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First published online December 1, 2006
Journal of Experimental Biology 209, 4984-4993 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02594

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Voices of the dead: complex nonlinear vocal signals from the larynx of an ultrasonic frog

Roderick A. Suthers^1,*, Peter M. Narins², Wen-Yu Lin³, Hans-Ulrich Schnitzler⁴, Annette Denzinger⁴, Chun-He Xu⁵ and Albert S. Feng³

¹ School of Medicine, Department of Biology, and Program for Neuroscience, Jordan Hall, 1001 E. Third Street, Indiana University, Bloomington, IN 47405, USA
² Departments of Physiological Science and Ecology and Evolutionary Biology, University of California, Los Angeles, CA 90095, USA
³ Department of Molecular and Integrative Physiology, University of Illinois, Urbana, IL 61801, USA
⁴ Lehrstuhl Tierphysiologie, Zoologisches Institut, Universität Tübingen, D-72076 Tübingen, Germany
⁵ Shanghai Institutes of Biological Sciences, The Chinese Academy of Science, Shanghai, P. R. China

^* Author for correspondence (e-mail: suthers{at}indiana.edu)

Accepted 16 October 2006

Most anurans are highly vocal but their vocalizations are stereotyped and simple with limited repertoire sizes compared with other vocal vertebrates, presumably because of the limited mechanisms for fine vocal motor control. We recently reported that the call of the concaveeared torrent frog (Amolops tormotus Fei) is an exception in its seemingly endless variety, musical warbling quality, extension of call frequency into the ultrasonic range and the prominence of subharmonics, chaos and other nonlinear features. We now show that the major spectral features of its calls, responsible for this frog's vocal diversity, can be generated by forcing pressurized air through the larynx of euthanized males. Laryngeal specializations for ultrasound appear to include very thin portions of the medial vocal ligaments and reverse sexual size dimorphism of the larynx - being smaller in males than in females. The intricate morphology of the vocal cords, which changes along their length, suggests that nonlinear phenomena probably arise from complex nonlinear oscillatory regimes of separate elastically coupled masses. Amolops is thus the first amphibian for which the intrinsic nonlinear dynamics of its larynx - a relatively simple and expedient mechanism - can account for the species' call complexity, without invoking sophisticated neuromuscular control.

Key words: vocal cord, nonlinear dynamics, vocal communication, Amolops

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