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First published online March 14, 2005
Journal of Experimental Biology 208, 1209-1217 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01511
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Directionality of the lizard ear

Jakob Christensen-Dalsgaard1,* and Geoffrey A. Manley2

1 Center for Sound Communication, SDU Odense University, Campusvej 55, DK-5230 Odense M, Denmark
2 Lehrstuhl für Zoologie, Technische Universität München, Lichtenbergstrasse 4, 85747 Garching, Germany



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  		Fig. 1. Eardrum vibration velocity spectra measured at different sound intensities in (A) Leiolepis and (B) Gekko and normalized by division by the sound spectrum measured at the eardrum. Under the assumption of linearity, the normalized spectra in each figure should be identical.

 


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  		Fig. 2. Eardrum vibration velocity spectra (amplitude, dB) for the four species studied: Mabuya (A), Leiolepis (B), Ctenosaura (C) and Gekko (D). The spectra are normalized by division by the sound spectra measured at the eardrum. Thick line: ipsilateral stimulation, broken line: contralateral stimulation. Values are in dB re 1 mm s-1 Pa-1, i.e. 0 dB corresponds to a vibration velocity of 1 mm s-1 at 1 Pa (94 dB SPL).

 


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  		Fig. 3. Cylinder surface plots (for details, see Materials and methods) of eardrum directionality in the four lizard species Mabuya (A), Leiolepis (B), Ctenosaura (C) and Gekko (D). The normalized velocities (colour scale, in dB re 1 mm s-1 Pa-1) are plotted as a function of direction (x-axis, contralateral angles on the left and ipsilateral angles on the right) and frequency (y-axis). The right and left row of figures show line plots of velocity (y-axis, dB re 1 mm s-1 Pa-1) as a function of direction (x-axis) at three frequencies (1000, 2000 and 3000 Hz, arrows), corresponding to three horizontal lines in the cylinder surface plot. (E,F) Eardrum directionality after occluding one eardrum by a dome of Vaseline. E, Mabuya; F, Leiolepis.

 


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  		Fig. 4. Diffraction measured as ipsilateral-contralateral sound pressure difference measured by a probe microphone at the eardrum in Mabuya (A), Leiolepis (B), Ctenosaura (C) and Gekko (D).

 


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  		Fig. 5. Interaural difference plot modeling the output of a binaural difference (EI) neuron in the four lizard species Mabuya (A), Leiolepis (B), Ctenosaura (C) and Gekko (D). The eardrum vibration data set is subtracted from its reflection along the body axis. Other details as in Fig. 3, except that the colour scale now is relative interaural differences in dB. The line plots in the left and right column show relative interaural differences (y-axis) as a function of direction (x-axis) at three frequencies (1000, 2000 and 3000 Hz, arrows). Note that the interaural differences include the effect of sound diffraction by the body of the lizard.

 


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  		Fig. 6. Cylinder surface plots of eardrum directionality (A) and interaural difference (B) of a grass frog, Rana temporaria. Other details in A as in Fig. 3A-D; in B as in Fig. 5A-D.

 


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  		Fig. 7. A simple model of lizard ear directionality (Fletcher 1992Go). (A) Diagram of a transverse section of a lizard head (Sceloporus; redrawn and altered from Wever, 1978Go). TM, tympanic membrane; C, columella; ET, Eustachian tube; MEC, middle ear cavity; RW, round window; OW, oval window. (B) The electrical analog circuit (ZV impedance of mouth cavity, ZT impedance of tympana, P1 and P2 sound inputs; details of parameters in Materials and methods). (C) Vibration velocity spectra for ipsilateral (red curve) and contralateral (black curve) sound directions (compare Fig. 2). (D) Cylinder surface plot of the model response (compare Fig. 3).

 

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© The Company of Biologists Ltd 2005