Comparative studies of the biophysics of directional hearing in anurans

Abstract

1) Laser vibrometry was used to study the frequency response of the eardrum as a function of the angle of sound incidence in four species of anurans. Furthermore, the transmission of sound via the body wall and lungs to the internal side of the eardrum and the influence of this lung input on the vibrations of the eardrum were studied. 2) The frequency response of the eardrum was directional in a broad frequency range around its peak. How-ever, the strongest directionality occurred in a narrow frequency range, which did not coincide with the main frequencies of the conspecific advertisement calls. 3) The maximum directionality in this narrow frequency range was 10–25 dB. The maximum directionality at the advertisement call frequencies never exceeded 5–10 dB. 4) Attenuation of the lung input by covering the body wall with vaseline had a significant influence on the frequency response of the eardrum in all species, albeit only in a relatively narrow frequency range. 5) An increase of lung volume caused the narrow frequency range of maximum directionality to move towards lower frequencies. The shift was not sufficient for the maximum directionality to reach the advertisement call frequencies.