Pinna-rim skin folds narrow the sonar beam in the lesser false vampire bat (Megaderma spasma)

False vampire bats (genus Megaderma) employ active as well as passive biosonars. In the present work, the acoustic impact of a conspicuous feature of the animals' ear morphology, skin folds of the pinna rim linking the two pinnae at the midline, has been studied using a numerical approach. Automated methods have been devised to measure the largest width of the beam patterns irrespective of beam orientation. A total of six pinna shapes from three individuals have been studied. For all these shapes, it was found that the reception biosonar beams had approximately elliptic cross-sections with the largest beamwidth being on average almost twice as large as the beamwidth in the orthogonal direction. The directions of the largest beamwidths were scattered around the azimuthal dimension. Removal of the skin folds resulted in significant widening of the beams along their widest dimensions with an increase in beamwidth of 9.2 degrees (a 30% change) on average. The strength and repeatability of this effect across individuals suggest the hypothesis that skin folds are functionally relevant to the animals' biosonar system. It may be a morphological adaptation to biosonar tasks that benefit from a narrow beam such as the detection of faint sounds or precise localization.