Dolphin short-term auditory fatigue and self-mitigation.

Auditory brainstem responses (ABRs) were measured at 57 kHz in two dolphins warned of an impending intense tone at 40 kHz. Over the course of testing, the duration of the intense tone was increased from 0.5 to 16 s to determine if changes in ABRs observed after cessation of the intense sound were the result of post-stimulatory auditory fatigue or conditioned hearing attenuation. One dolphin exhibited conditioned hearing attenuation after the warning sound preceding the intense sound, but little evidence of post-stimulatory fatigue after the intense sound. The second dolphin showed no conditioned attenuation before the intense sound, but auditory fatigue afterwards. The fatigue was observed within a few seconds after cessation of the intense tone: i.e., at time scales much shorter than those in previous studies of marine mammal noise-induced threshold shifts, which feature measurements on the order of a few minutes after exposure. The differences observed between the two individuals (less auditory fatigue in the dolphin that exhibited the conditioned attenuation) support the hypothesis that conditioned attenuation is a form of "self-mitigation."

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.

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.

Dolphin conditioned hearing attenuation in response to repetitive tones with increasing level.

All species of toothed whales studied to date can learn to reduce their hearing sensitivity when warned of an impending intense sound; however, the specific conditions under which animals will employ this technique are not well understood. The present study was focused on determining whether dolphins would reduce their hearing sensitivity in response to an intense tone presented at a fixed rate but increasing level, without an otherwise explicit warning. Auditory brainstem responses (ABRs) to intermittent, 57-kHz tone bursts were continuously measured in two bottlenose dolphins as they were exposed to a series of 2-s, 40-kHz tones at fixed time intervals of 20, 25, or 29 s and at sound pressure levels (SPLs) increasing from 120 to 160 dB re 1 µPa. Results from one dolphin showed consistent ABR attenuation preceding intense tones when the SPL exceeded ∼140-150 dB re 1 µPa and the tone interval was 20 s. ABR attenuation with 25- or 29-s intense tone intervals was inconsistent. The second dolphin showed similar, but more subtle, effects. The results show dolphins can learn the timing of repetitive noise and may reduce their hearing sensitivity if the SPL is high enough, presumably to "self-mitigate" the noise effects.

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.

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.

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.

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.

Metabolic response of dolphins to short-term fasting reveals physiological changes that differ from the traditional fasting model.

Bottlenose dolphins (Tursiops truncatus) typically feed on prey that are high in lipid and protein content and nearly devoid of carbohydrate, a dietary feature shared with other marine mammals. However, unlike fasted-adapted marine mammals that predictably incorporate fasting into their life history, dolphins feed intermittently throughout the day and are not believed to be fasting-adapted. To assess whether the physiological response to fasting in the dolphin shares features with or distinguishes them from those of fasting-adapted marine mammals, the plasma metabolomes of eight bottlenose dolphins were compared between post-absorptive and 24-h fasted states. Increases in most identified free fatty acids and lipid metabolites and reductions in most amino acids and their metabolites were consistent with the upregulation of lipolysis and lipid oxidation and the downregulation of protein catabolism and synthesis. Consistent with a previously hypothesized diabetic-like fasting state, fasting was associated with elevated glucose and patterns of certain metabolites (e.g. citrate, cis-aconitate, myristoleic acid) indicative of lipid synthesis and glucose cycling to protect endogenous glucose from oxidative disposal. Pathway analysis predicted an upregulation of cytokines, decreased cell growth and increased apoptosis including apoptosis of insulin-secreting ß-cells. Metabolomic conditional mutual information networks were estimated for the post-absorptive and fasted states and 'topological modules' were estimated for each using the eigenvector approach to modularity network division. A dynamic network marker indicative of a physiological shift toward a negative energy state was subsequently identified that has the potential conservation application of assessing energy state balance in at-risk wild dolphins.

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.

Effects of dolphin hearing bandwidth on biosonar click emissions.

