Sound reception pathway of the Indo-Pacific humpback dolphin

Abstract

The Indo-Pacific humpback dolphins (Sousa chinensis) are nearshore odontocetes, distributed in tropical and sub-tropical oceans. This species has been studied to unveil its ability to echolocate. Indo-Pacific humpback dolphin, like its Odontocetes companion, relies on echolocation system to navigate and detect targets, which contains a sound transmitting system in the forehead and a sound reception in the jaw. Their soft tissues present gradient sound speed and density distributions in the forehead. Solid skull, air structures and soft tissues form a natural multi-phase meta-material to modulate sounds into energy focused beams. This multi-phase property is also applied to the hearing system as revealed in current papers. Here in this work, the physical mechanism of sound reception in the Indo-Pacific humpback dolphin is studied by using the computed tomography (CT) scanning, physical measurements and numerical simulation. Hounsfield units (HUs) of the forehead tissues are extracted from CT scanning results. A linear relationship is revealed between HU and sound speed, HU and density, which are combined with HU distribution to reconstruct the sound speed and density distribution of the sound reception system. The CT scanning shows that the sound reception system located at lower head is composed of external mandibular fat, internal mandibular fat, mandible and hearing bones. Model of sound reception system is developed on the basis of CT scanning results and used in subsequent simulations. The physical process of sound reception reveals that the hearing system can guide sounds through variable pathways to reach hearing bones. Sounds can enter into the reception system along the acoustic pathways composed of mandible, external mandibular fat and internal mandibular fat. Mandibular fat and mandible form a unique sound pathway. In addition, another pathway which is composed of external mandibular fat, pan bone and internal mandibular fat can lead the sound to propagate and finally arrive at hearing bones. The diversity of acoustic pathways is applicable to a range of frequencies from 30 to 120 kHz. The variability of acoustic pathways in Indo-Pacific humpback dolphin shows the complexity of its biosonar system. The anatomy and simulation results can deepen our understanding of the mechanism of echolocation of Indo-Pacific humpback dolphin and provide references for designing man-made sound reception devices.