species, Pipistrellus abramus. The echolocation behavior was examined for two different flight tasks: (a) field recording while capturing insects in the open area, and (b) recording for a landing approach to a target wall in the laboratory. We acoustically compared these two echolocations by the bats while approaching a target. In the field and laboratory, repetition rates of pulse emission in the search phase were constant at approximately 10 pulses/s. When approach phase was started, the bats increased the repetition rate of the pulse emission to 140-190 pulses/s. We found that the pulse duration was dynamically decreased from 10 to 0.5 ms during prey capturing in the field, whereas it ranged from 0.5 to 3-4 ms in the laboratory. A CF-like portion (a narrow slope portion at the end of pulse) was observed to follow the initial FM sweep beyond approximately 2 m of the target distance in the laboratory. Interestingly, the CF-like portion was found to be extended by the bats in the field and such long pulse duration was never seen in the laboratory. This suggests that FM bats use not only broadband signals, but also narrowband signals for echolocation in the far target range as CF-FM bat species. Biosonar animals might have been supposed to adapt their echolocation to underlying physical law in nature or their environment through their evolutionary history. These comparative studies between the field and laboratory recordings are expected to help our understanding of bat's biosonar system, and various echolocation strategies employed by the bats will contribute to develop artificial SONAR system or new echo-sensing devices in the future.