Behavioral and Physiological Bases for Doppler Shift Compensation by Echolocating Bats

Abstract

Horseshoe bats (Rhinolophidae), Old World leaf-nosed bats (Hipposideridae), and several species of moustached bats (Mormoopidae), including Parnell’s moustached bat (Pteronotus parnellii), Paraguayan moustached bat (Pteronotus paraguayensis) and Mesoamerican moustached bats (Pteronotus mesoamericanus) emit pulses with long constant frequency (CF) components and have evolved mechanisms for separating the pulse and echo in frequency. This approach to echolocation—high duty cycle (HDC) echolocation—depends largely on Doppler shift compensation (DSC). In 1968, Schnitzler discovered that the greater horseshoe bat (Rhinolophus ferrumequinum) compensates for flight-induced Doppler shifts in the CF component of echoes by adjusting their call frequency, ensuring a stable echo frequency during flight. This significant behavioral adaptation is supported by an acoustic fovea, a striking morphological and physiological specialization occurring from the peripheral to the central auditory system in HDC bats. The auditory fovea has neurons with high sensitivity to the narrow frequency range of the CF components of echolocation calls of HDC bats. Doppler shift compensation of the frequency that dominates the echolocation calls maintains the frequency within the narrow range of the acoustic fovea. This combination of CF calls, HDC echolocation, and DSC allows fine-frequency analysis of acoustic glints in echoes from insects fluttering their wings, which makes these bats very effective at detecting flutter.