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Echoes of bat-pollinated bell-shaped flowers: conspicuous for nectar-feeding bats?

Dagmar v. Helversen^*, Marc W. Holderied and Otto v. Helversen

Zoological Institute, University of Erlangen, Staudtstr. 5, D-91058 Erlangen, Germany
Present address: School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 1UG, UK

View larger version (13K): [in a new window]   Fig. 1. Experimental setup for measuring the echoes of flowers (and other objects). Microphone (Brüel & Kjaer or GRAS 1/4'' condenser microphone) and loudspeaker (15 mm diameter) were as close to each other as possible, with their centers at a distance of 18 mm apart. The flower was fixed on top of a thin holder, which projected from a slender rod, at a distance of 20 cm from the loudspeaker-microphone array, and could be rotated by hand on a turntable.

View larger version (31K): [in a new window]   Fig. 2. Impulse response (A) and spectrum of the impulse function (B) of an echo of the flower of Markea neurantha, irradiated from 4° lateral to the longitudinal axis of the bell. The grey tone gradation corresponds to the attenuation steps of the echoes in Figs 3E, 4E, 5E and 6E.

View larger version (38K): [in a new window]   Fig. 3. Echo fingerprints of flowers in Amphitecna latifolia. (A) Sketch of the flower (scale bar, 20 mm). (B) Duration of the impulse response for 90 directions in the horizontal plane (-90° to +90° in 2° increments). (C) The directional pattern of the impulse response (black, positive amplitudes; white, negative amplitudes; see Fig. 2). (D) Overall amplitude of the echo. (E) Spectral directional pattern from 20 kHz to 140 kHz (black, 0 dB to -6 dB; grey tone gradation in steps of 6 dB corresponding to the scale given in Fig. 2).

View larger version (36K): [in a new window]   Fig. 4. Echo fingerprints of flowers in Markea neurantha. (A) Sketch of the flower (scale bar, 20 mm). (B) Duration of the impulse response for 90 directions in the horizontal plane (-90° to +90° in 2° increments). (C) The directional pattern of the impulse response (black, positive amplitudes; white, negative amplitudes; see Fig. 2). (D) Overall amplitude of the echo. (E) Spectral directional pattern from 20 kHz to 140 kHz (black, 0 dB to -6 dB; grey tone gradation in steps of 6 dB corresponding to the scale given in Fig. 2).

View larger version (38K): [in a new window]   Fig. 5. Echo fingerprints of flowers in Crescentia cujete. (A) Sketch of the flower (scale bar, 20 mm). (B) Duration of the impulse response for 90 directions in the horizontal plane (-90° to +90° in 2° increments). (C) The directional pattern of the impulse response (black, positive amplitudes; white, negative amplitudes; see Fig. 2). (D) Overall amplitude of the echo. (E) Spectral directional pattern from 20 kHz to 140 kHz (black, 0 dB to -6 dB; grey tone gradation in steps of 6 dB corresponding to the scale given in Fig. 2).

View larger version (35K): [in a new window]   Fig. 6. Echo fingerprints of flowers in Vriesea gladioliflora. (A) Sketch of the flower (scale bar, 20 mm). (B) Duration of the impulse response for 90 directions in the horizontal plane (-90° to +90° in 2° increments). (C) The directional pattern of the impulse response (black, positive amplitudes; white, negative amplitudes; see Fig. 2). (D) Overall amplitude of the echo. (E) Spectral directional pattern from 20 kHz to 140 kHz (black, 0 dB to -6 dB; grey tone gradation in steps of 6 dB corresponding to the scale given in Fig. 2).

View larger version (9K): [in a new window]   Fig. 7. Oscillogram (top) and sonagram (bottom) of a typical echolocation call of Glossophaga soricina and two echoes of the flower Markea neurantha (achieved by FIR-filtering the call with the impulse response at 0° and -30°). Sonagram parameters: window size (N); frame size (F); window overlap (O); window: flattop (FLT); resulting bandwidth (BW); software: SASLAB Pro 4.2; Avisoft.

View larger version (36K): [in a new window]   Fig. 8. Spectral directional pattern of a hollow hemisphere (A), a paraboloid (B) and an ellipsoid (C). All three hollow forms had the same circular opening with a diameter of 36 mm but different depths (for exact parameters, see text). Above each spectral pattern the relative intensity of the echo, averaged over the frequency range from 20 kHz to 140 kHz, is plotted as a function of the angle of sound incidence.