Toothed whales, including sperm whales, bottlenose dolphins, and harbor porpoises, use echolocation to navigate and locate prey. These odontocetes produce high frequency clicks that are reflected back as echoes when the click sounds strike an object(s). Differences between the original click and the returning echoes can provide the echolocating animal with information about the object’s size, shape, orientation, direction, speed, distance, and even its internal structure.

Animation illustrating echolocation by a dolphin. Copyright University of Rhode Island.

For echolocation to be successful, multiple features of the returning echoes must be analyzed. Humans have developed electronic sonar systems that operate similarly to echolocation, and must analyze different features of a signal to discern targets of interest. Engineers apply automatic gain control (AGC) to modify the strength of received signals, making returning echo energy levels independent of target distance. To accommodate transmission levels and receiving sensitivities, some echolocating animals may also use several mechanisms similar to AGC to compensate for varying echo intensity levels [1]Supin, A., & Nachtigall, P. E. (2013). Gain control in the sonar of odontocetes. Journal of Comparative Physiology A, 199(6), 471–478. https://doi.org/10.1007/s00359-012-0773-7.[2]Finneran, J. J., Mulsow, J., & Houser, D. S. (2013). Auditory evoked potentials in a bottlenose dolphin during moderate-range echolocation tasks. The Journal of the Acoustical Society of America, 134(6), 4532–4547. https://doi.org/10.1121/1.4826179.. One mechanism is to adjust the intensity and rate of clicks according to target range to ensure that echoes arrive before a subsequent click is produced. During the search phase of a dive, when an animal is most unsure about target range, signals are emitted at slower rates and higher source levels. Click rates increase and source levels decrease as the animal moves towards the target and the target range decreases [3]Au, W. W. L., & Benoit-Bird, K. J. (2003). Automatic gain control in the echolocation system of dolphins. Nature, 423(6942), 861–863. https://doi.org/10.1038/nature01727.[4]Jensen, F. H., Bejder, L., Wahlberg, M., & Madsen, P. T. (2009). Biosonar adjustments to target range of echolocating bottlenose dolphins( Tursiops sp.) in the wild. Journal of Experimental Biology, 212(8), 1078–1086. https://doi.org/10.1242/jeb.025619..

Recording of a Blainville’s beaked whale echolocation buzz in El Hierro, Canary Islands. Sound provided by M. Johnson, Woods Hole Oceanographic Institution and N. Aguilar de Soto University of La Laguna (ULL), Spain. Sounds obtained under permit from the Canary Island Government to ULL. Funding from the US National Oceanographic Partnership Program.

Research with animals in captivity has demonstrated that some species of toothed whales can also change their hearing sensitivity to optimize the detection of echoes. For example, auditory brainstem response (ABR) studies with a trained harbor porpoise showed the animal to not only increase the sound level of its outgoing signals with increased target distance, but it also decreased its ABR threshold by 6 dB, making its hearing more sensitive, when target distance increased[5]Linnenschmidt, M., Beedholm, K., Wahlberg, M., Højer-Kristensen, J., & Nachtigall, P. E. (2012). Keeping returns optimal: Gain control exerted through sensitivity adjustments in the harbour porpoise auditory system. Proceedings of the Royal Society B: Biological Sciences, 279(1736), 2237–2245. https://doi.org/10.1098/rspb.2011.2465. Auditory evoked potential (AEP) research with a trained false killer whale indicated this species also controls its hearing sensitivity while echolocating[6]Supin, A. Ya., Nachtigall, P. E., Au, W. W. L., & Breese, M. (2004). The interaction of outgoing echolocation pulses and echoes in the false killer whale’s auditory system: Evoked-potential study. The Journal of the Acoustical Society of America, 115(6), 3218–3225. https://doi.org/10.1121/1.1707088.[7] Supin, A. Ya., Nachtigall, P. E., & Breese, M. (2008). Hearing sensitivity during target presence and absence while a whale echolocates. The Journal of the Acoustical Society of America, 123(1), 534–541. https://doi.org/10.1121/1.2812593.[8]Nachtigall, P. E., & Supin, A. Y. (2008). A false killer whale adjusts its hearing when it echolocates. Journal of Experimental Biology, 211(11), 1714–1718. https://doi.org/10.1242/jeb.013862.. The whale changed its hearing sensitivities to its emitted clicks depending on target presence or absence[9]Supin, A. Ya., Nachtigall, P. E., & Breese, M. (2008). Hearing sensitivity during target presence and absence while a whale echolocates. The Journal of the Acoustical Society of America, 123(1), 534–541. https://doi.org/10.1121/1.2812593., as well as target strength and distance[10]Nachtigall, P.E. & Supin, A.Y. (2008). A false killer whale adjusts its hearing when it echolocations.  Journal of Experimental Biology, 211(11), 1714-1718. https://doi.org/10.1242/jeb.013862.[11]Supin, A. Ya., Nachtigall, P. E., Au, W. W. L., & Breese, M. (2004). The interaction of outgoing echolocation pulses and echoes in the false killer whale’s auditory system: Evoked-potential study. The Journal of the Acoustical Society of America, 115(6), 3218–3225. https://doi.org/10.1121/1.1707088.[12]Supin, A. Ya., Nachtigall, P. E., & Breese, M. (2008). Hearing sensitivity during target presence and absence while a whale echolocates. The Journal of the Acoustical Society of America, 123(1), 534–541. https://doi.org/10.1121/1.2812593.[13]Finneran, J. J., Mulsow, J., & Houser, D. S. (2013). Auditory evoked potentials in a bottlenose dolphin during moderate-range echolocation tasks. The Journal of the Acoustical Society of America, 134(6), 4532–4547. https://doi.org/10.1121/1.4826179.

