Male túngara frogs appear in choruses and have well-defined vocal cords at a size of 19 mm snout-vent length (SVL), but they do not begin producing advertisement calls until they reach approximately 24 mm SVL. Is this because the larynx is not fully mature until the latter point, or is it because the calls of small-bodied adults would not be attractive enough to outweigh the costs (energy, predation and parasitism) of producing them? We investigate this question by tuning bond graph models of the túngara frog larynx with parameter values across this critical body size range. Simulations were informed by a new set of diceCT-based measurements of key laryngeal morphological features from an ontogenetic series. Models produce sustained oscillations in airflow through the larynx (sound) with species-typical spectral features across the full range of the ontogenetic ‘silent zone’, suggesting that muteness during this period is facultative and not physically enforced. This result illuminates the morphological and behavioural consequences of an interplay of natural and sexual selection. Sexual selection and developmental constraints may lead the early development of laryngeal vibratory tissues, while natural selection (energetic costs and predation avoidance) and sexual selection (reduced call attractiveness) act to mute smaller adult frogs.
Vocalisation, ontogeny, morphology, sexual selection, diceCT, bond graph