Comparison of female and male vocalisation and larynx morphology in the size dimorphic foot-flagging frog species Staurois guttatus

In anurans, males have larger laryngeal structures than females and produce conspicuous species-specific calls in various social contexts. Knowledge of female vocalisations is not well established and we start by summarising available spectral and behavioural information on calls in females. We then present novel data on female and male calls in Staurois guttatus and ask how larynx morphology influences call characteristics. While there was no difference in the dominant frequency between the sexes, sound pressure of female calls was lower than in males suggesting that they could be masked by ambient stream noise in the natural habitat. In an experimental setup, unreceptive females started calling when approached by a male less than 30 cm away, indicating an agonistic function of calling behaviour. In accordance with the overall size dimorphism in S. guttatus , laryngeal muscles as analysed by microCT were larger in females than in males whereas a reverse dimorphism was reported for most anuran species with silent and vocal females. We argue that in noisy environments such as streams, small male larynx size associated with high frequency calls is advantageous due to reduced masking and discuss the functional differences and communalities in signalling behaviour between the sexes and in the genus Staurois.