Differences in odontocete biosonar emissions have been reported for animals with hearing loss compared to those with normal hearing. For example, some animals with high-frequency hearing loss have been observed to lower the dominant frequencies of biosonar signals to better match a reduced audible frequency range. However, these observations have been limited to only a few individuals and there has been no systematic effort to examine how animals with varying degrees of hearing loss might alter biosonar click properties. In the present study, relationships between age, biosonar click emissions, auditory evoked potentials (AEPs), and hearing bandwidth were studied in 16 bottlenose dolphins (Tursiops truncatus) of various ages and hearing capabilities. Underwater hearing thresholds were estimated by measuring steady-state AEPs to sinusoidal amplitude modulated tones at frequencies from 23 to 152 kHz. Input-output functions were generated at each tested frequency and used to calculate frequency-specific thresholds and the upper-frequency limit of hearing for each subject. Click emissions were measured during a biosonar aspect change detection task using a physical target. Relationships between hearing capabilities and the acoustic parameters of biosonar signals are described here and compared to previous experiments with fewer subjects.

Classification of biosonar target echoes based on coarse and fine spectral features in the bottlenose dolphin (Tursiops truncatus).

Previous bottlenose dolphin studies suggest that the coarse envelope of an echo spectrum ("macrostructure") has hierarchical dominance over finer-scale spectral features ("microstructure") during synthetic echo discrimination tasks. In this study, two dolphins listened to and discriminated between underwater sound stimuli consisting of pairs of clicks with different micro- and macrostructures. After conditioning dolphins to reliably discriminate between two "anchor" stimuli with different micro- and macrostructures, probe stimuli, which contained a macrostructure identical to one of the anchor stimuli and the microstructure of the alternate anchor, were infrequently presented. Dolphins responded to probes in a manner consistent with macrostructure primacy.

Endocrine response to simulated U.S. Navy mid-frequency sonar exposures in the bottlenose dolphin (Tursiops truncatus).

Little information exists on endocrine responses to noise exposure in marine mammals. In the present study, cortisol, aldosterone, and epinephrine levels were measured in 30 bottlenose dolphins (Tursiops truncatus) before and after exposure to simulated U.S. Navy mid-frequency sonar signals (3250-3450 Hz). Control and exposure sessions, each consisting of ten trials, were performed sequentially with each dolphin. While swimming across the experimental enclosure during exposure trials, each dolphin received a single 1-s exposure with received sound pressure levels (SPLs, dB re 1 µPa) of 115, 130, 145, 160, 175, or 185 dB. Blood samples were collected through behaviorally conditioned, voluntary participation of the dolphins approximately one week prior to, immediately following, and approximately one week after exposure were analyzed for hormones via radioimmunoassay. Aldosterone was below detection limits in all samples. Neither cortisol nor epinephrine showed a consistent relationship with received SPL, even though dolphins abandoned trained behaviors after exposure to the highest SPLs and the severity of behavioral changes scaled with SPL. It remains unclear if dolphins interpret high-level anthropogenic sound as stressful, annoying, or threatening and whether behavioral responses to sound can be equated to a physiological (endocrine) response.

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.

Spectral cues and temporal integration during cylinder echo discrimination by bottlenose dolphins (Tursiops truncatus).

Three bottlenose dolphins (Tursiops truncatus) participated in simulated cylinder wall thickness discrimination tasks utilizing electronic "phantom" echoes. The first experiment resulted in psychometric functions (percent correct vs wall thickness difference) similar to those produced by a dolphin performing the task with physical cylinders. In the second experiment, a wide range of cylinder echoes was simulated, with the time separation between echo highlights covering a range from <30 to >300 µs. Dolphin performance and a model of the dolphin auditory periphery suggest that the dolphins used high-frequency, spectral-profiles of the echoes for discrimination and that the utility of spectral cues degraded when the time separation between echo highlights approached and exceeded the dolphin's temporal integration time of ∼264 µs.

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.

Jittered echo-delay resolution in bottlenose dolphins (Tursiops truncatus).

Psychophysical methods similar to those employed with bats were used to examine jittered echo-delay resolution in bottlenose dolphins (Tursiops truncatus). Two dolphins were trained to produce echolocation clicks and report a change from electronic echoes with a fixed delay of ~ 12.6 ms (~ 9.4 m simulated range) to echoes with delays that alternated (jittered) between successive emitted signals. Jitter delays varied from 0 to 50 µs. Jittered echo-delay thresholds were between 1 and 2 µs-the lowest achievable (non-zero) values with the hardware configuration. Error functions matched the click autocorrelation function near zero jitter delay, and were well within the envelope of the autocorrelation function; however, measured jitter delay thresholds were larger than predictions for a coherent or semicoherent receiver at comparable signal-to-noise ratios. When one of the two alternating jittered echoes was inverted in polarity, both dolphins reliably discriminated echoes at all jittered echo delays, including 0 µs (i.e., only jittering in polarity, not delay). Finally, both dolphins used unusual patterns of click emissions, where groups of echolocation clicks were interspersed with silent gaps. Further tests with sub-microsecond jitter values and various echo signal-to-noise ratios would be necessary for proper direct comparison with jitter detection values obtained for bats.

