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

Chirps (i.e., whistles shorter than 0.25 s) 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. Dolphins emitted chirps either as single units or in trains. Chirp trains, defined as sequences of at least three chirps, averaged 69.64 ± 26.01 ms in duration and presented inter-chirp intervals of 124.87 ± 79.69 ms. Differences between single chirps and chirps produced within chirp trains included 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 events (single chirps and chirp trains). Significantly higher chirp train production rates were found in larger groups and during socializing than during other activities, suggesting that these vocalizations likely serve a role in social communication. Variations in chirp train production rate were noted across the three studied geographic 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. Delphinid species produce short duration whistles known as chirps. The production of chirps by Indo-Pacific humpback dolphins was analyzed and revealed that these dolphins could not only emit chirps as distinct units (e.g., single chirps), but also within chirp trains. Differences in duration, frequency, and structure were observed between single chirps and chirps produced within chirp trains suggested they represent distinct vocalization types. Dolphins produced chirp trains predominantly in large socializing groups, indicating a potential role in social communication. In addition, variations in chirp train production rates across populations may be attributed to environmental and social structure differences. These findings provide insights into the unique characteristics and potential social functions of chirp trains in Indo-Pacific humpback dolphins, informing our understanding of cetacean acoustic communication.