Asila Ghoul

Noise‐induced permanent threshold shift in a harbor seal

Investigation of auditory temporary threshold shift (TTS) in marine mammals has provided a means of predicting the harmful effects of underwater anthropogenic noise. As complete recovery of hearing is requisite in these studies, they have been considered appropriate in light of subject availability and ethical considerations. In our psychophysical studies of TTS in pinnipeds, we have employed methods designed to safely titrate from sound levels of noise inducing no TTS to levels of significant but completely recoverable hearing loss. In the present study, these methods were used with a harbor seal (Phoca vitulina) exposed to an underwater 4.1 kHz pure tone fatiguing stimulus. Sound levels and durations were gradually increased to a maximum received sound pressure of 184 dB re 1 μPa with a duration of 60 s (SEL=202 dB re 1 μPa2s). Upon the second exposure to this fatiguing stimulus, an initial threshold shift in excess of 50 dB was estimated at a test frequency of 5.8 kHz, a half‐octave above the fatiguing...

Temporal summation of airborne tones in a California sea lion (Zalophus californianus)

The trade-off between sound level and duration on hearing sensitivity (temporal summation) was investigated in a California sea lion (Zalophus californianus) using airborne pure-tone stimuli. Thresholds were behaviorally measured using the method of constant stimuli at 2.5, 5, and 10 kHz for nine signal durations ranging from 25 to 500 ms. In general, thresholds decreased as duration increased up to 300 ms, beyond which thresholds did not significantly improve. When these data were fitted separately to two versions of an exponential model, the estimated time constants (92-167 ms) were generally consistent between the two fits. However, the model with more free parameters generated fits with consistently higher R(2) values, while avoiding potential arbitrary decisions about which data to include. The time constants derived for the California sea lion were generally consistent with those reported for other mammals, including other pinnipeds. The current study did not show a clear correlation between time constant and test frequency. The results should be considered when conducting audiometric work, assessing communications ranges, and evaluating potential noise impacts of airborne tonal signals on California sea lions.

Temporal processing of low-frequency sounds by seals (L)a)

In a recent study, Kastelein et al. [(2010) J. Acoust. Soc. Am. 127, 1135-1145] reported auditory integration times for harbor seals (Phoca vitulina) exceeding 3000 ms for 200 Hz tonal signals. This finding is unexpected and potentially significant given that time constants measured in mammals for tones above 1 kHz are typically less than 500 ms. To further explore this result, the hearing of another harbor seal was measured in air and water for 200 Hz tones with durations of 500 and 2500 ms. Threshold comparisons, as well as reaction time measures, revealed no gain in audibility as signal duration increased above 500 ms.

Sound Production and Reception in Southern Sea Otters (Enhydra lutris nereis)

Because of their dependence on a highly restricted coastal habitat, Enhydra lutris is especially vulnerable to a variety of different environmental and anthropogenic threats. This species is presently listed as threatened and is protected throughout the northern and southern portions of its range.Resource managers are presently faced with uncertainty when responding to and prioritizing potential threats to these animals due to insufficient understanding of the factors that may disturb or disrupt normal behavior patterns both above and below the waters surface. The objective of these studies was to obtain direct measurements of the source characteristics of vocalizations and the limits of auditory reception in Enhydra lutris. These data are necessary to form a basic but essential under-standing of bioacoustics in this species. To further develop this knowledge base, psychoacoustic profiles of aerial and underwater hearing sensitivity as a function of sound frequency are imperative to adequately consider sea otters alongside other marine mammals within the issue of anthropogenic impacts. These studies are presently ongoing i n our laboratory. A s these coastal-living carnivores have only recently transitioned to a marine lifestyle, an improved understanding of their acoustic communication and auditory adaptations will also provide insight into their evolutionary biology and behavioral ecology as well as the evolutionary pressures shaping underwater perception in marine mammals.

Preliminary investigation of sound reception in southern sea otters (Enhydra lutris nereis).

Due to their dependence upon a highly restricted coastal habitat, sea otters are vulnerable to a variety of environmental and anthropogenic threats. Among these is the potential disturbance from human‐generated sources of noise. Presently, there are no data on the auditory sensitivity of sea otters, and little evidence to suggest what sounds may be most relevant to these animals. As an initial step toward describing the acoustic sense of sea otters, we conducted a controlled exposure experiment, adapted from sound exposure studies used in behavioral field research, to efficiently measure the aerial frequency range of hearing in four captive sea otters. This approach was designed to determine which frequencies were audible to each animal, rather than to provide direct measures of auditory sensitivity. The maximum range of aerial hearing determined using this method was 0.125 to 32 kHz. These are the first direct measurements of hearing obtained for sea otters, and the results are relevant to improving unde...

Auditory temporal summation in pinnipeds.

