Relatively few studies have used experimental manipulations to investigate the mechanics of vocal production in frogs and toads, even though many frogs produce complex signals with multiple components and/or nonlinearities. Modelling approaches can add to empirical studies by illuminating how various components of the vocal system interact to produce communication signals. In this study, we use bond graphs, a lumped-element modelling technique, to explore how the combination of active modulation of vocal production and the passive dynamics of a unique laryngeal structure result in the complex calls produced by males in an anuran model system. The túngara frog (Physalaemus (= Engystomops) pustulosus) produces advertisement calls with a ‘whine’ and a facultative ‘chuck’. Whines are amplitude and frequency modulated. The chuck is characterized by its spectral complexity and appears to contain a period doubling bifurcation resulting in subharmonics. In our model, we focus on how a fibrous mass attached to the vocal cords results in subharmonics in the chuck. Our models suggest that active (neural) modulation of the fibrous mass is not necessary for the transition between the spectral characteristics of the whine and chuck. Rather, it is possible that the vibratory mode of the fibrous mass and thus the vocal cords changes passively as a result of changes in airflow through the system.
Anuran, túngara frog, advertisement call, fibrous mass, vocal system model, nonlinear