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

Abstract

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.