Ears adapted to detect motion, or how echolocating bats exploit the capacities of the mammalian auditory system

Abstract

Bats avail themselves of the rich food resources of the night by specializing in audition. They emit short echolocation sounds and listen to the echoes returning from potential prey and the environment. The bat's auditory system analyzes spectral and temporal parameters of echoes for detecting, precisely locating, and identifying a target. Different bat species have solved the problem of acoustic target detection and pattern recognition even in heavily clustered situations by focusing on certain acoustical features of the targets. The specialized motion defection by Horseshoe bats, for instance, analyzes small echofrequency shifts modulated onto a long constant frequency echolocation signal. These frequency modulations are Doppler shifts of echoes returning from wing beating insects. For detecting modulations as small as 10 Hz or 0.01%, Horseshoe bats have in the cochlea an extremely narrow filter (Q = ca. 500) matched to the carrier frequency (i.e., echolocation sound) of 83 kHz. The filter is based on mechanical principles and realized by structural differentiations of the basilar membrane. We have called this specialized patch of the basilar membrane an “acoustical fovea.” In analogy to the visual fovea the “foveal frequencies” are largely over‐represented in the tonopical arrangement of the ascending auditory pathway. The bats have developed a feedback system which lowers the emitted echolocation frequency in such a way that the Doppler shifted echofrequency is kept precisely at a fixed reference frequency close to 83 kHz. This feedback system and additional neuronal data disclose an intricate coupling of the auditory and vocalizing system. The evolution of echolocation in bats has driven the analyzing capacities of audition close to theoretical limits. Investigations in such specialized systems give fascinating insights into possible general principles of auditory information processing.