Effects of dolphin hearing bandwidth on biosonar click emissions

Quantifying differences in dolphin hearing thresholds obtained with behavioral and auditory evoked potential methods

Different methods of producing the auditory steady state response (ASSR) are used to test dolphin hearing, but each method affects the resulting ASSR threshold. Since behavioral thresholds are often desired, this study, using common ASSR methods, compared differences between ASSR and behavioral hearing thresholds in five dolphins. Sinusoidal amplitude modulated (SAM) tones or tone pip trains were presented to the dolphins through a contact transducer while they were in air or partially submerged under water. Underwater behavioral hearing thresholds were obtained with pure tone stimuli on the same days as ASSR testing. Independent of the test medium, SAM tone stimuli yielded thresholds that consistently overestimated (i.e., were higher than) behavioral thresholds. Tone pip trains consistently underestimated thresholds when presented in air, and while they underestimated thresholds at lower test frequencies, they overestimated thresholds at higher test frequencies when presented under water. The mean differences between ASSR and behavioral thresholds were almost always lower when using tone pip train stimuli, but were exaggerated up to -47 dB when testing frequencies just above the upper-frequency limit of hearing. Knowing the relationship between ASSR and behavioral thresholds enables better approximations of behavioral thresholds in dolphins for which only ASSR thresholds exist.

Discrimination of double-click synthetic echoes by bottlenose dolphins: Effects of inter-highlight interval and phasea)

Two bottlenose dolphins (Tursiops truncatus) were trained to discriminate double-click synthetic "echoes" differing in inter-highlight interval (IHI). In the first experimental task, dolphins passively listened to background (S-) stimuli with constant IHI and responded on discriminating a change to target (S+) stimuli with a slightly increased IHI. The second task was similar, but the highlights were assigned random, frequency-independent phase angles. This phase randomization was designed to interfere with potential auditory cues from characteristic spectral interference patterns linked to IHI changes. Discrimination thresholds were higher with randomized phase when the S- stimuli had IHIs of 50-250 μs. Thresholds were highest and comparable at the longest S- IHIs of 375 and 500 μs and were independent of phase condition. Although dolphin detection of changes in highlight envelope timing can explain threshold patterns at 375 and 500 μs, this cue did not explain performance at IHIs less than the dolphin auditory temporal window of ∼250 μs. Modeling results suggested that phase manipulations eliminated the availability of a simple difference in spectral magnitudes at the shortest IHIs, but the perception of a time separation pitch cue may still explain the dolphins' observed threshold patterns.

The effects of range and echo-phase on range resolution in bottlenose dolphins (Tursiops truncatus) performing a successive comparison taska)

Echolocating bats and dolphins use biosonar to determine target range, but differences in range discrimination thresholds have been reported for the two species. Whether these differences represent a true difference in their sensory system capability is unknown. Here, the dolphin's range discrimination threshold as a function of absolute range and echo-phase was investigated. Using phantom echoes, the dolphins were trained to echo-inspect two simulated targets and indicate the closer target by pressing a paddle. One target was presented at a time, requiring the dolphin to hold the initial range in memory as they compared it to the second target. Range was simulated by manipulating echo-delay while the received echo levels, relative to the dolphins' clicks, were held constant. Range discrimination thresholds were determined at seven different ranges from 1.75 to 20 m. In contrast to bats, range discrimination thresholds increased from 4 to 75 cm, across the entire ranges tested. To investigate the acoustic features used more directly, discrimination thresholds were determined when the echo was given a random phase shift (±180°). Results for the constant-phase versus the random-phase echo were quantitatively similar, suggesting that dolphins used the envelope of the echo waveform to determine the difference in range.

Input compensation of dolphin and sea lion auditory brainstem responses using frequency-modulated up-chirps

Frequency-modulated "chirp" stimuli that offset cochlear dispersion (i.e., input compensation) have shown promise for increasing auditory brainstem response (ABR) amplitudes relative to traditional sound stimuli. To enhance ABR methods with marine mammal species known or suspected to have low ABR signal-to-noise ratios, the present study examined the effects of broadband chirp sweep rate and level on ABR amplitude in bottlenose dolphins and California sea lions. "Optimal" chirps were designed based on previous estimates of cochlear traveling wave speeds (using high-pass subtractive masking methods) in these species. Optimal chirps increased ABR peak amplitudes by compensating for cochlear dispersion; however, chirps with similar (or higher) frequency-modulation rates produced comparable results. The optimal chirps generally increased ABR amplitudes relative to noisebursts as threshold was approached, although this was more obvious when sound pressure level was used to equate stimulus levels (as opposed to total energy). Chirps provided progressively less ABR amplitude gain (relative to noisebursts) as stimulus level increased and produced smaller ABRs at the highest levels tested in dolphins. Although it was previously hypothesized that chirps would provide larger gains in sea lions than dolphins-due to the lower traveling wave speed in the former-no such pattern was observed.