In addition to improving the understanding of auditory processing in pinnipeds, direct measures of temporal summation are relevant to the selection of signal parameters when conducting audiometric research, assessing the effects of signal duration on communication ranges, and evaluating the potential auditory impacts of anthropogenic signals. In the present study, individuals from three pinniped species were tested to determine how signal duration influenced pure‐tone hearing thresholds. The psychophysical method of constant stimuli was used to obtain aerial thresholds for each subject at nine different signal durations ranging from 25 to 500 ms. Parameter estimates derived for a California sea lion (Zalophus californianus) from an exponential model of temporal summation yielded time constants (τ) of 176, 98, and 141 ms at frequencies of 2.5, 5, and 10 kHz, respectively. Preliminary results with a northern elephant seal (Mirounga angustirostris) at 5 kHz (this study), and a harbor seal (Phoca vitulina) at...

Low-frequency temporary threshold shift not observed in spotted or ringed seals exposed to single air gun impulsesa)

Underwater hearing thresholds were measured at 100 Hz in trained spotted (Phoca largha) and ringed seals (Pusa hispida) before and immediately following voluntary exposure to impulsive noise from a seismic air gun. Auditory responses were determined from psychoacoustic data and behavioral responses were scored from video recordings. Four successive exposure conditions of increasing level were tested, with received unweighted sound exposure levels from 165 to 181 dB re 1 μPa2 s and peak-to-peak sound pressures from 190 to 207 dB re 1 μPa. There was no evidence that these single seismic exposures altered hearing-including in the highest exposure condition, which matched previous predictions of temporary threshold shift (TTS) onset. Following training at low exposure levels, relatively mild behavioral responses were observed for higher exposure levels. This demonstrates that individuals can learn to tolerate loud, impulsive sounds, but does not necessarily imply that similar sounds would not elicit stronger behavioral responses in wild seals. The absence of observed TTS confirms that regulatory guidelines (based on M-weighting) for single impulse noise exposures are conservative for seals. However, additional studies using multiple impulses and/or higher exposure levels are needed to quantify exposure conditions that do produce measurable changes in hearing sensitivity.

Hearing in Sea Otters (Enhydra lutris): Audible Frequencies Determined from a Controlled Exposure Approach

The sea otter (Enhydra lutris) is an amphibious marine mammal that is vulnerable to coastal anthropogenic disturbance. Effective management of noise-generating activities within sea otter habitats requires information about hearing that is presently unavailable for this species. As an initial step toward describing the auditory capabilities of sea otters, we used a controlled exposure approach to conservatively estimate the aerial frequency range of hearing in four captive individuals. The study was designed to determine which frequencies were audible to each animal rather than to quantify auditory sensitivity. To this end, the sea otters were intermittently exposed to relatively highamplitude tones between 0.063 and 45.3 kHz— and to blank “control” events—during periods of sustained rest. Positive responses to both the sound exposure trials and the control trials were scored by experimentally blind observers and used to determine statistically reliable detections at each frequency. The widest confirmed hearing range measured for the sea otters was 0.125 to 32 kHz. Our results indicate that sea otters can detect a broad range of airborne sounds, similar to many terrestrial carnivores that have been studied. These are the first hearing measurements obtained for this species, and the results are relevant to improving understanding of sea otter acoustic communication, evolutionary biology, and behavioral ecology, as well as in supporting ongoing conservation efforts. This method can be adapted to examine the acoustic detection capabilities of species for which little data are available and for which conventional audiometry may prove challenging.

Psychophysical studies of hearing in sea otters (Enhydra lutris)

The sensory biology of sea otters is of special interest, given their amphibious nature and their recent evolutionary transition from land to sea. However, little is known about the acoustic sense of sea otters, including sensitivity to airborne and underwater sound. In this study, we sought to obtain direct measures of auditory function. We trained an adult-male southern sea otter to participate in audiometric testing in an acoustic chamber and an acoustically mapped pool. We used a psychoacoustic method of limits to determine absolute auditory thresholds in air and under water across the hearing range. In addition to obtaining aerial and underwater audiograms, we also evaluated hearing in the presence of noise. The otter’s aerial hearing closely resembled that of a sea lion, and showed reduced sensitivity to high-frequency (>22 kHz) and low-frequency (<2 kHz) sounds relative to terrestrial mustelids. Under water, hearing was less sensitive than sea lions and other pinnipeds, especially at frequencies be...

Psychophysical audiogram of a California sea lion (Zalophus californianus) listening for airborne tonal sounds in an acoustic chamber

Many species-typical audiograms for marine mammals are based on data from only one or a few individuals that are not always tested under ideal conditions. Here, we report auditory thresholds across the frequency range of hearing for a healthy, five-year-old female California sea lion identified as Ronan. Ronan was trained to enter a hemi-anechoic acoustic chamber to perform a go/no-go audiometric experiment. Auditory sensitivity was measured first by an adaptive staircase procedure and then by the method of constant stimuli. Minimum audible field measurements were obtained for 500 ms frequency-modulated tonal upsweeps with 10% bandwidth and 5% rise and fall times. Thresholds were measured at 13 frequencies: in one-octave frequency steps from 0.1 to 25.6 kHz, and additionally at 18.0, 22.0, 36.2, and 40.0 kHz. Sensitivity was greatest between ~0.9 and 23 kHz, with best hearing of 0 dB re 20 μPa at 12.8 kHz. Hearing range, determined at the 60 dB re 20 μPa level, extended from ~0.2 kHz to 38 kHz. Sensitivit...