In the absence of a strong echo (no perceived echo, low target strength with weak echo, or large distance to target), the whale’s hearing sensitivities increased, which would improve detection of fainter echoes. The whale also actively reduced its hearing sensitivity in response to its own echolocation signals. The whale decreased its sensitivity to its own echolocation clicks by approximately 40 dB compared to the playback of false killer whale clicks of equal received intensity[14]Nachtigall, P. E., & Supin, A. Y. (2008). A false killer whale adjusts its hearing when it echolocates. Journal of Experimental Biology, 211(11), 1714–1718. https://doi.org/10.1242/jeb.013862.[15]Supin, A. Ya., Nachtigall, P. E., & Breese, M. (2006). Source-to-sensation level ratio of transmitted biosonar pulses in an echolocating false killer whale. The Journal of the Acoustical Society of America, 120(1), 518–526. https://doi.org/10.1121/1.2202862.. Similarly, a bottlenose dolphin was found to display automatic gain control mechanisms as the target range was increased up to 80 m[16]Finneran, J. J., Mulsow, J., & Houser, D. S. (2013). Auditory evoked potentials in a bottlenose dolphin during moderate-range echolocation tasks. The Journal of the Acoustical Society of America, 134(6), 4532–4547. https://doi.org/10.1121/1.4826179..

The false killer whale’s hearing is being measured using an auditory brainstem response (ABR) test. The probes, attached to the animal’s head and back using suction cups, measure small electrical voltages produced by the brain in response to an acoustic stimulation. Photo courtesy of Paul E. Nachtigall, Hawaii Institute of Marine Biology.