Bottlenose dolphin temporary threshold shift following exposure to 10-ms impulses centered at 8 kHza)

Studies of marine mammal temporary threshold shift (TTS) from impulsive sources have typically produced small TTS magnitudes, likely due to much of the energy in tested sources lying below the subjects' range of best hearing. In this study of dolphin TTS, 10-ms impulses centered at 8 kHz were used with the goal of inducing larger magnitudes of TTS and assessing the time course of hearing recovery. Most impulses had sound pressure levels of 175-180 dB re 1 μPa, while inter-pulse interval (IPI) and total number of impulses were varied. Dolphin TTS increased with increasing cumulative sound exposure level (SEL) and there was no apparent effect of IPI for exposures with equal SEL. The lowest TTS onset was 184 dB re 1 μPa2s, although early exposures with 20-s IPI and cumulative SEL of 182-183 dB re 1 μPa2s produced respective TTS of 35 and 16 dB in two dolphins. Continued testing with higher SELs up to 191 dB re 1 μPa2s in one of those dolphins, however, failed to result in TTS greater than 14 dB. Recovery rates were similar to those from other studies with non-impulsive sources and depended on the magnitude of the initial TTS.

Temporary threshold shift in bottlenose dolphins exposed to steady-state, 1/6-octave noise centered at 0.5 to 80 kHza)

Temporary threshold shift (TTS) was measured in bottlenose dolphins after 1-h exposures to 1/6-octave noise centered at 0.5, 2, 8, 20, 40, and 80 kHz. Tests were conducted in netted ocean enclosures, with the dolphins free-swimming during noise exposures. Exposure levels were estimated using a combination of video-based measurement of dolphin position, calibrated exposure sound fields, and animal-borne archival recording tags. Hearing thresholds were measured before and after exposures using behavioral methods (0.5, 2, 8 kHz) or behavioral and electrophysiological [auditory brainstem response (ABR)] methods (20, 40, 80 kHz). No substantial effects of the noise were seen at 40 and 80 kHz at the highest exposure levels. At 2, 8, and 20 kHz, exposure levels required for 6 dB of TTS (onset TTS exposures) were similar to previous studies; however, at 0.5 kHz, onset TTS was much lower than predicted values. No clear relationships could be identified between ABR- and behaviorally measured TTS. The results raise questions about the validity of current noise exposure guidelines for dolphins at frequencies below ∼1 kHz and how to accurately estimate received noise levels from free-swimming animals.

Bats experience age-related hearing loss (presbycusis)

Hearing loss is a hallmark of aging, typically initially affecting the higher frequencies. In echolocating bats, the ability to discern high frequencies is essential. However, nothing is known about age-related hearing loss in bats, and they are often assumed to be immune to it. We tested the hearing of 47 wild Egyptian fruit bats by recording their auditory brainstem response and cochlear microphonics, and we also assessed the cochlear histology in four of these bats. We used the bats' DNA methylation profile to evaluate their age and found that bats exhibit age-related hearing loss, with more prominent deterioration at the higher frequencies. The rate of the deterioration was ∼1 dB per year, comparable to the hearing loss observed in humans. Assessing the noise in the fruit bat roost revealed that these bats are exposed to continuous immense noise-mostly of social vocalizations-supporting the assumption that bats might be partially resistant to loud noise. Thus, in contrast to previous assumptions, our results suggest that bats constitute a model animal for the study of age-related hearing loss.

Effects of echo phase on bottlenose dolphin jittered-echo detection

The ability of bottlenose dolphins to detect changes in echo phase was investigated using a jittered-echo paradigm. The dolphins' task was to produce a conditioned vocalization when phantom echoes with fixed echo delay and phase changed to those with delay and/or phase alternated ("jittered") on successive presentations. Conditions included: jittered delay plus constant phase shifts, ±45° and 0°-180° jittered phase shifts, alternating delay and phase shifts, and random echo-to-echo phase shifts. Results showed clear sensitivity to echo fine structure, revealed as discrimination performance reductions when jittering echo fine structures were similar, but envelopes were different, high performance with identical envelopes but different fine structure, and combinations of echo delay and phase jitter where their effects cancelled. Disruption of consistent echo fine structure via random phase shifts dramatically increased jitter detection thresholds. Sensitivity to echo fine structure in the present study was similar to the cross correlation function between jittering echoes and is consistent with the performance of a hypothetical coherent receiver; however, a coherent receiver is not necessary to obtain the present results, only that the auditory system is sensitive to echo fine structure.

