Numerical modeling of whistle reception directivity in Indo-Pacific humpback dolphins (Sousa chinensis)

Odontocetes produce diverse acoustic signals, among which whistles play a pivotal role in social communication. However, studies on whistle directivity remain limited, particularly regarding sound reception. In this study, we examined the reception directivity of a constant whistle using three-dimensional numerical models based on computed tomography scans of a calf and an adult Indo-Pacific humpback dolphin (Sousa chinensis). Our results showed that the mandibular fats gathered sounds from the front, while the air sinuses blocked sounds from behind, above, and the contralateral side, thereby enhancing directional reception. Across frequencies from the fundamental at 3.8 kHz to the seventh harmonic at 30.2 kHz, the binaural directivity indices (DIs) of the calf and the adult dolphin ranged from 1.7 to 5.1 dB and from 2.4 to 7.2 dB, respectively. The overall binaural DIs were 1.9 dB in the calf and 2.7 dB in the adult, exceeding those at the fundamental frequency by 0.2 and 0.3 dB, respectively, indicating that the harmonics enhanced the overall whistle reception directivity. The adult exhibited greater whistle reception directivity than the calf, primarily due to its larger size. This study advances our understanding of the mechanisms shaping whistle directivity and its size-dependent variability in odontocetes.