Role of air sinuses in sound reception of the Yangtze finless porpoise: A numerical study.

Abstract

Although air sinuses are prevalent in odontocetes and are an integral component of their sound reception system, the acoustic function of these air-filled structures remains largely unknown. To address this, we developed a numerical model using computed tomography data from a Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis) to investigate the role of the air sinuses in sound reception. By comparing sound reception characteristics between model cases with and without the air sinuses, we found that the air sinuses improved sound reception directivity. Across frequencies from 1 to 100 kHz, the directivity indexes for cases with and without the air sinuses ranged from 0.35 to 5.64 dB and 0.23 to 4.12 dB, respectively. Additionally, the air sinuses increased amplitude differences in received sounds, with maximum values of 2.05, 2.78, and -2.38 dB for the front-to-behind, ipsilateral-to-contralateral, and top-to-bottom aspects, respectively. These results indicate that the air sinuses effectively provided acoustic isolation for the bony ear complexes from the behind, contralateral, and top aspects, thereby enhancing asymmetric sound reception dominated by the front, ipsilateral, and bottom aspects. This study contributes to a deeper understanding of odontocete sound reception and sheds light on the significant role of the air sinuses in this context.