Echolocating Bottlenose Dolphin Research Articles

Spectral and temporal cues used by echolocating bottlenose dolphins to discriminate changes in inter-highlight intervals

Echolocating dolphins use spectral cues to discriminate complex echoes from targets with multiple reflective surfaces; however, the specific cues involved are not clear. This study investigated the role of spectral interference patterns on the dolphin’s ability to discriminate two-highlight echoes. In task one of the study, two dolphins were trained to echolocate, listen to returning electronic (“phantom”) echoes with two identical highlights, and produce a conditioned acoustic response if the inter-highlight interval (IHI) increased. The amount that the IHI increased varied across trials. Task two replicated task one but applied a random phase shift to all highlights. This varied the specific locations of notches along the frequency axis in the complex echo spectrum but preserved the notch spacing (equal to the inverse of the IHI). For IHIs less than 250 μs, discrimination thresholds increased with IHI and were larger with random phase shifts. For IHIs greater than 250 μs, thresholds plateaued and were similar for both tasks. Results agree with those from a previous study with passively listening dolphins. Namely, echolocating dolphins can use the spacing of notches in spectral interference patterns to detect IHI changes within the ∼250-μs temporal window, but temporal cues are used at longer IHIs. [Work supported by ONR]

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.

Forward masking recovery in bottlenose dolphins (Tursiops truncatus): Auditory brainstem responses to paired-click stimuli in high-pass masking noise

Adaptation is a fundamental process in the auditory system and underlies the automatic gain control system of echolocating bottlenose dolphins. As pulse-echo delay increases, auditory brainstem response (ABR) amplitudes to the echoes progressively increase. This study examined adaptation across cochlear frequency regions using a paired-click (i.e., forward masking) paradigm and high-pass masking noise. Bottlenose dolphins passively listened to paired click stimuli (20–160 kHz “pink” spectra). A “conditioning” click was followed by a “probe” click with equal amplitude and a time delay ranging from 125 to 750 μs. ABRs to click pairs were obtained with and without high-pass masking noise that precluded the basal turn of the cochlea from responding to the click stimuli. The ABR evoked by a single click (temporally aligned with the first click of the paired-click condition) was subtracted from the click-pair ABR to visualize the response evoked by the probe click. Probe ABR amplitudes recovered linearly with increasing delay relative to the conditioning click, and were approximately 70% of full response amplitude at 750-μs delay. Paired-click interval ABR amplitude recovery functions were similar for the unmasked and high-pass masked conditions. [Work sponsored by ONR.]

Origin of the off-axis double-pulse in an echolocating bottlenose dolphin

It has been proposed that two pairs of phonic lips might be utilized by a dolphin to adaptively manipulate the frequency content and directionality of the echolocation beam in response to acquired target information. The presence of two pulses appearing off-axis of the echolocation beam and separated in time has been proposed as evidence supporting this hypothesis. An array containing 35 hydrophones was used to measure the beam pattern of a bottlenose dolphin performing a phantom echo-change detection task. Simulated target ranges varied from 2.5 to 80 m and clicks were measured at 5–10° resolution from + /150°. At recording angles beyond + /30°, the click appeared as two distinct pulses that declined in amplitude and distorted as the off-axis angle increased. A simple model utilizing the time difference of arrival for the two pulses was used to compare the direct source-receiver path to one of two source-reflector-receiver paths. Assuming a range of constant sound speeds, distances traveled were compared to a CT scan of the same animal to predict anatomical regions potentially contributing to the second pulse. Results suggest that the second pulse is due to reflections from internal structures of the dolphin head and not a second sound source.

Auditory steady-state response measurement of the temporal dynamics of hearing sensitivity in an echolocating bottlenose dolphin (Tursiops truncatus)

Studies with some echolocating odontocetes demonstrate that receiver-based automatic gain control (AGC) compensates for reductions in echo strength resulting from acoustic spreading loss. This study examined AGC in an echolocating bottlenose dolphin by measuring changes in hearing sensitivity over time courses corresponding to single click-echo pairs. The electrophysiological auditory steady-state response (ASSR) elicited by a 113-kHz sinusoidally amplitude-modulated tone was recorded while the dolphin performed a target discrimination task. Auditory electrophysiological responses were extracted from the instantaneous electroencephalogram and coherently averaged using the modulation rate of the 113-kHz tone as a reference. A Fourier transform was then performed with a 10-ms sliding window to obtain the ASSR amplitude as a function of time relative to the dolphin’s outgoing click and received echo. The ASSR amplitude initially decreased at the time of click emission and then recovered over a course of 25 to 70 ms, depending on target range. This relatively long time course of recovery appears to be consistent with forward-masking, as opposed to an AGC mechanism based on the contraction and gradual release of middle ear muscles coincident with click emission. [Work funded by SSC Pacific Naval Innovative Science and Engineering (NISE) program.]

