Horseshoe bats use not changes in echo delay but Doppler shift to perceive approaching objects

Echolocating bats use echo delay for target ranging and Doppler shift information for relative velocity recognition. However, how they perceive moving objects remains unclear. To investigate this question, we played back echolocation pulses in real-time as virtual echoes to Japanese horseshoe bats (Rhinolophus ferrumequinum nippon) on a perch in a flight room. Since echoes coming back from an approaching object are theoretically characterized by both changes in echo delay and the presence of Doppler shift, we reproduced an artificial approaching object by encoding these two acoustic parameters in the virtual echoes. As a result, only Doppler shift evoked bats flight reaction, showing that they use only Doppler shift and not change in echo delay to perceive approaching objects. Also, we played back only constant frequency (CF) component and confirmed that they use the CF component to detect Doppler shift. Furthermore, as a response to the Doppler shift in the perceived echo, bats increased the bandwidth of the terminal component of their pulse. Surprisingly, this response occurred in the very first pulse after Doppler shift, which indicates bats can adapt their echolocation pulse characteristics to changing situations within a pulse. [This work was supported by JSPS KAKENHI Grant Nos. 18H03786 and 16H06542.]