Some current projects include:
Baseline hearing measurements in Alaskan belugasDue to the opening of the Northwest Passage and interest in Arctic resources, human naval activitie and consequent ocean noise (e.g., resource exploration, shipping and sonars)are increasing in northerly beluga waters. In this project we are measuring the hearing abilities of temporarily captured wild belugas from Bristol Bay. Our work is part of a larger health assessment of this population and is in collaboration with the National Marine Mammal Lab, Alaska Dept of Fish and Game, the Georgia Aquarium and Alaska SeaLife Center. Our hearing data are valuable because with these results we are substantially increasing the sample size and consequent knowledge of how this protected species naturally detects and utilizes sound. This work examines the frequencies and sound levels to which wild belugas are sensitive. A standard audiogram is being determined from the wild samples, noting the variation between animals and the audiogram of maximal sensitivity. This will be compared to available hearing data from captive belugas, evaluating any differences and potentially combining the two data sets. The hearing curves will be appraised relative to demographic and health-related meta-data from the animals from which the measurements were made. Through these data analyses we seek to: 1) define the natural and baseline hearing abilities and variability in belugas, 2) place the results in the context of potential ecological influences and that of anthropogenic noise, and 3) evaluate the validity of captive-based hearing data in relation to wild animals.
Our work was funded by WHOI's Ocean Life Institute, the Arctic Research Initiative and the Office of Naval Research.
(Image: a wild beluga temporarily maintained during an evoked potential hearing test as part of a larger health study. Study and image were under NMFS permit #14245, TAM photo)
Form and Function in Odontocete HearingBy in large, almost all our hearing data has come from just a few "representative" species such as the bottlenose dolphin. But there are dolphins and whales of all shapes and sizes. We've been investigating how other species such as the Yangtze finless porpoise (Neophocaena phocaenoides) and the Risso's dolphin (Grampus griseus) hears. Dolphins generally receive sound through their lower jaw, just as our pinnae gathers and funnels sound to our middle and inner ear. The Risso's has a unique shaped head, with a blunt rosturm and melon (forehead) with a groove down the middle. Finless porpoise have a shorter rostrum (compared to dolphins). This subspecies also lives in fresh water. Both of these points suggeste that there are subtle differences in how these animals receive sound. If this is true, it means they might use sound, and be affected by human-produced noise, somewhat differently than other dolphins. That's why it's important to investigate hearing diversity and how different species hear.
Risso's dolphin during a physiological hearing test.
This work is supported by a WHOI Interdisciplinary Award.
Marine Mammals and Noise
I have investigated how noise impacts marine mammal hearing. This includes demonstrating sonar induces temporary hearing loss (temporary threshold shifts - TTS) and the relationship of sound duration and intensity short duration sound must be of very high amplitude to induce TTS).
Sonar induced temporary hearing loss in dolphins
There is increasing concern that human-produced ocean noise is adversely affecting marine mammals, as several recent cetacean mass strandings may have been caused by animals’ interactions with naval “mid-frequency” sonar. However, it has yet to be empirically demonstrated how sonar could induce these strandings or cause physiological effects. In controlled experimental studies, we show that mid-frequency sonar can induce temporary hearing loss in a bottlenose dolphin (Tursiops truncatus). Mild behavioural alterations were also associated with the exposures. The auditory effects were only induced by repeated exposures to intense sonar pings with total sound exposure levels of 214 dB re: 1 μPa2∙s. Data support an increasing energy model to predict temporary noise-induced hearing loss and indicate that odontocete noise exposure effects bear trends similar to terrestrial mammals. Thus, sonar can induce physiological and behavioural effects in at least one species of odontocete; however, exposures must be of prolonged, high sound exposures levels to generate these effects.
Mooney, TA, Nachtigall, PE, Vlachos, S. 2009. Sonar-induced temporary hearing loss in dolphins. 5: 565-567. Biology Letters.
Predicting temporary threshold shifts in a bottlenose dolphin (Tursiops truncatus): the effects of noise level and duration
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 pressure (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.