Long-term evidence of noise-induced permanent threshold shift in a harbor seal (Phoca vitulina).

In psychophysical studies of noise-induced hearing loss with marine mammals, exposure conditions are often titrated from levels of no effect to those that induce significant but recoverable loss of auditory sensitivity [temporary threshold shift (TTS)]. To examine TTS from mid-frequency noise, a harbor seal was exposed to a 4.1-kHz underwater tone that was incrementally increased in sound pressure level (SPL) and duration. The seal's hearing was evaluated at the exposure frequency and one-half octave higher (5.8 kHz) to identify the noise parameters associated with TTS onset. No reliable TTS was measured with increasing sound exposure level until the second exposure to a 60-s fatiguing tone of 181 dB re 1 µPa SPL (sound exposure level 199 dB re 1 µPa2s), after which an unexpectedly large threshold shift (>47 dB) was observed. While hearing at 4.1 kHz recovered within 48 h, there was a permanent threshold shift of at least 8 dB at 5.8 kHz. This hearing loss was evident for more than ten years. Furthermore, a residual threshold shift of 11 dB was detected one octave above the tonal exposure, at 8.2 kHz. This hearing loss persisted for more than two years prior to full recovery.

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

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.

Auditory Sensitivity and Masking Profiles for the Sea Otter (Enhydra lutris).

Sea otters are threatened marine mammals that may be negatively impacted by human-generated coastal noise, yet information about sound reception in this species is surprisingly scarce. We investigated amphibious hearing in sea otters by obtaining the first measurements of absolute sensitivity and critical masking ratios. Auditory thresholds were measured in air and underwater from 0.125 to 40 kHz. Critical ratios derived from aerial masked thresholds from 0.25 to 22.6 kHz were also obtained. These data indicate that although sea otters can detect underwater sounds, their hearing appears to be primarily air adapted and not specialized for detecting signals in background noise.

Hearing in the sea otter (Enhydra lutris): auditory profiles for an amphibious marine carnivore.

In this study we examine the auditory capabilities of the sea otter (Enhydra lutris), an amphibious marine mammal that remains virtually unstudied with respect to its sensory biology. We trained an adult male sea otter to perform a psychophysical task in an acoustic chamber and at an underwater apparatus. Aerial and underwater audiograms were constructed from detection thresholds for narrowband signals measured in quiet conditions at frequencies from 0.125-40 kHz. Aerial hearing thresholds were also measured in the presence of octave-band masking noise centered at eight signal frequencies (0.25-22.6 kHz) so that critical ratios could be determined. The aerial audiogram of the sea otter resembled that of sea lions and showed a reduction in low-frequency sensitivity relative to terrestrial mustelids. Best sensitivity was -1 dB re 20 µPa at 8 kHz. Under water, hearing sensitivity was significantly reduced when compared to sea lions and other pinniped species, demonstrating that sea otter hearing is primarily adapted to receive airborne sounds. Critical ratios were more than 10 dB higher than those measured for pinnipeds, suggesting that sea otters are less efficient than other marine carnivores at extracting acoustic signals from background noise, especially at frequencies below 2 kHz.

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).

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 water's 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.