Animals' sound-producing organs often act as an integrated whole — particular vocal structure are not directly associated with the creation of discrete syllables^{1, 2, 3, 4}. But here we show that the 'chuck' of the 'whine-chuck' mating call of the túngara frog, Physalaemus pustulosus, is caused by a fibrous mass attached to the vocal folds; the chuck is eliminated by removal of this structure, although the frog still tries to produce the sound. Sexual selection affects the acoustic complexity of the frog's call^{5, 6, 7}, so evolution may have shaped this unusual vocalization, which is akin to the two-voiced song of songbirds^{6, 8}.

1. Departamento de Zoologia, Instituto de Biociências, Universidade Estadual Paulista, Rio Claro, São Paulo 13506-900, Brazil
2. Smithsonian Tropical Research Institute, Apartado 2072, Balboa, Panama
3. Section of Integrative Biology, University of Texas, Austin, Texas 78712, USA
4. Present address: Department of Physiological Science, University of California, Los Angeles, California 90095, USA

Correspondence to: M. J. Ryan^2,3 Email: mryan@mail.utexas.edu

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