Performance of an echolocating bottlenose dolphin in the presence of anthropogenic masking noise

To study the effects of acoustic masking from anthropogenic noise, bottlenose dolphin (Tursiops truncatus) echolocation performance was assessed in the presence of different masking noise types using Navy relevant source transmissions. Echolocation clicks produced by the dolphin were detected with a hydrophone, then digitized within a phantom echo generator (PEG). The PEG converted the received clicks into echoes, delayed appropriately for the simulated target range. The echoes were then broadcast to the dolphin via a sound projector while masking noise transmissions were held constant. Using an acoustic response and a modified method of constants procedure, the echolocation performance of the dolphin was computed as a function of range between 3 and 17 m. Comparative echolocation performance to different masking noise type categories was analyzed between intermittent and continuous noise, direct path transmissions and multipath exposure, and mid frequency versus high frequency bands. These results expand the limited understanding of biosonar processing capability and signal characteristic alterations used to discriminate and resolve changes in small scale features while exposed to potential noise interference types. [Work supported through the Office of Naval Research.]

Continuous vigilance by echolocating bottlenose dolphins (Tursiops truncatus) for up to fourteen days.

The dolphin’s ability to actively detect randomly presented targets via echolocation for three, continuous, 5‐day sessions and a single 14‐day session was investigated. During each experimental session, a dolphin was required to ensonify the perimeter of its floating enclosure and report the presence of phantom echo generated (PEG) targets by pressing a response paddle. PEG targets were generated by convolving the dolphin’s echolocation signal with the impulse response from a metallic sphere. The resulting PEG target was delayed in time and projected back to the dolphin to simulate the echo of a “real target.” PEG targets varied in simulated range, location, and time of occurrence. Percent correct for target detection were 97, 99, and 100 for dolphin A and 75, 78, and 86 for dolphin B, during each 5‐day session, respectively. During a 14‐day session, dolphin A reported 98% of the targets correctly. False alarm rates were low and there was no significant effect of range or inter‐trial‐interval on percent correct or response time. The results indicate that dolphins are capable of continuously monitoring their environment via echolocation for multiple days (at least 14 days) and can accurately report the presence of regularly occurring targets.

Modeling the detection range of fish by echolocating bottlenose dolphins and harbor porpoises

The target strength as a function of aspect angle were measured for four species of fish using dolphin-like and porpoise-like echolocation signals. The polar diagram of target strength values measured from an energy flux density perspective showed considerably less fluctuation with azimuth than would a pure tone pulse. Using detection range data obtained from dolphin and porpoise echolocation experiments, the detection ranges for the Atlantic cod by echolocating dolphins and porpoises were calculated for three aspect angles of the cod. Maximum detection ranges occurred when the fish was broadside to the odontocete and minimum detection ranges occurred when the cod was in the tail aspect. Maximum and minimum detection ranges for the bottlenose dolphin in a noise-limited environment was calculated to be 93 and 70 m, respectively. In a quiet environment, maximum and minimum detection ranges for the bottlenose dolphin were calculated to be 173 and 107 m, respectively. The detection ranges for the harbor porpoise in a quiet environment were calculated to be between 15 and 27 m. The primary reason for the large differences in detection ranges between both species was attributed to the 36 dB higher source level of the bottlenose dolphin echolocation signals.

Human listeners provide insights into echo features used by dolphins (Tursiops truncatus) to discriminate among objects.

