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What determines the tuning of hearing organs and the frequency of calls? A comparative study in the katydid genus Neoconocephalus (Orthoptera, Tettigoniidae)

Johannes Schul^* and Adam C. Patterson

Division of Biological Sciences, University of Missouri, 207 Tucker Hall, Columbia, MO 65211, USA

View larger version (41K): [in a new window]   Fig. 1. Spectra of male calls of five species of Neoconocephalus: (A) N. nebrascensis (N=10), (B) N. bivocatus (N=8), (C) N. robustus (N=10), (D) N. retusus (N=9) and (E) N. ensiger (N=3). The thick line represents the averaged spectrum of each species, while the thin lines denote the individual spectra contributing to the averaged spectrum. Spectra are calculated as fast Fourier transform (1024 points window length) averaged over 1 s.

View larger version (19K): [in a new window]   Fig. 5. Response amplitudes of the averaged tympanic nerve responses in Neoconocephalus nebrascensis. (A) Example of the averaged responses recorded from the tympanic nerve to 12 kHz and 50 kHz sine waves (10 ms duration; 1 ms rise/fall time) of various amplitudes. Stimulus amplitudes are given relative to the threshold at each frequency. (B) Stimulus amplitude/response function of the mean response amplitudes (mean ± S.E.M., N=15 in nine animals) of N. nebrascensis during stimulation with 12 kHz and 50 kHz. Stimulus amplitudes are given relative to the threshold at both frequencies. Response amplitudes are given as arbitrary units. (C) Breadth of the first oscillations (see inset) of the averaged responses (mean ± S.E.M., N=15 in nine animals) of N. nebrascensis during stimulation with 12 kHz and 50 kHz. Stimulus amplitudes are given relative to the threshold at both frequencies.

View larger version (15K): [in a new window]   Fig. 7. Attenuation of pure-tone sound pulses (7-40 kHz) over distance, as measured in a typical biotope of the five Neoconocephalus species. The curve fittings (solid lines) were calculated using the formula y=a*x+b*log(x)+c. The dotted line indicates the theoretical attenuation due to spherical spreading alone (6 dB per double distance).

View larger version (9K): [in a new window]   Fig. 2. Position and width of the low-frequency band of the calls of five Neoconocephalus species at -3 dB (thick bar) and -10 dB (thin bar): N. nebrascensis (N=10), N. bivocatus (N=8), N. robustus (N=10), N. retusus (N=9) and N. ensiger (N=3). The position of the mean peak frequency is indicated by a vertical line.

View larger version (27K): [in a new window]   Fig. 3. Hearing threshold (mean ± S.D.) of males (filled squares) and females (open circles) of five species of Neoconocephalus: (A) N. nebrascensis males (N=10 in seven insects) and females (N=8 in five insects), (B) N. bivocatus males (N=11 in six insects) and females (N=8 in five insects), (C) N. robustus males (N=9 in six insects) and females (N=5 in three insects), (D) N. retusus males (N=9 in five insects) and females (N=10 in five insects) and (E) N. ensiger males (N=8 in five insects) and females (N=4 in two insects).

View larger version (16K): [in a new window]   Fig. 4. Hearing thresholds (mean ± S.D.) of five species of Neoconocephalus relative to the highest sensitivity. Absolute sensitivity at this point ranged between 27.5 dB SPL and 31.8 dB SPL. The peak frequency (vertical lines) and the -3 dB band (horizontal lines) of the low-frequency component of the calls are indicated by vertical lines. N. robustus, N=14 in nine animals; N. bivocatus, N=19 in 11 animals; N. nebrascensis, N=18 in 12 animals; N. ensiger, N=12 in seven animals; N. retusus, N=19 in 10 animals.

View larger version (40K): [in a new window]   Fig. 6. Iso-intensity response function of the averaged responses recorded from the tympanic nerve in five species of Neoconocephalus (mean ± S.E.M.). Response amplitudes were measured at 9 dB, 18 dB, 27 dB and 36 dB above threshold of each frequency. (A) N. nebrascensis, N=15 in nine animals; (B) N. bivocatus, N=19 in 11 animals; (C) N. robustus, N=14 in nine animals; (D) N. retusus, N=19 in 10 animals; (E) N. ensiger, N=12 in seven animals.

View larger version (20K): [in a new window]   Fig. 8. Attenuation of pure-tone sound pulses (5-40 kHz) exceeding the spherical attenuation of 6 dB per double distance (`excess attenuation') at various distances. The attenuation was measured in a typical grassland habitat of the five Neoconocephalus species; the data shown here are taken from the best-fit curves for each frequency (see Materials and methods; Fig. 6).

View larger version (12K): [in a new window]   Fig. 9. Sound intensity above hearing threshold of pure-tone signals of 9 kHz and 18 kHz, as perceived by a female Neoconocephalus nebrascensis, over distance between sender and receiver. Data were calculated by assuming a signal amplitude of 110 dB SPL at 20 cm distance and using the measured data for the attenuation during sound transmission of pure tones (Figs 6, 7) and the hearing thresholds (Fig. 4). At approximately 1 m distance, both pure-tone signals are perceived at the same relative intensity (`break-even' distance; arrow).

View larger version (19K): [in a new window]   Fig. 10. Break-even distances for pure-tone signals relative to an 18 kHz signal. At the break-even distance, both signals are perceived by a female with the same intensity above threshold (see Fig. 8). For longer distances, the lower frequency is perceived louder, whereas for shorter distances, 18 kHz is perceived louder.