Do mounting vocalisations in tortoises have a communication function? A comparative analysis

We review the occurrence of vocalisations in tortoise courtship in order to investigate their functions, if any, taking into account evolutionary history, habitat and body size. Courtship behaviour has been described for 101 (41%) of all chelonian species. Among them, mount vocalisations occur in 35 species belonging to families Testudinidae Batsch, 1788 (n=29), Trionychidae Fitzinger, 1826 (n=3), Emydidae Rafinesque, 1815 (n=2), and Bataguridae = Geoemydidae Theobald, 1868 (including Batagurinae Gray, 1869) (n=1). The mapping of vocalisation evolution along the phylogenetic tree revealed that mount vocalisations are an ancestral trait, being present from the origin of Cryptodira, and calls mainly occur in terrestrial or semi-terrestrial species. In the species and subspecies of Testudinidae we considered, mounting-calls show an harmonic structure with frequency and amplitude modulation, which are acoustic features not compatible with mechanisms of sound production based simply on air flow through respiratory tracts. Moreover, the call fundamental frequency was negatively related to body size. Since in birds and amphibians such size-frequency relationship is due to a correlation existing between body size and mass of the vibrating device used to produce sounds (a greater device produces longer wavelengths and consequently low frequencies), in tortoises vocalisations might also be produced by vibrating structures not yet described. All these findings strongly suggest that mounting-calls might have the function to convey information on the signaller to conspecifics, possibly influencing the outcome of sexual interactions.