Echolocating bottlenose dolphins (Tursiops truncatus) discriminate between objects on the basis of the echoes reflected by the objects. However, it is not clear which echo features are important for object discrimination. To gain insight into the salient features, the authors had a dolphin perform a match-to-sample task and then presented human listeners with echoes from the same objects used in the dolphin's task. In 2 experiments, human listeners performed as well or better than the dolphin at discriminating objects, and they reported the salient acoustic cues. The error patterns of the humans and the dolphin were compared to determine which acoustic features were likely to have been used by the dolphin. The results indicate that the dolphin did not appear to use overall echo amplitude, but that it attended to the pattern of changes in the echoes across different object orientations. Human listeners can quickly identify salient combinations of echo features that permit object discrimination, which can be used to generate hypotheses that can be tested using dolphins as subjects.

Discriminating between the clicks produced by a bottlenose dolphin when searching for and identifying an object during a search task

Clicks collected from an echolocating bottlenose dolphin completing a search task were compared in order to determine if the clicks produced when the dolphin was acquiring the target differed from the clicks produced when the dolphin was searching for the target. The clicks produced by a free-swimming dolphin completing the search task were recorded using a biosonar measurement tool (BMT), an instrumentation package carried by the dolphin that collected both the outgoing clicks and the returning echoes. A discriminant function analysis classified the clicks as search or acquisition using the variables of peak-to-peak amplitude, duration, peak frequency, center frequency, and bandwidth. The acquisition clicks were classified more accurately than the search clicks. Acquisition clicks and search clicks were significantly different across all five of the variables. These results suggest that the clicks produced by bottlenose dolphins acquiring a target are different than those produced by dolphins searching for a target.

Acoustic features of objects matched by an echolocating bottlenose dolphin

The focus of this study was to investigate how dolphins use acoustic features in returning echolocation signals to discriminate among objects. An echolocating dolphin performed a match-to-sample task with objects that varied in size, shape, material, and texture. After the task was completed, the features of the object echoes were measured (e.g., target strength, peak frequency). The dolphin's error patterns were examined in conjunction with the between-object variation in acoustic features to identify the acoustic features that the dolphin used to discriminate among the objects. The present study explored two hypotheses regarding the way dolphins use acoustic information in echoes: (1) use of a single feature, or (2) use of a linear combination of multiple features. The results suggested that dolphins do not use a single feature across all object sets or a linear combination of six echo features. Five features appeared to be important to the dolphin on four or more sets: the echo spectrum shape, the pattern of changes in target strength and number of highlights as a function of object orientation, and peak and center frequency. These data suggest that dolphins use multiple features and integrate information across echoes from a range of object orientations.

Transmitting beam patterns of an echolocating bottlenose dolphin

Measurements on a free-swimming subject, echlocating under a simple go paradigm, were conducted at SSC San Diego, with a linear array of seven simultaneously and individually processed wideband hydrophones (response to 400 kHz), arraigned either vertically or horizontally, and centered on a small underwater video camera, with the video output synchronized with the recording of acoustic data. The measurement apparatus itself served as the test target. Lowering it into the water provided the cue for the blindfolded subject to locate its position in the test pen, swim to the apparatus, and touch it with the tip of its rostrum; whereupon the trainer provided a bridge signal, indicating reward due. During this process, acoustic beampattern measurements were made on echolocation clicks, as a function of frequency, which support previous, non-simultaneous, statistical beampattern measurements. Early in the experiment, the subject emitted echolocation clicks that peaked at 78 kHz, with −3 dB beamwidths of 8 to 10 degrees; but also containing high frequency components (above 135 kHz), with beamwidths that narrowed with increasing frequency, ranging to only a few degrees around 300 kHz, far beyond the subjects hearing range. After several days, the task appeared to become easier and there was much less high frequency content, but the click repetition frequency increased. Wideband noise was then introduced into the test pen, at frequencies below 135 kHz; and as the task became more difficult, the subject resumed transmitting clicks with high frequency components, although the peak frequency remained at 78 kHz, and click rates again increased, to much higher values.

Discrimination of complex synthetic echoes by an echolocating bottlenose dolphin.

