Predicting temporary threshold shifts in a bottlenose dolphin (Tursiops truncatus) : the effects of noise level and duration

Author Posting. © Acoustical Society of America, 2009. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 125 (2009): 1816-1826, doi:10.1121/1.3068456.Noise levels in the ocean are increasing and are expected to affect marine mammals. To examine the auditory effects of noise on odontocetes, a bottlenose dolphin (Tursiops truncatus) was exposed to octave-band noise (4–8 kHz) of varying durations (<2–30 min) and sound pressures (130–178 dB re 1 µPa). Temporary threshold shift (TTS) occurrence was quantified in an effort to (i) determine the sound exposure levels (SELs) (dB re 1 µPa2 s) that induce TTS and (ii) develop a model to predict TTS onset. Hearing thresholds were measured using auditory evoked potentials. If SEL was kept constant, significant shifts were induced by longer duration exposures but not for shorter exposures. Higher SELs were required to induce shifts in shorter duration exposures. The results did not support an equal-energy model to predict TTS onset. Rather, a logarithmic algorithm, which increased in sound energy as exposure duration decreased, was a better predictor of TTS. Recovery to baseline hearing thresholds was also logarithmic (approximately −1.8 dB/doubling of time) but indicated variability including faster recovery rates after greater shifts and longer recoveries necessary after longer duration exposures. The data reflected the complexity of TTS in mammals that should be taken into account when predicting odontocete TTS.This work was funded by the Office of Naval Research Grant No. 00014-098-1-687 to P.E.N. and the support of Bob Gisiner and Mardi Hasting is noted. Additional support came from SeaSpace to T.A.M

Hearing Sensation Levels of Emitted Biosonar Clicks in an Echolocating Atlantic Bottlenose Dolphin

Emitted biosonar clicks and auditory evoked potential (AEP) responses triggered by the clicks were synchronously recorded during echolocation in an Atlantic bottlenose dolphin (Tursiops truncatus) trained to wear suction-cup EEG electrodes and to detect targets by echolocation. Three targets with target strengths of −34, −28, and −22 dB were used at distances of 2 to 6.5 m for each target. The AEP responses were sorted according to the corresponding emitted click source levels in 5-dB bins and averaged within each bin to extract biosonar click-related AEPs from noise. The AEP amplitudes were measured peak-to-peak and plotted as a function of click source levels for each target type, distance, and target-present or target-absent condition. Hearing sensation levels of the biosonar clicks were evaluated by comparing the functions of the biosonar click-related AEP amplitude-versus-click source level to a function of external (in free field) click-related AEP amplitude-versus-click sound pressure level. The results indicated that the dolphin's hearing sensation levels to her own biosonar clicks were equal to that of external clicks with sound pressure levels 16 to 36 dB lower than the biosonar click source levels, varying with target type, distance, and condition. These data may be assumed to indicate that the bottlenose dolphin possesses effective protection mechanisms to isolate the self-produced intense biosonar beam from the animal's ears during echolocation

Schematic of the dolphin's relative position, data recording equipments, and data flow chart.

<p>AS, Acoustic screen; h, recording hydrophone; HS, Hoop station; PC, Laptop computer; t, Target during echolocation sessions or transducer during AEP recording to external “dolphin-like” clicks; VS, Visual screen.</p

Basic statistics of the regression line shift (dB re: 1 μPa) measured by point-by-point click level differences along the regression lines between emitted biosonar click-related AEP amplitude-versus-click SL functions and external click-related AEP amplitude-versus-click SPL function, and comparison between target-present and target-absent conditions (Paired T-test, i.e., T-test for dependent samples).

<p>*, and ***, difference is significant at p-level<0.05.</p

Distributions of emitted click SLs for different target types, distances, and target-present (a) and target-absent (b) conditions.

<p>M is the mean value ± s.d. of the click levels, N is number of collected clicks.</p

AEP amplitude-versus-click level functions.

<p>Comparison between functions of the biosonar click-related AEP amplitude-versus-click SL and the external click-related AEP amplitude-versus-click SPL for the target with −22-dB target strength at distances of 2 (a), 3.5 (b), and 6.5 m (c), respectively, under both target-present and target-absent conditions. For each function, a least-square linear regression line, the corresponding equation and the correlation coefficient are indicated.</p

Waveform (a) and spectrum (b) of the external “dolphin-like” clicks.

<p>Waveform (a) and spectrum (b) of the external “dolphin-like” clicks.</p

Experimental facilities and setup (top view).

<p>1, Hoop station; 2, Recording hydrophone; 3, Target during echolocation sessions or transducer during AEP recording to external “dolphin-like” clicks; 4, Acoustic screen; 5, Visual screen; 6, Response ball; 7, Video camera; 8, Instrument shack; 9, Window; 10, Stationing pad; 11, Trainer position; a, the animal swam to the hoop station from the stationing pad; b, the animal got out of the hoop station to touch the response ball to report that the target was present; c, the animal swam back to the stationing pad.</p

Same as in <b>Fig. 5</b> but for the target with −28-dB target strength.

<p>Same as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0029793#pone-0029793-g005" target="_blank"><b>Fig. 5</b></a> but for the target with −28-dB target strength.</p

Same as in <b>Fig. 5</b> but for the target with −34-dB target strength.

<p>Same as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0029793#pone-0029793-g005" target="_blank"><b>Fig. 5</b></a> but for the target with −34-dB target strength.</p