Lois A. Dankiewicz

Beamwidth control and angular target detection in an echolocating bottlenose dolphin (Tursiops truncatus)

Bottlenose dolphin (Tursiops truncatus) echolocation beams are typically characterized as symmetrical -3 dB beamwidths; however, the functional width of the beam during target detection has not been explored. Angular target detection thresholds of an echolocating dolphin were examined to more fully describe the functional characteristics of the echolocation beam. The dolphin performed an echolocation detection task with its head held in a fixed orientation. Targets were placed 9 m in front of the dolphin [0 degrees position (P(0))] and systematically moved right or left until target detection reached chance probability. A 24-element hydrophone array placed 1 m in front of the dolphin was used to measure vertical and horizontal echolocation beamwidths. Detection thresholds were 26 degrees left and 21 degrees right of P(0) for a sphere target and 19 degrees left and 13 degrees right of P(0) for a cylinder target. Estimates of maximum horizontal and vertical beamwidths ranged up to 40 degrees and 29 degrees , respectively, and exhibited large variability. The dolphin nominally steered the maximum response axis of the echolocation beam up to 18 degrees in the horizontal, 12 degrees in the upward vertical, and 4 degrees in the downward vertical. These results suggest that the dolphin can steer and modify the width of the echolocation beam.

Assessment of dolphin (Tursiops truncatus) auditory sensitivity and hearing loss using jawphones

Devices known as jawphones have previously been used to measure interaural time and intensity discrimination in dolphins. This study introduces their use for measuring hearing sensitivity in dolphins. Auditory thresholds were measured behaviorally against natural background noise for two bottlenose dolphins (Tursiops truncatus); a 14-year-old female and a 33-year-old male. Stimuli were delivered to each ear independently by placing jawphones directly over the pan bone of the dolphins lower jaw, the assumed site of best reception. The shape of the female dolphins auditory functions, including comparison measurements made in the free field, favorably matches that of the accepted standard audiogram for the species. Thresholds previously measured for the male dolphin at 26 years of age indicated a sensitivity difference between the ears of 2-3 dB between 4-10 kHz, which was considered unremarkable at the time. Thresholds for the male dolphin reported in this study suggest a high-frequency loss compared to the standard audiogram. Both of the males ears have lost sensitivity to frequencies above 55 kHz and the right ear is 16-33 dB less sensitive than the left ear over the 10-40 kHz range, suggesting that males of the species may lose sensitivity as a function of age. The results of this study support the use of jawphones for the measurement of dolphin auditory sensitivity.

Multiecho processing by an echolocating dolphin

Bottlenose dolphins (Tursiops truncatus) use short, wideband pulses for echolocation. Individual waveforms have high-range resolution capability but are relatively insensitive to range rate. Signal-to-noise ratio (SNR) is not greatly improved by pulse compression because each waveform has small time-bandwidth product. The dolphin, however, often uses many pulses to interrogate a target, and could use multipulse processing to combine the resulting echoes. Multipulse processing could mitigate the small SNR improvement from pulse compression, and could greatly improve range-rate estimation, moving target indication, range tracking, and acoustic imaging. All these hypothetical capabilities depend upon the animals ability to combine multiple echoes for detection and/or estimation. An experiment to test multiecho processing in a dolphin measured detection of a stationary target when the number N of available target echoes was increased, using synthetic echoes. The SNR required for detection decreased as the number of available echoes increased, as expected for multiecho processing. A receiver that sums binary-quantized data samples from multiple echoes closely models the N dependence of the SNR required by the dolphin. Such a receiver has distribution-tolerant (nonparametric) properties that make it robust in environments with nonstationary and/or non-Gaussian noise, such as the pulses created by snapping shrimp.

Acoustic backscatter from a diving dolphin

The acoustic backscatter of a diving Atlantic bottlenose dolphin (Tursiops truncatus) was measured using Simrad EK‐60 echosounders operating at 70, 120, and 200 kHz. The animal was trained to dive to a small platform suspended with a nylon rope from a small boat to various depths, with the maximum depth being 100 m. When the dolphin reached the platform, it was required to station on a bite plate attached to the platform until an acoustic signal was sent to release the animal. A video camera on the platform with the signal sent up on a separate line was used to monitor the animal on the bite plate. A time‐depth recorder was also attached to the dolphin to determine the average rate of decent and ascent. Echosounding records were collected while the dolphin descended to and ascended from the platform. A total of 15 echosounding records with the dolphin swimming within the beam of the transducers were collected. Preliminary analysis of the data indicated that target strength varied from about −26 dB to −40 ...

Mapping acoustic sensitivity about the dolphin’s head: A look at the peripheral hearing system

The hypothesis that echolocating dolphins best receive acoustic signals over the pan bones of the lower jaw is widely accepted. Studies in echolocation and hearing have assumed that those areas serve as the dolphin’s peripheral hearing system. The research that established that model, however, does not exclude other potential sound reception sites and suggests that additional areas of the head may be acoustically sensitive and perhaps frequency dependent. Using jawphones, relative hearing thresholds for representative frequencies (10, 30, 60, and 90 kHz) were behaviorally measured at over 40 sites on a dolphin’s head. Iso‐sensitivity curves were constructed and projected onto the image of a dolphin’s head based on these measurements. The results suggest sensitivity to high frequency along the lower jaw with greater sensitivity forward of the pan bone area, sensitivity to low frequency around the external auditory meatus, and an acoustic asymmetry with greater sensitivity favoring the right side of the hea...

