Kristen A. Taylor

Hearing measurements from a stranded infant Risso's dolphin, Grampus griseus.

SUMMARY An infant Rissos dolphin (Grampus griseus) was rescued from the beach in Southern Portugal, and an audiogram was measured using auditory evoked potentials (AEP) and envelope following response (EFR) techniques for frequencies from 4 to 150 kHz. The stimuli used were custom sinusoidally amplitude-modulated (SAM) tone-bursts, and the AEP responses were collected, averaged and analyzed to quantify the animals physiological response and, thereby, hearing thresholds. The infant animal showed a wide range of best sensitivity, with the lowest threshold of 49.5 dB re. 1 μPa at 90 kHz. The audiogram showed a typical mammalian ∪-shape with a gradual, low-frequency slope of 16.4 dB octave-1 and a sharp high-frequency increase of 95 dB octave-1. When compared with an audiogram of an older Rissos dolphin obtained using behavioral methods, the threshold values at upper frequencies were much lower for this infant animal, and this infant heard higher frequencies. These results redefine the hearing capabilities of Rissos dolphins by demonstrating very high-frequency sensitivity.

Hearing and Auditory Evoked Potential Methods Applied to Odontocete Cetaceans

Auditory evoked potential (AEP) procedures have been increasingly used to measure hearing processes in aquatic mammals. They have been demonstrated to be useful in measuring the audiograms of stranded animals like infant sperm whales (Physeter macrocephalus) and Risso’s dolphins (Grampus griseus). Modulation rate transfer functions (MRTF) demonstrating appropriate stimulus presentation rates are usually measured prior to recording audiograms with odontocetes. Measures comparing behavioral and AEP audiograms with the same animals have generally shown good correspondence between data gathered using the two procedures. AEPs and acoustic brainstem responses (ABRs) also have been used to measure hearing while an animal is actively echolocating. This technique of measuring the animal’s ability to hear its own outgoing signals, as well as the returning echoes, allows experimenters to develop a new understanding of the processes underlying echolocation.

Polar bear Ursus maritimus hearing measured with auditory evoked potentials

While there has been recent concern about the effects of sound on marine mammals, including polar bears, there are no data available measuring the hearing of any bear. The in-air hearing of three polar bears was measured using evoked auditory potentials obtained while tone pips were played to three individually anaesthetized bears at the Kolmården Djurpark. Hearing was tested in half-octave steps from 1 to 22.5 kHz. Measurements were not obtainable at 1 kHz and best sensitivity was found in the range from 11.2–22.5 kHz. Considering the tone pips were short and background noise measurements were available, absolute measurements were estimated based on an assumed mammalian integration time of 300 ms. These data show sensitive hearing in the polar bear over a wide frequency range and should cause those concerned with the introduction of anthropogenic noise into the polar bears environment to operate with caution.

A Portable System for the Evaluation of the Auditory Capabilities of Marine Mammals

We have created a portable system that is capable of measuring the hearing thresholds of marine mammals. It was designed for the purpose of testing the auditory capabilities for a wide range of marine mammal individuals and species. This system consists of multiple individual components, independently purchased or assembled. The major component of the system is a standard laptop computer with custom software that is able to both generate outgoing signals and acquire the corresponding brain potential measurements in response to those outgoing signals. The system has been, and still is, in an ongoing state of improvement and optimization with the goal of having a final system that could be used in nearly all field conditions.

Evaluation of the auditory capabilities of marine mammals using a portable auditory‐evoked potential system

There is growing concern about the increasing amounts of acoustic pollution in our oceans. Anthropogenic noise sources span a large range of frequencies and amplitudes. The extents to which these noise sources adversely impact marine mammals is poorly understood yet they are of particular significance given that the acoustic environment is crucial for many aspects of marine mammal life. Past and present experiments have addressed the effects of different sounds on the hearing capabilities of captive dolphins. However, only a few individuals from a small number of species have been tested. Our research group has created a portable system that is capable of measuring the hearing thresholds of marine mammals using auditory‐evoked potential (AEP) techniques. This portable system has enabled us to obtain audiograms of otherwise inaccessible marine mammal species. This system will also allow us to measure the hearing of stranded animals as well as begin to quantify the normal variations in hearing between diffe...