Classification of simulated complex echoes based on highlight time separation in the bottlenose dolphin (Tursiops truncatus)

Previous studies suggested that dolphins perceive echo spectral features on coarse (macrospectrum) and fine (microspectrum) scales. This study was based on a finding that these auditory percepts are, to some degree, dependent on the dolphin's ∼250-μs auditory temporal window (i.e., "critical interval"). Here, two dolphins were trained to respond on passively detecting a simulated "target" echo complex [a pair of echo "highlights" with a characteristic 120-μs inter-highlight interval (IHI)]. This target had unique micro- and macrospectral features and was presented among "distractor" echoes with IHIs from 50 to 500 μs (i.e., microspectra) and various highlight durations (i.e., macrospectra). Following acquisition of this discrimination task, probe echo complexes with the macrospectrum of the target but IHIs matching the distractors were infrequently presented. Both dolphins initially responded more often to probes with IHIs of 80-200 μs. Response strategies diverged with increasing probe presentations; one dolphin responded to a progressively narrower range of probe IHIs while the second increased response rates for probes with IHIs > 250 μs. These results support previous conclusions that perception of macrospectra for complex echoes is nonconstant as the IHI decreases below ∼100 μs, but results approaching and exceeding 250 μs-the temporal window upper boundary-were more ambiguous.

Spatial acuity of the bottlenose dolphin (Tursiops truncatus) biosonar system with a bat and human comparison

Horizontal angular resolution was measured in two bottlenose dolphins using a two-alternative forced-choice, biosonar target discrimination paradigm. The task required a stationary dolphin positioned in a hoop to discriminate two physical targets at a range of 4 m. The angle separating the targets was manipulated to estimate an angular discrimination threshold of 1.5°. In a second experiment, a similar two-target biosonar discrimination task was conducted with one free-swimming dolphin, to test whether its emission beam was a critical factor in discriminating the targets. The spatial separation between two targets was manipulated to measure a discrimination threshold of 6.7 cm. There was a relationship between differences in acoustic signals received at each target and the dolphin's performance. The results of the angular resolution experiment were in good agreement with measures of the minimum audible angle of both dolphins and humans and remarkably similar to measures of angular difference discrimination in echolocating dolphins, bats, and humans. The results suggest that horizontal auditory spatial acuity may be a common feature of the mammalian auditory system rather than a specialized feature exclusive to echolocating auditory predators.

Output compensation of auditory brainstem responses in dolphins and sea lions

Cochlear dispersion causes increasing delays between neural responses from high-frequency regions in the cochlear base and lower-frequency regions toward the apex. For broadband stimuli, this can lead to neural responses that are out-of-phase, decreasing the amplitude of farfield neural response measurements. In the present study, cochlear traveling-wave speed and effects of dispersion on farfield auditory brainstem responses (ABRs) were investigated by first deriving narrowband ABRs in bottlenose dolphins and California sea lions using the high-pass subtractive masking technique. Derived-band ABRs were then temporally aligned and summed to obtain the "stacked ABR" as a means of compensating for the effects of cochlear dispersion. For derived-band responses between 8 and 32 kHz, cochlear traveling-wave speeds were similar for sea lions and dolphins [∼2-8 octaves (oct)/ms for dolphins; ∼3.5-11 oct/ms for sea lions]; above 32 kHz, traveling-wave speed for dolphins increased up to ∼30 oct/ms. Stacked ABRs were larger than unmasked, broadband ABRs in both species. The amplitude enhancement was smaller in dolphins than in sea lions, and enhancement in both species appears to be less than reported in humans. Results suggest that compensating for cochlear dispersion will provide greater benefit for ABR measurements in species with better low-frequency hearing.

Cetacean Acoustic Welfare in Wild and Managed-Care Settings: Gaps and Opportunities

Simple Summary

Whales and dolphins in managed-care and wild settings are exposed to human-made, anthropogenic sounds of varying degrees. These sounds can lead to potential negative welfare outcomes if not managed correctly in zoos or in the open ocean. Current wild regulations are based on generally broad taxa-based hearing thresholds, but there is movement to take other contextual factors into account, partially informed by researchers familiar with work in zoological settings. In this spirit, we present more nuanced future directions for the evaluation of acoustic welfare in both wild and managed-care settings, with suggestions for how research in both domains can inform each other as a means to address the paucity of research available on this topic, especially in managed-care environments.

