Production of chirp trains by Indo-Pacific humpback dolphins (Sousa chinensis) in the northern South China Sea

Chirps (i.e., whistles shorter than 0.25s) have been reported across various delphinid species but the context of their production and function remains underexplored. In this study, the production of chirps was examined in free-ranging Indo-Pacific humpback dolphins across three locations in the northern South China Sea. The chirps produced by dolphins could be emitted as distinct units (i.e., single chirps) or within chirp trains. These chirp trains were composed of at least three chirps that lasted 69.64 ± 26.01 ms in average and presented inter-chirp intervals of 124.87 ± 79.69 ms. Differences presented between single chirps and individual chirps produced within chirp trains including a longer duration (179.37 ± 45.10 ms) for single chirps as well as frequency and contour differences suggesting the existence of two distinct whistle types (single chirps and chirp trains). Higher chirp train production rates were found in larger groups ( p  < 0.05) and during socializing than during other activities ( p  < 0.05), suggesting that these vocalizations likely serve a role in social communication. The presence of young in the group was not linked with higher chirp train production, therefore these signals may be primarily used for inter-adult communication. Furthermore, variations in chirp train production rate were noted across different populations, potentially due to the geographic separation of populations, habitat differences, and differences in social structure, and communication. These findings provide insights into the unique characteristics and potential social functions of chirp trains in Indo-Pacific humpback dolphins, addressing a gap in cetacean acoustic communication.