Auditory evoked potential measurement methodology for odontocetes and a comparison of measured thresholds with those obtained using psychophysical techniques

Most measurements of the hearing capabilities of toothed whales and dolphins have been taken using traditional psychophysical procedures in which the animals have been maintained in laboratory environments and trained to behaviorally report the sensation or difference of acoustic stimuli. Because of the advantage of rapid data collection, increased opportunities, and new methods, Auditory Evoked Potentials (AEPs) have become increasingly used to measure audition. The use of this new procedure calls to question the comparability of the established literature and the new results collected with AEPs. The results of behavioral and AEP methods have been directly compared with basic audiogram measurements and have been shown to produce similar (but not exactly the same) values when the envelope following response procedure has been used and the length of the stimulus is taken into account. The AEP methods allow possible audiometric opportunities beyond those available with conventional psychophysics including: ...

Hearing thresholds of a stranded infant Rissos dolphin

The underwater hearing of an infant male Rissos dolphin that stranded off the coast of southern Portugal was measured using evoked auditory potentials (AEPs). Hearing thresholds were measured from envelope following responses to amplitude modulated pure tones ranging from 4 to 150 kHz. Acoustic signals were presented within the calibrated rehabilitation pool. Evoked responses were passively gathered from human EEG electrode sensors imbedded within rubber suction cups that were gently attached to the animal with slight suction and conductor gel. One sensor was placed behind the blowhole and the other reference sensor on the back. Tones were presented 1 m directly in front of the animal under water while the animal was held in a neutrally buoyant position. Unlike the previously published audiogram for an older Rissos dolphin, the audiogram, obtained by presenting 18 different frequencies, showed that this very young animal heard tones up to 150 kHz in a manner similar to other odontocetes. These data, colle...

Directional sensitivity and hearing pathways in the beluga whale, Delphinapterus leucas

Although much variation exists in jaw morphology among species, odontocetes are believed to receive sound primarily through the pan bone region of the lower jaw. In order to further examine this jaw hearing hypothesis, we tested the head receiving sensitivity and directional hearing of a beluga whale, Delphinapterus leucas. Hearing measurements were conducted with a 9‐yr‐old female beluga using the auditory evoked potential technique. A preliminary audiogram indicated that the subject had very sensitive hearing (45‐55dB from 32‐80 kHz) and heard up to 128 kHz. The pathway investigation used a piezo‐electric transducer to present the click stimuli, whereas the hearing directivity was measured in the far field, also using broadband clicks. Like the bottlenose dolphin, the subject had a region of high sensitivity around the pan bone region (78 dB), however, unlike the bottlenose dolphin, we found that the region of maximum sensitivity was at the tip of the lower jaw (75dB). The subject also proved to have hi...

Auditory evoked potential hearing measures of a group of bottlenosed dolphins (Tursiopstruncatus)

The hearing of the bottlenosed dolphins at Kolmrdens Djurpark, Sweden, was examined using a portable auditory evoked potential (AEP) system in order to examine the hearing capabilities of dolphins housed in quiet environments with an emphasis on age‐related hearing variation. Audiograms were measured on two dolphins and modulation rate transfer functions (MRTF) were also measured on two animals. A complete audiogram (4–200 kHz) was obtained for the oldest dolphin, a 34 year old female named Vicky. Her audiogram showed no evidence of high‐frequency hearing loss. Notwithstanding this finding, her click evoked auditory brainstem response (ABR) revealed a marked increase in interpeak latency that could be indicative of an underlying hearing defect. The audiogram obtained from for the male dolphin, a 24 year old named Pichi (measured from 4–107 kHz), demonstrated profound hearing loss at all frequencies. The measured MRTF results indicated typical odontocete following responses to rates exceeding 1000 Hz.