Noseleaf Furrows in a Horseshoe Bat Act as Resonance Cavities Shaping the Biosonar Beam

Horseshoe bats emit their ultrasonic biosonar pulses through nostrils surrounded by intricately shaped protuberances (noseleaves). While these noseleaves have been hypothesized to affect the sonar beam, their physical function has never been analyzed. Using numerical methods, we show that conspicuous furrows in the noseleaf act as resonance cavities shaping the sonar beam. This demonstrates that (a) animals can use resonances in external, half-open cavities to direct sound emissions, (b) structural detail in the faces of bats can have acoustic effects even if it is not adjacent to the emission sites, and (c) specializations in the biosonar system of horseshoe bats allow for differential processing of subbands of the pulse in the acoustic domain.

Figure 1

Noseleaf boundary of the finite-element mesh on which the numerical calculations were carried out. (a) Mesh boundary of the representation of the natural shape of the noseleaf: the location of the point sources is marked by black voxels; the inner surfaces of the lancet furrows (arrows) are marked by dark gray voxels. (b) Mesh boundary for the modified shape with filled-in lancet furrows: the voxels representing added material are rendered in white; the remainder of the mesh boundary and the location of the sources remained unchanged. The length of the scale bar is 2 mm.

Figure 2

Spatial selectivity of the resonance: sound pressure amplitude (magnitude of the time-harmonic solution to the Helmholtz equation) for 60 kHz in the near field coded by gray (color) value on a linear scale. Arrows indicate the local spatial maxima inside the lancet furrows.

Figure 3

Frequency selectivity of the resonance: normalized sound pressure amplitudes in the lower (dashed line) and upper (thin solid line) right lancet furrow as a function of frequency. The frequency band displayed extends beyond the range known to be used by the bat (marked by downward triangles) in order to show the resonance behavior unequivocally. The spacing of the data points is 500 Hz, and the resonance amplitude value at each frequency was estimated based on 20 (lower furrow) or 19 (upper furrow) sample nodes located within a sphere of 0.2 mm radius; the maximum normalized standard deviation for these samples was 0.03. Superimposed on the resonance curves is the maximum change in the directivity function relative to its global maximum in percent (thick solid line). The inset shows a schematic spectrogram of a CF-FM biosonar pulse; spectrograms of pulses measured in Rhinolophus rouxi are given in 25.

Figure 4

Orthographic map projections of the numerical estimates of the directivity function at 60 kHz. (a) For the unaltered noseleaf shape [Fig. 1a]; the arrow indicates the location of the upward extension of the beam. (b) For a modified noseleaf with all lancet furrows filled [Fig. 1b]. The amplitude of the directivity function is linearly encoded by the gray scale, where black represents the maximum value. Contour lines are spaced 10% of the range apart. The contour estimates are based on 65 160 function values (resolution of 1° in azimuth and elevation) each.