The slippery slope of a Johnsonian ear: Natural variability versus natural loss

The normal audiogram of the bottlenosed dolphin, first established by Johnson [C. S. Johnson, in Marine Bio‐Acoustics, edited by W. N. Tavolga (1967), pp. 247–260], has been pivotal to nearly three decades of underwater echolocation research, most of which focused on the mechanisms and analytical limits of dolphin sonar. One important aspect of dolphin hearing that has escaped investigation is what are normal audiometric ranges, or, more important, how to determine whether differences seen in audiometric responses of a new animal represent normal variants or a pathologically impaired ear. In this study, radiologic data and audiograms from animals tested by Brill et al. (this session) were compared with results from postmortem histologic analyses of ears from dolphins with known hearing losses. Structural alterations in the ear x rays of the older, male dolphins were consistent with obstructive disease processes and demineralization of temporal bones from dolphins with conductive and progressive sensorineu...

ULTRASOUND INSPECTION FOR INTRAVASCULAR BUBBLES IN A REPETITIVELY DIVING DOLPHIN

Nitrogen (N2) bubbles resulting from behavioural responses to sonar exposure have been postulated as a mechanism contributing to beaked whale strandings. According to this hypothesis, sequential deep diving of beaked whales produces tissue N2 gas supersaturation that manifests when the animal is near the surface. During certain sonar exposure conditions, altered dive behaviour is postulated to produce a degree of supersaturation that results in intravascular N2 bubble formation and symptoms similar to those observed in decompression sickness (DCS). Under normal diving conditions, this supersaturation is mitigated via behavioural (e.g., diving) or physiological means (e.g., blood flow redistribution) and is insufficient to produce symptomatic N2 bubbles. Although intravascular bubble formation is unlikely to be evaluated in beaked whales, such experiments are feasible in captive bottlenose dolphins. Because N2 uptake is limited to the depths above which lung collapse occurs, dolphins can be trained to perform dive profiles that permit N2 uptake to approximate that expected to occur in diving beaked whales. Therefore, in this study, postdive ultrasound scans and blood sampling were used to evaluate intravascular bubble formation and blood N2 levels in bottlenose dolphins trained to perform dive sequences that purposely increased tissue N2 saturation.

Evidence of hearing loss in an Atlantic bottlenose dolphin (Tursiopstruncatus)

Auditory thresholds were behaviorally measured for two Atlantic bottlenose dolphins (Tursiops truncatus); a 14‐year‐old female, and a 33‐year‐old male. Stimuli were delivered directly to the lateral sides of the lower jaw via jawphones as opposed to free‐field broadcasts. The female’s audiogram clearly reflects the standard for this species [C. S. Johnson, in Marine Bio‐Acoustics, edited by W. N. Tavolga, pp. 247–260 (1967)]. Previous thresholds for the male measured at age 26 indicated a hearing loss in the left ear of approximately 2 to 3 dB [re: 1μPa] between 4 to 10 kHz, which were considered unremarkable. At age 33, the same male demonstrates distinctive losses. The right ear shows a 16–33‐dB loss over 10–40 kHz, the range of best sensitivity. Above 55 kHz, the right ear is 2–3 dB more sensitive than the left. Both ears then decline to an upper frequency cutoff of approximately 70 kHz below the standard 120 kHz. Hearing losses due to age have been reported for this species [S. H. Ridgway and D. A. Ca...

Evidence for acoustic imaging capability in a bottlenose dolphin

A necessary condition for acoustic imaging (e.g., synthetic aperture sonar processing) is the capability to sum echo samples from the same point in the environment over different signal-echo pairs. To decide whether a dolphin has this capability, a limited number N of electronically simulated echoes with constant delay were transmitted back to an echolocating dolphin. The experiment was performed in San Diego Bay, which has a large indigenous population of snapping shrimp. The signal-to-noise ratio (SNR) of the simulated echoes was controlled by adding artificial Gaussian noise and by varying echo amplitude. If the dolphin is capable of SAS-like imaging, then the SNR required for detection should decrease as the number of available echoes N is increased. This phenomenon was indeed observed. The best receiver model for describing the dependence of SNR on N uses binary summation (an M-out-of-N detector). Binary summation is robust against the strong impulsive interference produced by snapping shrimp. The effect of binary summation on SAS-like processing is assessed by creating SAS images from binary-quantized data at the output of a broadband, dolphin-like sonar, and comparing these images to those obtained without binary quantization.

Dolphin detection of targets as a function of angular presentation and target strength.

As part of an ongoing study, bottlenose dolphin (Tursiops truncatus) echolocation beam steering was assessed as a dolphin performed a target detection task (4‐ and 7.62‐cm spheres) while echolocating with its head held in a fixed orientation and monitored via underwater camera. Targets were placed approximately 4.0 m in front of the dolphin [zero degree position (P0)] and at 34 deg to the left and right of P0 (PL and PR). Echolocation clicks and echoes were digitized using a high‐speed 32 channel analog‐to‐digital card, sampling 29 calibrated hydrophones with analog filter‐amplifiers arranged in a diamond‐shaped array supported in a hemispherical web in front of the animal. Using a yes‐no response and a modified method of constants procedure with 0.50 target presence/absence probability, detection ability (d) and response bias (B) of the dolphin were computed for both targets at all positions. Detection thresholds (d) for the 4‐ and 7.62‐cm spheres at P0, PL, and PR were 4.65, 2.08, and 2.62, and 4.08, 2....