The external ears as receiving antennas in echolocating bats

Abstract

The external ears of bats appear to vary greatly in size and shape, but they in fact have dimensions specifically related to the structure of sonar sounds in different species. They determine the directionality of hearing and produce cues for vertical target localization. The pinna of broadband (FM) bats is equal in length to the wavelength of the lowest first‐harmonic frequencies in the sonar signals, even if the bat only broadcasts higher‐order harmonics into the air. The bladelike tragus focuses the directional sensitivity of hearing more sharply and cleanly to the front than the pinna alone can achieve. It also produces secondary reflections within the external ear which encode target elevation and which are necessary for accurate vertical localization of targets. Narrow‐band (CF) bats have a pinna 5 to 8 times longer than the wavelength of predominant signal frequencies and have sharper directional sensitivity than broadband bats, which have a pinna only 3 to 4 times the predominant wavelengths. The configuration of the external ear matches the timing of tragus reflections to the period (autocorrelation) structure of FM sonar sounds to avoid average‐phase ambiguity in perception of target elevation.