Marine Mammal HearingThere are several projects that fall under our investigation of marine mammal hearing diversity. They include (i) investigating the hearing range and sensitivity of previously untested species or populations and (ii) examining the diversity of cetacean auditory form and function.
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.
Temporal Resolution in Marine Mammals
Odontocete Auditory Temporal ResolutionLocalization of sounds is perhaps the most important function of hearing and identifying a sound's location is often aided by temporal cues. I am investigating how fast various species can recieve and process short sounds and the comparative relationship between marine species. We've looked the temporal processing and its relationship to hearing and echolocation. The papers are summarized below.
Auditory temporal resolution and evoked responses to pulsed sounds for the Yangtze finless porpoises (Neophocaena phocaenoides asiaeorientalis)
Temporal cues are important for some forms of auditory processing, such as echolocation. Among odontocetes (toothed whales, dolphins, and porpoises), it has been suggested that porpoises may have temporal processing abilities which differ from other odontocetes because of their relatively narrow auditory filters and longer duration echolocation signals. This study examined auditory temporal resolution in two Yangtze finless porpoises (Neophocaena phocaenoides asiaeorientalis) using auditory evoked potentials (AEPs) to measure: (i) rate following responses and modulation rate transfer function for 100 kHz centered pulse sounds and (ii) hearing thresholds and response amplitudes generated by individual pulses of different durations. The animals followed pulses well at modulation rates up to 1250 Hz, after which response amplitudes declined until extinguished beyond 2500 Hz. The subjects had significantly better hearing thresholds for longer, narrower-band pulses similar to porpoise echolocation signals compared to brief, broadband sounds resembling dolphin clicks. Results indicate that the Yangtze finless porpoise follows individual acoustic signals at rates similar to other odontocetes tested. Relatively good sensitivity for longer duration, narrow-band signals suggests that finless porpoise hearing is well-suited to detect their unique echolocation signals.
Mooney, TA, Li, SH, Ketten, DK, Wang, K, and Wang D. 2011. Auditory temporal resolution and evoked responses to pulsed sounds for the Yangtze finless porpoises (Neophocaena phocaenoides asiaeorientalis). Journal of Comparative Physiology A. 197:1149–1158. doi: 10.1007/s00359-011-0677-y
Wild white-beaked dolphin
Auditory temporal resolution of a wild white-beaked dolphin (Lagenorhynchus albirostris)
Adequate temporal resolution is required across taxa to properly utilize amplitude modulated acoustic signals. Among mammals, odontocete marine mammals are considered to have relatively rapid temporal resolution, which is a selective advantage when processing fast traveling underwater sound. However, multiple methods used to estimate auditory temporal resolution have left comparisons among odontocetes and other mammals somewhat vague. Here we present the estimated auditory temporal resolution of an adult male white-beaked dolphin, (Lagenorhynchus albirostris), using auditory evoked potentials and click stimuli. Ours is the first of such studies performed on a wild dolphin in a capture-and-release scenario. The white-beaked dolphin followed rhythmic clicks up to a rate of approximately 1125-1250 Hz, after which the modulation rate transfer function (MRTF) cut-off steeply. However, 10% of the maximum response was still found at 1450 Hz indicating high temporal resolution. The MRTF was similar in shape and bandwidth to that of other odontocetes. The estimated maximal temporal resolution of white-beaked dolphins and other odontocetes was approximately twice that of pinnipeds and manatees, and more than ten-times faster than humans and gerbils. The exceptionally rapid temporal resolution abilities of odontocetes are likely due primarily to echolocation capabilities that require rapid processing of acoustic cues.
Mooney, TA, Nachtigall, PE, Taylor, KA, Rasmussen, MH, and Miller, LA. 2009. Auditory temporal resolution of a wild white-beaked dolphin (Lagenorhynchus albirostris). 195:375-384. Journal of Comparative Physiology A.
Rapid auditory evoked responses and high temporal resolution in a Risso’s dolphin,Grampus griseus
Toothed whales and dolphins (Odontocetes) are known to echolocate, producing short, broadband clicks and receiving the corresponding echoes, at extremely rapid rates. Auditory evoked potentials (AEP) and broadband click stimuli were used to determine the modulation rate transfer function (MRTF) of a neonate Risso’s dolphin,Grampus griseus, thus estimating the dolphin’s temporal resolution, and quantifying its physiological delay to sound stimuli. The Risso’s dolphin followed sound stimuli up to 1000 Hz with a second peak response at 500 Hz. A weighted MRTF reflected that the animal followed a broad range of rates from 100-1000 Hz, but beyond 1250 Hz, the animal’s hearing response was simply and onset/offset response. Similar to other mammals, the dolphin’s AEP response to a single stimulus was a series of waves. The delay of the first wave, PI, was 2.76 ms and the duration of the multi-peaked response was 4.13 ms. The MRTF was similar in shape to other marine mammals, except that the response delay was among the fastest measured. Results predicted that the Risso’s dolphin should have the ability to follow clicks and echoes while foraging up to an extremely close range.
Mooney TA, Nachtigall PE, Yuen MMY. 2006. Temporal resolution of the Risso's dolphin, Grampus griseus, auditory system. Journal of Comparative Physiology A. 192: 373-380.
Cephalopod auditgory temporal resolution...?
We've been examining if and how squid can follow individual pulsed sounds and the maximum rate they can follow.