Acoustic property reconstruction of a pygmy sperm whale (Kogia breviceps) forehead based on computed tomography imaging

Abstract

Computed tomography (CT) imaging and sound experimental measurements were used to reconstruct the acoustic properties (density, velocity, and impedance) of the forehead tissues of a deceased pygmy sperm whale (Kogia breviceps). The forehead was segmented along the body axis and sectioned into cross section slices, which were further cut into sample pieces for measurements. Hounsfield units (HUs) of the corresponding measured pieces were obtained from CT scans, and regression analyses were conducted to investigate the linear relationships between the tissues' HUs and velocity, and HUs and density. The distributions of the acoustic properties of the head at axial, coronal, and sagittal cross sections were reconstructed, revealing that the nasal passage system was asymmetric and the cornucopia-shaped spermaceti organ was in the right nasal passage, surrounded by tissues and airsacs. A distinct dense theca was discovered in the posterior-dorsal area of the melon, which was characterized by low velocity in the inner core and high velocity in the outer region. Statistical analyses revealed significant differences in density, velocity, and acoustic impedance between all four structures, melon, spermaceti organ, muscle, and connective tissue (p < 0.001). The obtained acoustic properties of the forehead tissues provide important information for understanding the species' bioacoustic characteristics.

Evolution of cranial telescoping in echolocating whales (Cetacea: Odontoceti)

“…; Goold and Clarke ; Song et al. ; Thornton et al. suggest acoustic specializations linked to this unusual structure, but some of the unique cranial and soft tissue structures of the head of Physeter may be derived for ramming behaviors (Panagiotopoulou et al.…”

“…Beyond the bony anatomy, physeteroids also are the only living odontocetes to lack a melon, instead possessing a spermaceti organ, and uniquely retain distinct left and right nasal tracks that lead to a single blowhole (Cranford 1999;Heyning 1989). Studies of the roles of skull and soft tissues of Physeter (Huggenberger et al 2014;Song et al 2017) and Kogia (Karol et al 1978;Goold and Clarke 2000;Song et al 2015;Thornton et al 2015 suggest acoustic specializations linked to this unusual structure, but some of the unique cranial and soft tissue structures of the head of Physeter may be derived for ramming behaviors (Panagiotopoulou et al 2016;Alam et al 2016) as well. The bizarre cranial morphology described above has resulted in Kogia having a significant influence on the identification of major sources of variation in our facial morphology dataset; Kogia forms an extreme outlier for PC 4 of the whole skull dataset and principle components 3, 4, and 6 for the half skull dataset.…”

The influence of air-filled structures on wave propagation and beam formation of a pygmy sperm whale (Kogia breviceps) in horizontal and vertical planes

“…These hypotheses were supported in part by a subsequent study by Song et al, through the reconstruction of the acoustic property distributions of a pygmy sperm whale forehead. 27 The results suggested that the sounds generated at the phonic lips would propagate into the spermaceti organ. These sounds would likely be reflected by air-filled structures, soft tissues, and the skull before forming sound beams.…”

“…Pygmy sperm whale beam formation studies could deepen our understanding of its biosonar system. Although the sound propagation of this species has been proposed and discussed in previous studies by interpreting the species' lipid topography and morphology, [24][25][26][27] no direct study has been conducted to verify the proposals. The sounds produced at the source were proposed to be divided into two parts.…”

“…[14][15][16][17][18][19][20][21] However, few were conducted on the pygmy sperm whale, mainly from morphology perspectives. [22][23][24][25][26][27] The pygmy sperm whale (Kogia breviceps), has a larger head than those of bottlenose dolphins (Tursiops truncatus) and the Indo-Pacific humpback dolphin (Sousa chinensis), and K. breviceps was reported to produce narrowband and high frequency clicks, with peak frequencies ranging from 60 to 160 kHz. 6,7 The high frequency clicks, with faster attenuation than low frequency clicks, hinder long range echolocation, and seem to be a disadvantage for pygmy sperm whales, which forage in deep waters.…”