Bottlenose dolphins (Tursiops truncatus) detect and discriminate underwater objects by interrogating the environment with their native echolocation capabilities. Study of dolphins' ability to detect complex (multihighlight) signals in noise suggest echolocation object detection using an approximate 265-micros energy integration time window sensitive to the echo region of highest energy or containing the highlight with highest energy. Backscatter from many real objects contains multiple highlights, distributed over multiple integration windows and with varying amplitude relationships. This study used synthetic echoes with complex highlight structures to test whether high-amplitude initial highlights would interfere with discrimination of low-amplitude trailing highlights. A dolphin was trained to discriminate two-highlight synthetic echoes using differences in the center frequencies of the second highlights. The energy ratio (delta dB) and the timing relationship (delta T) between the first and second highlights were manipulated. An iso-sensitivity function was derived using a factorial design testing delta dB at -10, -15, -20, and -25 dB and delta T at 10, 20, 40, and 80 micros. The results suggest that the animal processed multiple echo highlights as separable analyzable features in the discrimination task, perhaps perceived through differences in spectral rippling across the duration of the echoes.

Features of echoes bottlenose dolphins use to perceive object properties

Echolocating bottlenose dolphins can discriminate among objects that vary in size, shape, and structure. Our purpose in this project is to investigate the features of echoes dolphins may use to determine each of these object properties (e.g., target strength, number, and position of highlights). A dolphin performed a cross-modal matching task where he was presented with an object in one modality (e.g., vision) and then asked to choose the same object from among a group of three objects using another modality (e.g., echolocation). The dolphin was presented with object sets in which the objects within the set varied along one feature (size or shape or material or texture). These objects were later ensonified with dolphin-like clicks and the object echoes were collected with a binaural (two hydrophone) system. To mimic the dolphin’s ability to scan across objects, echoes were captured as the objects were rotated. Differences in echoic features within object sets were examined along with the dolphin’s error. In addition, an artificial neural network (ANN) was used to classify objects using the object echoes. The ANN was utilized to explore the importance of spectral versus time domain features, and single versus multiple object orientations in object classification.

Discrimination of amplitude-modulated synthetic echo trains by an echolocating bottlenose dolphin.

Bottlenose dolphins (Tursiops truncatus) have an acute ability to use target echoes to judge attributes such as size, shape, and material composition. Most target recognition studies have focused on features associated with individual echoes as opposed to information conveyed across echo sequences (feature envelope of the multi-echo train). One feature of aspect-dependent targets is an amplitude modulation (AM) across the return echoes in the echo train created by relative movement of the target and dolphin. The current study examined whether dolphins could discriminate targets with different AM envelopes. "Electronic echoes" triggered by a dolphin's outgoing echolocation clicks were manipulated to create sinusoidal envelopes with varying AM rate and depth. Echo trains were equated for energy, requiring the dolphin to extract and retain information from multiple echoes in order to detect and report the presence of AM. The dolphin discriminated amplitude-modulated echo trains from those that were not modulated. AM depth thresholds were approximately 0.8 dB, similar to other published amplitude limens. Decreasing the rate of modulation from approximately 16 to 2 cycles per second did not affect the dolphin's AM depth sensitivity. The results support multiple-echo processing in bottlenose dolphin echolocation. This capability provides additional theoretical justification for exploring synthetic aperture sonar concepts in models of animal echolocation that potentially support theories postulating formation of images as an ultimate means for target identification.

Atlantic bottlenose dolphin (Tursiops truncatus) hearing threshold for brief broadband signals.

The hearing sensitivity of an Atlantic bottlenose dolphin (Tursiops truncatus) to both pure tones and broadband signals simulating echoes from a 7.62-cm water-filled sphere was measured. Pure tones with frequencies between 40 and 140 kHz in increments of 20 kHz were measured along with broadband thresholds using a stimulus with a center frequency of 97.3 kHz and 88.2 kHz. The pure-tone thresholds were compared with the broadband thresholds by converting the pure-tone threshold intensity to energy flux density. The results indicated that dolphins can detect broadband signals slightly better than a pure-tone signal. The broadband results suggest that an echolocating bottlenose dolphin should be able to detect a 7.62-cm diameter water-filled sphere out to a range of 178 m in a quiet environment.

Detection distances of bottom-set gillnets by harbour porpoises ( Phocoena phocoena) and bottlenose dolphins ( Tursiops truncatus)

Many odontocetes die annually in gillnet fisheries. Why they become entangled is not yet clear. Maybe some species detect the nets too late to avoid collision. Therefore, the target strength of 11 types of bottom-set gillnets was measured under 0 and 45° angles of incidence. From these target strengths and from knowledge on the echolocation abilities of two odontocete species (harbour porpoises, bottlenose dolphins), the detection ranges of the nets by these small cetaceans could be estimated. The 90% detection range by echolocating harbour porpoises, approaching the nets at right (perpendicular) angles under low noise level conditions, varied between 3 and 6 m depending on the net type. For bottlenose dolphins, under high noise conditions, the 90% detection range varied between 25 and 55 m. At other angles of approach, the estimated detection ranges are shorter. The study suggests that echolocating bottlenose dolphins can detect nets in time to avoid collision, whereas echolocating harbour porpoises cannot in most cases. Suggestions for future research to reduce small cetacean bycatch by improving the nets' detectability by echolocation are given.

