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

Validated 3D finite-element model of the Risso's dolphin (Grampus griseus) head anatomy demonstrates gular sound reception and channelling through the mandibular fats

Like other odontocetes, Risso's dolphins actively emit clicks and passively listen to the echoes during echolocation. However, the head anatomy of Risso's dolphins differs from that of other odontocetes by a unique vertical cleft along the anterior surface of the forehead and a differently-shaped lower jaw. In this study, 3D finite-element sound reception and production models were constructed based on computed tomography (CT) data of a deceased Risso's dolphin. Our results were verified by finding good agreement with experimental measurements of hearing sensitivity. Moreover, the acoustic pathway for sounds to travel from the seawater into the dolphin's tympanoperiotic complexes (TPCs) was computed. The gular reception mechanism, previously discovered inDelphinus delphisandZiphius cavirostris, was also found in this species. The received sound pressure levels and relative displacement at TPC surfaces were compared between the cases with and without the mandibular fats or mandible. The results demonstrate a pronounced wave-guiding role of the mandibular fats and a limited bone-conductor role of the mandible. For sound production modelling, we digitally filled the cleft with neighbouring soft tissues, creating a hypothetical 'cleftless' head. Comparison between sound travelling through a 'cleftless' head vs. an original head indicates that the distinctive cleft plays a limited role in biosonar sound propagation.

From the morphospace to the soundscape: Exploring the diversity and functional morphology of the fish inner ear, with a focus on elasmobranchsa)

Fishes, including elasmobranchs (sharks, rays, and skates), present an astonishing diversity in inner ear morphologies; however, the functional significance of these variations and how they confer auditory capacity is yet to be resolved. The relationship between inner ear structure and hearing performance is unclear, partly because most of the morphological and biomechanical mechanisms that underlie the hearing functions are complex and poorly known. Here, we present advanced opportunities to document discontinuities in the macroevolutionary trends of a complex biological form, like the inner ear, and test hypotheses regarding what factors may be driving morphological diversity. Three-dimensional (3D) bioimaging, geometric morphometrics, and finite element analysis are methods that can be combined to interrogate the structure-to-function links in elasmobranch fish inner ears. In addition, open-source 3D morphology datasets, advances in phylogenetic comparative methods, and methods for the analysis of highly multidimensional shape data have leveraged these opportunities. Questions that can be explored with this toolkit are identified, the different methods are justified, and remaining challenges are highlighted as avenues for future work.

Does rotation increase the acoustic field of view? Comparative models based on CT data of a live dolphin versus a dead dolphin

Rotational behaviour has been observed when dolphins track or detect targets, however, its role in echolocation is unknown. We used computed tomography data of one live and one recently deceased bottlenose dolphin, together with measurements of the acoustic properties of head tissues, to perform acoustic property reconstruction. The anatomical configuration and acoustic properties of the main forehead structures between the live and deceased dolphins were compared. Finite element analysis (FEA) was applied to simulate the generation and propagation of echolocation clicks, to compute their waveforms and spectra in both near- and far-fields, and to derive echolocation beam patterns. Modelling results from both the live and deceased dolphins were in good agreement with click recordings from other, live, echolocating individuals. FEA was also used to estimate the acoustic scene experienced by a dolphin rotating 180° about its longitudinal axis to detect fish in the far-field at elevation angles of -20° to 20°. The results suggest that the rotational behaviour provides a wider insonification area and a wider receiving area. Thus, it may provide compensation for the dolphin's relatively narrow biosonar beam, asymmetries in sound reception, and constraints on the pointing direction that are limited by head movement. The results also have implications for examining the accuracy of FEA in acoustic simulations using recently deceased specimens.

The Distinctive Forehead Cleft of the Risso's Dolphin (Grampus griseus) Hardly Affects Biosonar Beam Formation

The Risso's dolphin (Grampus griseus) has a distinctive vertical crease (or cleft) along the anterior surface of the forehead. Previous studies have speculated that the cleft may contribute to biosonar beam formation. To explore this, we constructed 2D finite element models based on computer tomography data of the head of a naturally deceased Risso's dolphin. The simulated acoustic near-field signals, far-field signals, and transmission beam patterns were compared to corresponding measurements from a live, echolocating Risso's dolphin. To investigate the effect of the cleft, we filled the cleft with neighboring soft tissues in our model, creating a hypothetical "cleftless" forehead, as found in other odontocetes. We compared the acoustic pressure field and the beam pattern between the clefted and cleftless cases. Our results suggest that the cleft plays an insignificant role in forehead biosonar sound propagation and far-field beam formation. Furthermore, the cleft was not responsible for the bimodal click spectrum recorded and reported from this species.