Given that toothed whales in captivity have demonstrated active hearing control during echolocation, scientists hypothesized that these whale species could learn to “self-mitigate” the potential effects of anthropogenic underwater sound by reducing their hearing sensitivity. Scientists investigated the ability of trained odontocetes to change their hearing sensitivity as a conditioned response to a warning signal presented prior to a loud sound[17]Nachtigall, P. E., Supin, A. Ya., Pacini, A. F., & Kastelein, R. A. (2018). Four odontocete species change hearing levels when warned of impending loud sound. Integrative Zoology, 13(2), 160–165. https://doi.org/10.1111/1749-4877.12286.[18]Nachtigall, P. E., Supin, A. Ya., Pacini, A. F., & Kastelein, R. A. (2016). Conditioned hearing sensitivity change in the harbor porpoise (Phocoena phocoena). The Journal of the Acoustical Society of America, 140(2), 960–967. https://doi.org/10.1121/1.4960783[19]Nachtigall, P. E., Supin, A. Y., Estaban, J.-A., & Pacini, A. F. (2016). Learning and extinction of conditioned hearing sensation change in the beluga whale (Delphinapterus leucas). Journal of Comparative Physiology A, 202(2), 105–113. https://doi.org/10.1007/s00359-015-1056-x.[20]Nachtigall, P. E., & Supin, A. Y. (2015). Conditioned frequency-dependent hearing sensitivity reduction in the bottlenose dolphin (Tursiops truncatus). Journal of . Biology, 218(7), 999–1005. https://doi.org/10.1242/jeb.114066.[21]Nachtigall, P. E., & Supin, A. Y. (2013). A false killer whale reduces its hearing sensitivity when a loud sound is preceded by a warning. Journal of Experimental Biology, 216(16), 3062–3070. https://doi.org/10.1242/jeb.085068.. Species that have been trained and tested include false killer whale, bottlenose dolphin, beluga whale, and harbor porpoise [22]Nachtigall, P. E., Supin, A. Ya., Pacini, A. F., & Kastelein, R. A. (2016). Conditioned hearing sensitivity change in the harbor porpoise (Phocoena phocoena). The Journal of the Acoustical Society of America, 140(2), 960–967. https://doi.org/10.1121/1.4960783[23]Nachtigall, P. E., Supin, A. Ya., Smith, A. B., & Pacini, A. F. (2016). Expectancy and conditioned hearing levels in the bottlenose dolphin (Tursiops truncatus). Journal of Experimental Biology, jeb.133777. https://doi.org/10.1242/jeb.133777.[24]Nachtigall, P. E., Supin, A. Y., Estaban, J.-A., & Pacini, A. F. (2016). Learning and extinction of conditioned hearing sensation change in the beluga whale (Delphinapterus leucas). Journal of Comparative Physiology A, 202(2), 105–113. https://doi.org/10.1007/s00359-015-1056-x.[25]Nachtigall, P. E., Supin, A. Ya., Pacini, A. F., & Kastelein, R. A. (2018). Four odontocete species change hearing levels when warned of impending loud sound. Integrative Zoology, 13(2), 160–165. https://doi.org/10.1111/1749-4877.12286.[26]Nachtigall, P. E., & Supin, A. Y. (2015). Conditioned frequency-dependent hearing sensitivity reduction in the bottlenose dolphin (Tursiops truncatus). Journal of Experimental Biology, 218(7), 999–1005. https://doi.org/10.1242/jeb.114066.[27]Finneran, J. J. (2018). Conditioned attenuation of auditory brainstem responses in dolphins warned of an intense noise exposure: Temporal and spectral patterns. The Journal of the Acoustical Society of America, 143(2), 795–810. https://doi.org/10.1121/1.5022784.[28]Finneran, J. J. (2020). Conditioned attenuation of dolphin monaural and binaural auditory evoked potentials after preferential stimulation of one ear. The Journal of the Acoustical Society of America, 147(4), 2302–2313. https://doi.org/10.1121/10.0001033..

AEP studies with trained odontocetes showed hearing thresholds to increase (by 13 – 17 dB) relative to baseline, pre-conditioned sensitivities, when a test warning signal of tone pips preceded a loud sound[29]Nachtigall, P. E., Supin, A. Ya., Pacini, A. F., & Kastelein, R. A. (2018). Four odontocete species change hearing levels when warned of impending loud sound. Integrative Zoology, 13(2), 160–165. https://doi.org/10.1111/1749-4877.12286.. Whales and dolphins have learned to reduce their hearing sensitivity when a warning sound precedes a loud sound. For example, in one study with a trained false killer whale, the loud sound was a tone of 20 kHz, at 170 dB re1μPa, with a duration of 5 seconds. The rhythmic trains contained 17 pips each, and the trains were presented at a rate of 20 per second. The levels of the warning signals varied from 80 to 120 dB re1μPa rms. If the warning signals occurred 1 to 9 s or 5 to 35 s prior to the louder sound, the animal’s sensitivity shifted and hearing thresholds increased.

It is important to note that a change in hearing sensation levels when the warning sound was paired with a loud sound did not occur on the first trial in any of the experiments. Hearing sensitivity was gradually reduced across trials for all species. Also, after several sessions in which the animals no longer received the loud sound following the warning sound, hearing sensitivities returned to baseline, pre-conditioned levels.