Abstract

Cetaceans are potentially at risk of poor welfare due to the animals’ natural reliance on sound and the persistent nature of anthropogenic noise, especially in the wild. Industrial, commercial, and recreational human activity has expanded across the seas, resulting in a propagation of sound with varying frequency characteristics. In many countries, current regulations are based on the potential to induce hearing loss; however, a more nuanced approach is needed when shaping regulations, due to other non-hearing loss effects including activation of the stress response, acoustic masking, frequency shifts, alterations in behavior, and decreased foraging. Cetaceans in managed-care settings share the same acoustic characteristics as their wild counterparts, but face different environmental parameters. There have been steps to integrate work on welfare in the wild and in managed-care contexts, and the domain of acoustics offers the opportunity to inform and connect information from both managed-care settings and the wild. Studies of subjects in managed-care give controls not available to wild studies, yet because of the conservation implications, wild studies on welfare impacts of the acoustic environment on cetaceans have largely been the focus, rather than those in captive settings. A deep integration of wild and managed-care-based acoustic welfare research can complement discovery in both domains, as captive studies can provide greater experimental control, while the more comprehensive domain of wild noise studies can help determine the gaps in managed-care based acoustic welfare science. We advocate for a new paradigm in anthropogenic noise research, recognizing the value that both wild and managed-care research plays in illustrating how noise pollution affects welfare including physiology, behavior, and cognition.

Auditory oddball responses in Tursiops truncatus

Two previous studies suggest that bottlenose dolphins exhibit an "oddball" auditory evoked potential (AEP) to stimulus trains where one of two stimuli has a low probability of occurrence relative to another. However, they reported oddball AEPs at widely different latency ranges (50 vs 500 ms). The present work revisited this experiment in a single dolphin to report the AEPs in response to two tones each assigned probabilities of 0.2, 0.8, and 1 across sessions. The AEP was further isolated from background EEG using independent component analysis, and showed condition effects in the 40-60 ms latency range.

Measuring auditory cortical responses in Tursiops truncatus

Auditory neuroscience in dolphins has largely focused on auditory brainstem responses; however, such measures reveal little about the cognitive processes dolphins employ during echolocation and acoustic communication. The few previous studies of mid- and long-latency auditory-evoked potentials (AEPs) in dolphins report different latencies, polarities, and magnitudes. These inconsistencies may be due to any number of differences in methodology, but these studies do not make it clear which methodological differences may account for the disparities. The present study evaluates how electrode placement and pre-processing methods affect mid- and long-latency AEPs in (Tursiops truncatus). AEPs were measured when reference electrodes were placed on the skin surface over the forehead, the external auditory meatus, or the dorsal surface anterior to the dorsal fin. Data were pre-processed with or without a digital 50-Hz low-pass filter, and the use of independent component analysis to isolate signal components related to neural processes from other signals. Results suggest that a meatus reference electrode provides the highest quality AEP signals for analyses in sensor space, whereas a dorsal reference yielded nominal improvements in component space. These results provide guidance for measuring cortical AEPs in dolphins, supporting future studies of their cognitive auditory processing.

The offset auditory brainstem response in bottlenose dolphins (Tursiops truncatus): Evidence for multiple underlying processes

The auditory brainstem response (ABR) to stimulus onset has been extensively used to investigate dolphin hearing. The mechanisms underlying this onset response have been thoroughly studied in mammals. In contrast, the ABR evoked by sound offset has received relatively little attention. To build upon previous observations of the dolphin offset ABR, a series of experiments was conducted to (1) determine the cochlear places responsible for response generation and (2) examine differences in response morphologies when using toneburst versus noiseburst stimuli. Measurements were conducted with seven bottlenose dolphins (Tursiops truncatus) using tonebursts and spectrally "pink" broadband noisebursts, with highpass noise used to limit the cochlear regions involved in response generation. Results for normal-hearing and hearing-impaired dolphins suggest that the offset ABR contains contributions from at least two distinct responses. One type of response (across place) might arise from the activation of neural units that are shifted basally relative to stimulus frequency and shares commonalities with the onset ABR. A second type of response (within place) appears to represent a "true" offset response from afferent centers further up the ascending auditory pathway from the auditory nerve, and likely results from synchronous activity beginning at or above the cochlear nucleus.