Acoustic basis for recognition of aspect-dependent three-dimensional targets by an echolocating bottlenose dolphin

The relationships between acoustic features of target echoes and the cognitive representations of the target formed by an echolocating dolphin will influence the ease with which the dolphin can recognize a target. A blindfolded Atlantic bottlenose dolphin (Tursiops truncatus) learned to match aspect-dependent three-dimensional targets (such as a cube) at haphazard orientations, although with some difficulty. This task may have been difficult because aspect-dependent targets produce different echoes at different orientations, which required the dolphin to have some capability for object constancy across changes in echo characteristics. Significant target-related differences in echo amplitude, rms bandwidth, and distributions of interhighlight intervals were observed among echoes collected when the dolphin was performing the task. Targets could be classified using a combination of energy flux density and rms bandwidth by a linear discriminant analysis and a nearest centroid classifier. Neither statistical model could classify targets without amplitude information, but the highest accuracy required spectral information as well. This suggests that the dolphin recognized the targets using a multidimensional representation containing amplitude and spectral information and that dolphins can form stable representations of targets regardless of orientation based on varying sensory properties.

A comparison of signal detection between an echolocating dolphin and an optimal receiver.

An electronic simulated target apparatus was used in a two-experiment study to compare the target detection performance of an echolocating bottlenose dolphin with an optimal receiver. Random Gaussian noise with a relatively flat spectrum from 20 to 160 kHz was used as a masking source. Experiment I was conducted to establish a technique for estimating the echo energy-to-noise ratio, Ee/N, at the dolphin's threshold of detection. Dolphins typically vary the amplitude of their emitted signal over a large range making it difficult to estimate Ee/N. In the first part of experiment I, the simulated echo was a double click, the pulses separated by 200 microseconds, with each pulse being a replica of the dolphin's transmitted signal. A staircase psychophysical procedure was used to obtain the detection threshold, and the echo energy-to-noise ratio based on the highest amplitude click emitted per trial, (Ee/N)max, was determined at each reversal point. The second echo type consisted of one of the animal's echolocation clicks, previously measured, digitized and stored in an erasable programmable read-only memory (EPROM). The electronic target simulator was modified so that every time the dolphin emitted an echolocation signal, the EPROM was triggered to produce two pulses separated by 200 microseconds. On any trial, the EPROM signal was played back at a fixed amplitude, regardless of the amplitude of the dolphin's emitted signal. The Ee/N obtained with the EPROM signal at threshold was found to be 2.9 dB lower than (Ee/N)max obtained with the normal phantom target. Therefore an estimate of Ee/N can be obtained by subtracting 2.9 dB from (Ee/N)max.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of extended reinforcement schedules on the receiver operating characteristic (ROC) of an echolocating bottlenose dolphin (Tursiops truncatus)

A bottlenose dolphin was conditioned to report via two manipulanda the presence or absence of a solid steel sphere suspended 80 cm beneath the water's surface, 7 m (minimum) from his position. The dolphin's biosonar target detection and trained binary reporting behavior were tested utilizing variable primary reinforcement schedules (VR) from VR1 (fish reward every correct response) to VR100 (fish reward on one trial, randomly selected from 100 trials). Tests were also conducted with a fixed reinforcement schedule (FR) of FR100 (6.8 kg of fish reward after correctly responding on the last trial of a 100-trial session). The probability of target presentation was 0.50 for all schedules tested and the number and amount of reinforcement was kept equal for “present” and “absent” responses during all tests. The dolphin's ROC remained essentially unchanged and Neyman-Pearson at all reinforcement schedules. However, if reinforcement was kept at the more extended schedules (VR20 to VR100 and FR100) for more than eight consecutive 100-trial sessions all responses became “target-absent.” Response topography clearly showed the difference between detection and response bias.