The role of various structures in the head on the formation of the biosonar beam of the baiji (Lipotes vexillifer)

Abstract

The relative role of the various structures in the head of the baiji (Lipotes vexillifer) is examined. A finite element approach was applied to numerically simulate the acoustic propagation through a dolphin's head to examine the relative role of the skull, air sacs, and melon in the formation of the biosonar beam in the vertical plane. The beam pattern obtained with the whole head in place is compared with the beam pattern when the air sac is removed and the other structures (skull and melon) are in place, with only the skull removed, and finally with only the melon removed. The beam pattern with the air sacs and skull intact and the melon removed closely resembled the beam pattern for the complete head, suggesting that the melon has a minor role in the formation of the beam. The beam pattern for the other two cases had very little resemblance to the beam pattern for the whole head. The air sacs seem to have a role of directing propagation of the signal toward the front and the skull prevents the sound propagating below the rostrum. The beam patterns along with a correlation analysis showed that the melon had only a slight influence on the shape and direction of the beam. The resultant beam exiting the head of the dolphin is the result of complex reflection processes within the head of the animal.

Biosonar signal propagation in the harbor porpoise's (Phocoena phocoena) head: The role of various structures in the formation of the vertical beam

“…Previous studies have used numerical models and experimental measurements to display that the internal structures in the dolphin's head such as the air sacs, melon and skull contribute to the formation of biosonar beam (Aroyan et al, 1992;Houser et al, 2004;Au et al, 2010;Cranford et al, 2014;Finneran et al, 2014;Wei et al, 2014;Song et al, 2016;Wei et al, 2016). Aroyan et al (1992) suggested that the air sacs and skull were the dominant factors in shaping the beam and the melon might be capable of mild focusing in the formation of the short-beaked common dolphin's biosonar beam.…”

“…A vibroacoustic finite element model was employed in Cranford et al (2014) to report the air spaces and the shape of the skull played important roles in the formation of the sound transmission beam in the bottlenose dolphin's head. Wei et al (2016) used a broadband transit signal as the driving source to model the propagation of the echolocation clicks in the baiji's head, indicating that the air sacs and skull were the major contributor to the formation of the baiji's biosonar beam in vertical plane. The results shown in this paper are in line with the prior studies to report that the air sac and skull are the major components to form the harbor porpoise's vertical beam.…”

“…Au et al (2010) experimentally measured the acoustic field on the forehead of echolocating Atlantic bottlenose dolphins with suction cup hydrophones that were placed on the side of the forehead and found the biosonar signals being directed (probably by the air sacs and skull) before the signals arrived at the melon. Wei et al (2016) used a finite element model and a broadband transient signal at the source which more closely represented an echolocation dolphin than the study of Aroyan et al (1992) which used a continuous tonal source. The numerical simulation results demonstrated that the melon in the baiji's head had only a slight influence on the shape and direction of its outgoing biosonar beam in the vertical plane.…”

“…An accurate model requires high accuracy image reconstruction technology such as the computed tomography (CT) scan because the anatomical features in the animal's head are extremely complex. The numerical modeling has been used for investigating sound production, transmission and reception on different species of odontocetes, including short-beaked common dolphin (Delphinus delphis) (Aroyan et al, 1992;Aroyan, 2001), Cuvier's beaked whale (Ziphius cavirostris) (Cranford, 2000), humpback whales (Megaptera novaeangliae) (Adam et al, 2013), bottlenose dolphin (Tursiops truncatus) (Cranford et al, 2014), baiji (Lipotes vexillifer) (Wei et al, 2014;Wei et al, 2016). However, the physiological mechanism of the biosonar beam formation in an echolocating harbor porpoise's head is still not well understood.…”

Finite element simulation of broadband biosonar signal propagation in the near- and far-field of an echolocating Atlantic bottlenose dolphin (Tursiops truncatus)

“…In order to investigate the specific details of how these structures function and further increase our understanding of the mechanics of biosonar sound production and propagation, numerical modeling techniques have been applied to simulate the biosonar systems of odontocetes (Aroyan et al, 1992;Aroyan et al, 2001;Krysl et al, 2006;Cranford et al, 2014;Wei et al, 2014;Wei et al, 2016;Song et al, 2016;Wei et al, 2017;Wei et al, 2018). There is a general consensus on the roles of the skull and air sacs from the results of prior numerical simulation research and experimental measurements (Aroyan et al, 1992;Houser et al, 2004;Au et al, 2010;Finneran et al, 2014;Cranford et al, 2014;Wei et al, 2014;Song et al, 2016;Wei et al, 2016;Wei et al, 2017;Wei et al, 2018).…”