Echolocating marine mammals are not the only animals that can actively adjust their auditory sensitivity. The acoustic reflex in echolocating bats is well studied. All tested bat species have been found to employ a middle ear reflex mechanism that tightens the muscles attached to their middle ear bones just prior to producing an echolocation signal. This reflex can reduce sensitivity by as much as 30 dB[30]Henson, O. W. (1965). The activity and function of the middle-ear muscles in echo-locating bats. The Journal of Physiology, 180(4), 871–887. https://doi.org/10.1113/jphysiol.1965.sp007737.[31]Suga, N., & Jen, P.H. (1975). Peripheral control of acoustic signals in the auditory system of echolocating bats. Journal of Experimental Biology, 62(2), 277-311. https://doi.org/10.1242/jeb.62.2.277.[32]Kick, S., & Simmons, J. (1984). Automatic gain control in the bat’s sonar receiver and the neuroethology of echolocation. The Journal of Neuroscience, 4(11), 2725–2737. https://doi.org/10.1523/JNEUROSCI.04-11-02725.1984..The bats are able to then rapidly release the tension of the muscles to optimize the detection of returning echoes[33]Kick, S., & Simmons, J. (1984). Automatic gain control in the bat’s sonar receiver and the neuroethology of echolocation. The Journal of Neuroscience, 4(11), 2725–2737. https://doi.org/10.1523/JNEUROSCI.04-11-02725.1984.[34]Hartley, D. J. (1992). Stabilization of perceived echo amplitudes in echolocating bats. II. The acoustic behavior of the big brown bat, Eptesicus fuscus, when tracking moving prey. The Journal of the Acoustical Society of America, 91(2), 1133–1149. https://doi.org/10.1121/1.402640.. Some species have been shown to synchronize this contraction and relaxation sequence with long strings of repeated echolocation calls.

Odontocetes share similar adaptations to bats, however, the physiological mechanisms for hearing attenuation in the trained odontocete experiments are not fully understood. AEP work with a captive bottlenose dolphin showed that conditioned hearing dampened within a limited frequency band at frequencies equal to or higher than the frequency of the loud sound[35]Nachtigall, P. E., & Supin, A. Y. (2015). Conditioned frequency-dependent hearing sensitivity reduction in the bottlenose dolphin (Tursiops truncatus). Journal of Experimental Biology, 218(7), 999–1005. https://doi.org/10.1242/jeb.114066.. Researchers hypothesize that in the trained animals, the conditioned response was controlled primarily by efferent neural signals that inhibited hair cell responses [36]Finneran, J. J. (2018). Conditioned attenuation of auditory brainstem responses in dolphins warned of an intense noise exposure: Temporal and spectral patterns. The Journal of the Acoustical Society of America, 143(2), 795–810. https://doi.org/10.1121/1.5022784[37]Finneran, J. J. (2020). Conditioned attenuation of dolphin monaural and binaural auditory evoked potentials after preferential stimulation of one ear. The Journal of the Acoustical Society of America, 147(4), 2302–2313. https://doi.org/10.1121/10.0001033.. Efferent control of hearing is a slower but longer lasting effect on sensitivity than middle ear reflexes. However, many cetaceans have been found to possess the well-developed middle ear muscles and ligaments that are required for a middle ear reflex. One report demonstrated that a captive harbor porpoise decreased its hearing without training in response to loud test signals with no warning sounds[38]Kastelein, R. A., Helder-Hoek, L., Cornelisse, S. A., von Benda-Beckmann, A. M., Lam, F.-P. A., de Jong, C. A. F., & Ketten, D. R. (2020). Lack of reproducibility of temporary hearing threshold shifts in a harbor porpoise after exposure to repeated airgun sounds. The Journal of the Acoustical Society of America, 148(2), 556–565. https://doi.org/10.1121/10.0001668.. It may be that both inner ear neural and middle ear mechanisms are operating simultaneously or sequentially. More research is needed to understand these mechanisms in marine mammal hearing.