Numerical modeling of the impacts of acoustic stimulus on fish otoliths from two directions

Previous experiments have shown (1) evidence that exposure to high-intensity sounds (e.g., air-gun signals) may cause damage to the sensory hair cells of the fish ears and impair fish hearing and (2) evidence that in some circumstances such exposures cause minimal structural damage. The contradictory results regarding the damage accrued suggested that the angle of sound energy arrivals at the fish ears may play a part in the propensity of the sound to cause damage to sensory hair cells. To further study this and gain insight into specific details of the differential motion of the otolith relative to the sensory macula when incident sounds arrive from different directions, three-dimensional finite element models were constructed based on the micro-computed tomography imaging of the sagittal otoliths of the bight redfish (Centroberyx gerrardi). We used the models to study the response of fish sagittal otoliths to sounds arriving from horizontal and vertical directions. Sound pressure levels, relative displacement, acceleration, and shear stress of the otoliths and/or otolith-water boundary were calculated and compared. The results suggest that the angle of sound energy arrivals at the otoliths and the geometry of the otolith lead to different magnitudes of the differential motion between the macula and otoliths, with sound arriving in the vertical potentially creating more damage than the same sound arriving from the horizontal.

Possible limitations of dolphin echolocation: a simulation study based on a cross-modal matching experiment

Dolphins use their biosonar to discriminate objects with different features through the returning echoes. Cross-modal matching experiments were conducted with a resident bottlenose dolphin (Tursiops aduncus). Four types of objects composed of different materials (water-filled PVC pipes, air-filled PVC pipes, foam ball arrays, and PVC pipes wrapped in closed-cell foam) were used in the experiments, respectively. The size and position of the objects remained the same in each case. The data collected in the experiment showed that the dolphin’s matching accuracy was significantly different across the cases. To gain insight into the underlying mechanism in the experiments, we used finite element methods to construct two-dimensional target detection models of an echolocating dolphin in the vertical plane, based on computed tomography scan data. The acoustic processes of the click’s interaction with the objects and the surrounding media in the four cases were simulated and compared. The simulation results provide some possible explanations for why the dolphin performed differently when discriminating the objects that only differed in material composition in the previous matching experiments.

Modeling of the near to far acoustic fields of an echolocating bottlenose dolphin and harbor porpoise

Echolocation signals emitted by odontocetes can be roughly classified into three broad categories: broadband echolocation signals, narrowband high-frequency echolocation signals, and frequency modulated clicks. Previous measurements of broadband echolocation signal propagation in the bottlenose dolphin (Tursiops truncatus) did not find any evidence of focusing as the signals travel from the near-field to far-field. Finite element analysis (FEA) of high-resolution computed tomography scan data was used to examine signal propagation of broadband echolocation signals of dolphins and narrowband echolocation signals of porpoises. The FEA results were used to simulate the propagation of clicks from phonic lips, traveling through the forehead, and finally transmission into the water. Biosonar beam formation in the near-field and far-field, including the amplitude contours for the two species, was determined. The finite element model result for the simulated amplitude contour in the horizontal plane was consistent with prior direct measurement results for Tursiops, validating the model. Furthermore, the simulated far-field transmission beam patterns in both the vertical and horizontal planes were also qualitatively consistent with results measured from live animals. This study indicates that there is no evidence of convergence for either Tursiops or Phocoena as the sound propagates from the near-field to the far-field.

Three-dimensional finite element simulation of acoustic propagation in spiral bubble net of humpback whale

In 2004, Leighton hypothesized that the acoustic calls emitted by humpback whales when feeding using bubble nets, may enhance the effectiveness of the net in confining prey (such as herring) by forming a "wall of sound" with a quiet zone within. Modelling of the acoustics of this phenomenon was previously restricted to 2D; this paper conducts a 3D model of the propagation of signals resembling those emitted by humpback whales when bubble netting, projected into an upward spiral bubble net which data to date suggest is the accurate form for the bubble net in 3D space. In this study, the feeding calls were analyzed in the time-frequency domain to extract acoustic information sufficient to allow modeling of the resulting spatial distribution of acoustic pressure and particle velocity, and how they vary over the duration of the call. Sound propagation in the bubble net was described by using a linear steady-state formulation for an effective medium of bubbly water. Using the predicted attenuation, phase velocity and density in bubbly water, a 3D finite element model was constructed to numerically simulate the upward-spiral bubble net which consists of a mixture of bubbles that exhibit a range of radii. The acoustic pressure field and particle motion field were both calculated within the bubble net. The simulation results show that the energy of the whale feeding call could be effectively focused in the bubble net, generating intensive sound pressure and particle motion fields in the bubbly arm of the net, but with some "quiet" regions closer to the center of the net, as Leighton hypothesized. Furthermore, when the hearing ability of herring is taken into consideration, the results suggest that this acoustic focusing effect could be a plausible factor in trapping them in the bubble net. It also allows speculation on the possible enhancements that the time-varying nature of the call during feeding could give to the whale in this mechanism for the bubble net feeding by humpback whales.