Key Laboratory of Underwater Acoustic Communication and Marine Information Technology, Ministry of Education, College of Ocean and Earth Sciences, Xiamen University, Xiamen 361000, China
Fund Project:Project supported by the National Key R&D Program of China (Grant No. 2018YFC1407504) and the National Natural Science Foundation of China (Grant Nos. 41676023, 41276040)
Received Date:22 June 2020
Accepted Date:16 July 2020
Available Online:25 November 2020
Published Online:05 December 2020
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. Keywords:Indo-Pacific humpback dolphin/ sound reception/ bioacoustics/ acoustic fat
通过CT影像数据可以读取其CT扫描结果单位, 亨氏单位(Hounsfield unit, HU). 通过实体解剖可获取中华白海豚回声定位系统的软组织样本, 结合超声测量可获取软组织的声速, 进而重建出中华白海豚整个回声定位系统的声速分布[13]. 中华白海豚的软组织样本数量有限, 无法覆盖整个头部. 实际测量中, 首先利用CT影像数据获取中华白海豚整个头部的HU分布, 如图1(b)与图1(c)所示. 解剖过程中, 详细记录海豚皮肤的厚度、海豚头部回声定位系统组织样本的尺寸. 随后, 根据记录在CT影像中定位样本, 读取样本HU值. 将样本的HU值与样本的声速、密度进行回归分析, 获取HU与声速、HU与密度之间的关系. 此后, 根据HU分布以及HU与声速、HU与密度之间的关系, 重建整个中华白海豚头部的声速、密度分布. 图2展示了中华白海豚声接收系统在水平截面(xz截面)和垂直截面(yz截面)声速、密度分布. 图2(a)与图2(c)显示的头部听觉系统的声参数分布表明下颌脂肪的声速与密度小于外层的肌肉和结缔组织. 下颌骨内侧脂肪与下颌骨外侧脂肪的声速在1300—1400 m/s之间, 密度在900—1000 kg/m3之间. 下颌内部脂肪与下颌外部脂肪分布于盘骨的内、外两侧, 组成低声速通道. 图 2 中华海豚头部声接收系统不同截面的声速、密度分布 (a) 水平截面(xz)声速分布; (b) 垂直截面(yz)声速分布; (c) 水平截面(xz)密度分布; (d) 垂直截面(yz)密度分布 Figure2. Distributions of sound speed and density in different planes of reception system in Indo-Pacific humpback dolphin: (a) Sound speed distribution in horizontal plane; (b) sound speed distribution in vertical plane; (c) density distribution in horizontal plane; (d) density distribution in vertical plane.