Whitlow W. Au

Dolphin hearing: Relative sensitivity as a function of point of application of a contact sound source in the jaw and head region

The auditory input area of the dolphin head was investigated in an unrestrained animal trained to beach itself and to accept noninvasive electroencephalograph (EEG) electrodes for the recording of the auditory brain-stem response (ABR). The stimulus was a synthetic dolphin click, transmitted from a piezo-electric transducer and coupled to the skin via a small volume of water. The results conform with earlier experiments on acute preparations that show best auditory sensitivity at the middle of the lower jaw. Minimum latency was found at the rear of the lower jaw. A shaded receiver configuration for the dolphin ear is proposed.

Masked Hearing Abilities in a False Killer Whale (Pseudorca Crassidens)

Underwater audiograms are available for only a few odontocete species: Phocoena phocoena (Andersen, 1970), Inia geoffrensis (Jacobs and Hall, 1972), Tursiops truncatus (Johnson, 1967), Delphinapterus leucas (White et al., 1978), Orcinus orca (Hall and Johnson, 1971), and Pseudorca crassidens (Thomas et al., 1988). All odontocetes studied have the typical U-shaped mammalian hearing curve and hear over a wide range of frequencies (up to 120 kHz in belugas and up to 140 kHz in bottlenose dolphins). Low-frequency hearing among these species is comparable, but the high frequency cut-off is species-specific.

The echolocation ability of the beluga (Delphinapterus leucas) to detect targets in clutter

A beluga (Delphinapterus leucas) was trained to detect different length cylinders in front of a clutter screen at five separation distances. The clutter screen consisted of 300, 5.1‐cm‐diam cork spheres located behind the targets. The clutter screen was used in two positions: perpendicular to the body axis of the beluga (90° grazing angle) and rotated 22° (68° grazing angle). Five hollow stainless‐steel cylinders (3.2‐cm‐diameter and 0.38‐cm wall thickness) of different lengths (14, 10, 7, 5, and 3 cm) were used as targets. Detection data were collected on the belugas performance as a function of the separation between targets and the clutter screen. The belugas performance was above 80% correct detection for the 14‐ and 10‐cm cylinders as the separation distance decreased from 10.1 to 5.1 cm for both grazing angles. For all targets except the 3‐cm cylinder, the belugas detection was higher at 0‐cm separation than at 2.5‐cm separation. The belugas detection performance was higher at 68° than at 90° gr...

Target detection by the beluga using a surface‐reflected path

During an echolocation-in-noise experiment we suspected that a beluga (Delphinapterus leucas) was using a surface-reflected path to maximize detection performance. We tested and confirmed this suspicion by acoustically and mechanically denying access to surface path information. The whales performance varied as the surface-reflected path was denied.

Cylinder and Cube Shape Discrimination by an Echolocating Blindfolded Bottlenosed Dolphin

An Atlantic bottlenosed dolphin was trained to wear rubber eyecup blindfolds and to choose a cylinder as compared to a cube when the two targets were simultaneously presented. The animal was housed and tested within a circular fiberglass tank filled with filtered sea water. As may be seen in Fig. 1, the animal was trained to station within the tank and echolocate down a water-filled trough.

Recognizing objects from multiple orientations using dolphin echoes

Object constancy, the ability to recognize objects despite changes in orientation, has not been well studied in the auditory modality. Dolphins use echolocation for object recognition, and objects ensonified by dolphins produce echoes that can vary significantly as a function of orientation. In four experiments, human listeners had to classify echoes from objects ensonified with dolphin signals. Participants were trained to discriminate among the objects using an 18-echo stimulus from a 10 degree range of aspect angles, then tested with novel aspect angles across a 60 degree range. In the first two experiments, the three objects varied in material, size, and shape. Participants were typically successful recognizing the objects at all angles (M = 78%). In experiment 3, the three objects had the same material but different shapes. Participants were often unsuccessful recognizing the objects at all angles (M = 46%). In experiment 4, participants had to classify echoes from four fish species across a wider ra...

Relative abundance of sound scattering organisms in the Northwestern Hawaiian Islands is a driver for some odontocete foragers

Previous studies in the Northwestern Hawaiian Islands (NWHI) focused on shallower communities in and near reefs and did not investigate the organisms living in deeper waters that some apex predators rely on for food, e.g., some odontocetes forage at depths greater than 400 m. To examine the relationship between deep-diving odontocete predators and prey, a Simrad EK60 echosounder operating at 70 kHz collected acoustic abundance throughout the NWHI from May 7 to June 4, 2013. Visual and passive acoustic surveys for marine mammal presence were conducted concurrently with the echosounder. Two broad scattering layers were found, a deep layer from 325 to 670 m and a shallow layer from 0 to 195 m. The highest densities of both deep and shallow scattering organisms were associated with deep slopes of banks and atolls. Beaked and short-finned pilot whale sightings occurred in locations of high scattering density associated with slopes of atolls and banks. It is hypothesized that the high scattering organisms assoc...

Control ultrasound beam by tissues in the head of finless porpoise acting as a tunable gradient index material

Porpoises are well known to emit directional ultrasound beams for detecting and tracking preys; however, how they produce and manipulate directional beams are challenging. Here, we investigated physical mechanism of ultrasound beam formation and control of finless porpoise (N. a. sunameri) by using an integrated scheme of computed tomography, tissue and field measurements, and numerical modeling. The results showed that complex acoustic structures in the porpoise’s forehead contributed to producing directional acoustic field. Furthermore, we demonstrated that the skull, air sacs, connective tissue, muscle, and melon constituted a gradient index (GRIN) structure whose density and sound velocity are positively correlated, and thus regulated the directional beam. The removal or compression deformation of the forehead tissues decentralizes energy and widens sound beam, indicating that the forehead tissues as a tunable natural GRIN material significantly impact beam patterns of the finless porpoise. The result...

Biosonar radiation field on the forehead of a Risso’s dolphin during prey capture

On-animal suction cups with embedded hydrophones allow examination of how signals on the forehead of echolocating odontocetes relate to the internal anatomical structure and the transmission beampattern. Risso’s dolphin (Grampus griseus) is an interesting species for this investigation due to the presence of a unique vertical groove in the middle of their forehead. In this study, a linear array of six broadband suction cup hydrophones were attached along the forehead groove of an adult female Risso’s dolphin trained to catch freshly thawed dead squid in front of an eight-element far-field hydrophone array. The animal’s movement was simultaneously observed using an underwater video camera. A total of nine successful prey captures were recorded. During each catch, the animal first emitted regular echolocation clicks, which quickly transitioned into buzzes (clicks with distinctively high repetition rate) until prey capture. The amplitude and relative time of arrival of these signals across all channels were ...

Snapping shrimp: Changes in environmental noise in Laguna de Términos, Campeche, México, after hurricanes

Laguna de Terminos, Campeche, Mexico, is one of the largest coastal lagoons in Mexico where snapping shrimp is the main source of biological sound. We measured the environmental noise from 2004 to 2008 in stations distributed homogeneously in this lagoon during 17 sampling periods using a digital audio tape recorder sampling at 48 kHz with 16 bits for 1 minute at each station. The environmental noise in the recordings was measured with a semi-automatic MATLAB routine designed for this purpose. Results indicate that environmental noise in the lagoon changes from one sampling to another, but it changed drastically after the hurricanes of 2005 in some of the stations. This changes in submarine environmental noise helps us in understanding the effects of natural phenomena in the distribution and abundance of marine wildlife, such as snapping shrimps and bottlenose dolphins. [Work supported by CONACyT-Campeche and PAPIIT&PASPA-UNAM.]