Dolphin echolocation behaviour during active long-range target approaches.

Echolocating toothed whales generally adjust click intensity and rate according to target range to ensure that echoes from targets of interest arrive before a subsequent click is produced, presumably facilitating range estimation from the delay between clicks and returning echoes. However, this click-echo-click paradigm for the dolphin biosonar is mostly based on experiments with stationary animals echolocating fixed targets at ranges below ∼120 m. Therefore, we trained two bottlenose dolphins instrumented with a sound recording tag to approach a target from ranges up to 400 m and either touch the target (subject TRO) or detect a target orientation change (subject SAY). We show that free-swimming dolphins dynamically increase interclick interval (ICI) out to target ranges of ∼100 m. TRO consistently kept ICIs above the two-way travel time (TWTT) for target ranges shorter than ∼100 m, whereas SAY switched between clicking at ICIs above and below the TWTT for target ranges down to ∼25 m. Source levels changed on average by 17log10(target range), but with considerable variation for individual slopes (4.1 standard deviations for by-trial random effects), demonstrating that dolphins do not adopt a fixed automatic gain control matched to target range. At target ranges exceeding ∼100 m, both dolphins frequently switched to click packet production in which interpacket intervals exceeded the TWTT, but ICIs were shorter than the TWTT. We conclude that the click-echo-click paradigm is not a fixed echolocation strategy in dolphins, and we demonstrate the first use of click packets for free-swimming dolphins when solving an echolocation task.

Frequency-modulated up-chirp stimuli enhance the auditory brainstem response of the killer whale (Orcinus orca).

Previous studies suggested that frequency-modulated tonal stimuli where the frequency sweeps upward (up-chirps) may enhance auditory brainstem response (ABR) amplitudes in mammals. In this study, ABRs were measured in response to up-chirps in three killer whales (Orcinus orca) and compared to ABRs evoked by broadband clicks. Chirp durations ranged from 125 - 2000 µs. Chirp spectral content was either "uncompensated," meaning the spectrum paralleled the transmitting response of the piezoelectric transducer, or "compensated," where the spectral density level was flat (+/-4 dB) across the stimulus bandwidth (10 - 130 kHz). Compensated up-chirps consistently produced higher amplitude ABRs than uncompensated clicks with the same peak equivalent sound pressure level. ABR amplitude increased with up-chirp duration up to 1400 µs, although there was considerable variability between individuals. Results suggest that compensating stimuli for the response of transducers can have a dramatic effect on broadband ABRs, and that compensated up-chirps might be useful for testing whale species where large size makes far-field recording of ABRs at the skin surface difficult.

Detailed analysis of two detected overlaying transient components within the echolocation beam of a bottlenose dolphin (Tursiops truncatus).

Dolphin echolocation clicks measured far off-axis contain two time-separated components. Whether these components overlap and appear as a single signal on axis has received little attention. Here, the scaled reassigned spectrogram analysis was used to examine if bottlenose dolphin (Tursiops truncatus) clicks measured near- or on-axis of the echolocation beam contained overlapping components. Across click trains, the number of overlapping components spatially varied within the echolocation beam. Two overlapping components were found to predominantly occur in the upper portion of the beam, whereas the lower portion of the beam predominantly contained a single component. When components overlapped, the trailing component generally had a higher center frequency and arrived less than 5 µs after the leading component. The spatial relationship of components was consistent with previous findings of two vertically distinct beam lobes with separated frequency content. The two components in the upper portion of the beam possibly result from a single transient click propagating through a geometrically dispersive media; specifically, the slower sound speed of the dolphin melon's core slightly delays the more directional, high frequency energy of the click, whereas the less directional, lower frequency energy propagates through more peripheral but higher sound speed portions of the melon.