Acoustic properties of a short-finned pilot whale head with insight into temperature influence on tissues' sound velocity

Abstract

Acoustic properties of odontocete head tissues, including sound velocity, density, and acoustic impedance, are important parameters to understand dynamics of its echolocation. In this paper, acoustic properties of head tissues from a freshly dead short-finned pilot whale (Globicephala macrorhynchus) were reconstructed using computed tomography (CT) and ultrasound. The animal's forehead soft tissues were cut into 188 ordered samples. Sound velocity, density, and acoustic impedance of each sample were either directly measured or calculated by formula, and Hounsfield Unit values (HUs) were obtained from CT scanning. According to relationships between HUs and sound velocity, HUs and density, as well as HUs and acoustic impedance, distributions of acoustic properties in the head were reconstructed. The inner core in the melon with low-sound velocity and low-density is an evidence for its potential function of sound focusing. The increase in acoustic impedance of forehead tissues from inner core to outer layer may be important for the acoustic impedance matching between the outer layer tissue and seawater. In addition, temperature dependence of sound velocity in soft tissues was also examined. The results provide a guide to the simulation of the sound emission of the short-finned pilot whale.

Numerical-modeling-based investigation of sound transmission and reception in the short-finned pilot whale (Globicephala macrorhynchus)

“…In 2016, a female short-finned pilot whale was found dead in Lingshui waters, Hainan Province, China; it was immediately frozen at À20 C. The whale was considered to be freshly dead, because no obvious decomposition was noted (Dong et al, 2017). After 20 days, the specimen was removed to be naturally thawed.…”

“…The forehead soft tissue pack was extracted and cut into small pieces for sound speed and density measurements. More details can be found in our previous study (Dong et al, 2017). Here, a brief introduction to the model development is provided.…”

“…This species has an almost negligible beak and bulbous melon. The sound speed and density of soft tissues in its bio-sonar transmission system are within the ranges 1224-1749 m/s and 874-1090 kg/m 3 , respectively (Dong et al, 2017). This species emits echolocation clicks, which predominately peak below 40 kHz (Baumann-Pickering et al, 2015); meanwhile, they can hear up to high frequencies over 100 kHz, with a peak sensitivity in the range 20-40 kHz (Greenhow et al, 2014).…”