Role of the temporal window in dolphin auditory brainstem response onset

Auditory brainstem responses (ABRs) to linear-enveloped, broadband noisebursts were measured in six bottlenose dolphins to examine relationships between sound onset envelope properties and the ABR peak amplitude. Two stimulus manipulations were utilized: (1) stimulus onset envelope pressure rate-of-change was held constant while plateau pressure and risetime were varied and (2) plateau duration was varied while plateau pressure and risetime were held constant. When the stimulus onset envelope pressure rate-of-change was held constant, ABR amplitudes increased with risetime and were fit well with an exponential growth model. The model best-fit time constants for ABR peaks P1 and N5 were 55 and 64 μs, respectively, meaning ABRs reached 99% of their maximal amplitudes for risetimes of 275-320 μs. When plateau pressure and risetime were constant, ABR amplitudes increased linearly with stimulus sound exposure level up to durations of ∼250 μs. The results highlight the relationship between ABR amplitude and the integral of some quantity related to the stimulus pressure envelope over the first ∼250 μs following stimulus onset-a time interval consistent with prior estimates of the dolphin auditory temporal window, also known as the "critical interval" in hearing.

Offset auditory brainstem response (ABR) amplitude in bottlenose dolphins

Although commonly recorded as onset responses, the auditory brainstem response (ABR) can also be elicited at stimulus offset. The offset ABR has not been extensively investigated in marine mammals. Three normal hearing (NH) and three hearing impaired (HI) dolphins were assessed while fully submerged in sea water. Stimulus spectrum, level, rise/fall time (RFT), and plateau duration were manipulated. Onset and offset ABR amplitude were quantified as the rms voltage 1-7 ms following stimulus onset or offset, respectively. For the same stimulus conditions, onset and offset responses were often larger for NH than HI dolphins, and offset responses were typically smaller than onset responses. For the level series, offset response amplitude typically increased with increasing stimulus level, although offset responses were not 3 dB above the noisefloor for 113-kHz tonebursts. Increasing RFT decreased onset and offset response amplitude. For the 40-kHz tonebursts, a RFT of 32 μs produced a large amplitude offset ABR in NH dolphins. Offset responses for 113-kHz tonebursts were 3 dB above the noisefloor at the shortest RFTs. Offset responses were largest for 4 ms duration stimuli (likely due to overlapping onset and offset response analysis windows), but otherwise, offset responses changed little with increasing duration.

Detection of simulated patterned echo packets by bottlenose dolphins (Tursiops truncatus)

Dolphins performing long-range biosonar tasks sometimes use "packets" of clicks, where inter-click-intervals within each packet are less than the two-way acoustic travel time from dolphin to target. The multi-echo nature of packets results in lower detection thresholds than single echoes; however, other potential benefits of packet use remain unexplored. The present study investigated whether structured temporal patterns observed in click packets impart some advantage in detecting echo-like signals embedded in noise. Two bottlenose dolphins were trained to passively listen and detect simulated packets of echoes in background noise consisting of either steady-state broadband Gaussian noise, or Gaussian noise containing randomly presented impulses similar to dolphin clicks. Four different inter-stimulus-interval (ISI) patterns (constant, random, increasing, or decreasing ISI within each packet) were tested. It was hypothesized that decreasing ISIs-found naturally in dolphin packets-would result in the lowest thresholds, while random, unlearnable patterns would result in the highest. However, no biologically significant differences in threshold were found among the four ISI patterns for either noise condition. Thus, the bottlenose dolphin's stereotypical pattern of decreasing ISI during active echolocation did not appear to provide an advantage in packet detection in this passive listening task.

Dolphin echo-delay resolution measured with a jittered-echo paradigm

Biosonar echo delay resolution was investigated in four bottlenose dolphins (Tursiops truncatus) using a "jittered" echo paradigm, where dolphins discriminated between electronic echoes with fixed delay and those whose delay alternated (jittered) on successive presentations. The dolphins performed an echo-change detection task and produced a conditioned acoustic response when detecting a change from non-jittering echoes to jittering echoes. Jitter delay values ranged from 0 to 20 μs. A passive listening task was also conducted, where dolphins listened to simulated echoes and produced a conditioned acoustic response when signals changed from non-jittering to jittering. Results of the biosonar task showed a mean jitter delay threshold of 1.3 μs and secondary peaks in error functions suggestive of the click autocorrelation function. When echoes were jittered in polarity and delay, error functions shifted by approximately 5 μs and all dolphins discriminated echoes that jittered only in polarity. Results were qualitatively similar to those from big brown bats (Eptesicus fuscus) and indicate that the dolphin biosonar range estimator is sensitive to echo phase information. Results of the passive listening task suggested that the dolphins could not passively detect changes in timing and polarity of simulated echoes.