Courtship and underwater communication in the Sunda gharial (Tomistoma schlegelii)

Many threatened crocodilians are shy and difficult to observe in the wild, limiting the current knowledge of their behaviour and ecology. Although acoustic communication is commonly used by adults during courtship and territory defence, the signals produced vary between taxa. Here we provide the first classification of adult vocalisations in Tomistoma schlegelii–a rare gavialid inhabiting swamp forests in Southeast Asia–and describe the behaviour produced during courtship. Using video and passive underwater acoustic monitoring we recorded a breeding captive pair of T. schlegelii housed indoors, as well as monitored 12 captive adults kept in a semi-natural outdoor enclosure over a period of two months. Courtship behaviour during 18 recorded mating events followed that observed in other crocodilians, but acoustic signals were restricted to mating activity and were only recorded in the two animals housed indoors. We identified seven signal types produced by T. schlegelii, which differed in duration, dominant frequency, and the presence of harmonic structure. These results suggest the species uses short-range, underwater acoustic signals during courtship and mating, with little vocal activity outside of those events, and provide foundation for future studies of this rare crocodilian.