Many other mammals, such as humans, cats, dogs, guinea pigs, and chinchillas, also exhibit an acoustic reflex to actively attenuate hearing[39]Galambos, R. (1956). Suppression of auditory nerve activity by stimulation of efferent fibers to cochlea. Journal of Neurophysiology, 19(5), 424–437. https://doi.org/10.1152/jn.1956.19.5.424.[40]Ward, W. D. (1962). Studies on the Aural Reflex. III. Reflex Latency as Inferred from Reduction of Temporary Threshold Shift from Impulses. The Journal of the Acoustical Society of America, 34(8), 1132–1137. https://doi.org/10.1121/1.1918260.[41]Hung, I. J., & Dallos, P. (1972). Study of the Acoustic Reflex in Human Beings. I. Dynamic Characteristics. The Journal of the Acoustical Society of America, 52(4B), 1168–1180. https://doi.org/10.1121/1.1913229.[42]Borg, E., & Counter, S.A.(1989). The Middle-Ear Muscles. Scientific American, 261(2), 74-81. JSTOR.[43]McFadden, S.L., Henderson, D., & Shen, Y.-H. (1997). Low-frequency ‘conditioning’ provides long-term protection from noise-induced threshold shifts in chinchillas. Hearing Research, 103(1-2), 142-150. https://doi.org/10.1016./S0378-5955(96)00170-0.[44]Canlon, B., Fransson, A. & Viberg, A. (199). Medial olivocochlear efferent terminals are protected by sound conditioning. Brain Research, 850(1-2), 253-260. https://doi.org/10.1016/S0006-8993(99)02091-0.[45]Møller, A. R. (2000). Hearing: Its Physiology and Pathophysiology. Academic Press.. In most humans a middle ear reflex that dampens hearing in response to loud external sounds may also occur right before they produce loud vocalizations[46]Ward, W. D. (1962). Studies on the Aural Reflex. III. Reflex Latency as Inferred from Reduction of Temporary Threshold Shift from Impulses. The Journal of the Acoustical Society of America, 34(8), 1132–1137. https://doi.org/10.1121/1.1918260.[47]Hung, I. J., & Dallos, P. (1972). Study of the Acoustic Reflex in Human Beings. I. Dynamic Characteristics. The Journal of the Acoustical Society of America, 52(4B), 1168–1180. https://doi.org/10.1121/1.1913229.[48]Borg, E., & Counter, S.A.(1989). The Middle-Ear Muscles. Scientific American, 261(2), 74-81. JSTOR.[49]Møller, A. R. (2000). Hearing: Its Physiology and Pathophysiology. Academic Press.. 

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References

  • Finneran, J. J., Echon, R., Mulsow, J., & Houser, D. S. (2016). Short-term enhancement and suppression of dolphin auditory evoked responses following echolocation click emission. The Journal of the Acoustical Society of America, 140(1), 296–307. https://doi.org/10.1121/1.4955093.
  • Kloepper, L. N., Smith, A. B., Nachtigall, P. E., Buck, J. R., Simmons, J. A., & Pacini, A. F. (2014). Cognitive adaptation of sonar gain control in the bottlenose dolphin. PLoS ONE, 9(8), e105938. https://doi.org/10.1371/journal.pone.0105938.
  • Li, S., Nachtigall, P. E., & Breese, M. (2011). Dolphin hearing during echolocation: Evoked potential responses in an Atlantic bottlenose dolphin (Tursiops truncatus). Journal of Experimental Biology, 214(12), 2027–2035. https://doi.org/10.1242/jeb.053397.
  • Supin, A. Ya., Nachtigall, P. E., & Breese, M. (2010). Target distance-dependent variation of hearing sensitivity during echolocation in a false killer whale. The Journal of the Acoustical Society of America, 127(6), 3830–3836. https://doi.org/10.1121/1.3425733.

 

 

 

Cited References

Cited References
1 Supin, A., & Nachtigall, P. E. (2013). Gain control in the sonar of odontocetes. Journal of Comparative Physiology A, 199(6), 471–478. https://doi.org/10.1007/s00359-012-0773-7.
2, 13, 16 Finneran, J. J., Mulsow, J., & Houser, D. S. (2013). Auditory evoked potentials in a bottlenose dolphin during moderate-range echolocation tasks. The Journal of the Acoustical Society of America, 134(6), 4532–4547. https://doi.org/10.1121/1.4826179.
3 Au, W. W. L., & Benoit-Bird, K. J. (2003). Automatic gain control in the echolocation system of dolphins. Nature, 423(6942), 861–863. https://doi.org/10.1038/nature01727.
4 Jensen, F. H., Bejder, L., Wahlberg, M., & Madsen, P. T. (2009). Biosonar adjustments to target range of echolocating bottlenose dolphins( Tursiops sp.) in the wild. Journal of Experimental Biology, 212(8), 1078–1086. https://doi.org/10.1242/jeb.025619.
5 Linnenschmidt, M., Beedholm, K., Wahlberg, M., Højer-Kristensen, J., & Nachtigall, P. E. (2012). Keeping returns optimal: Gain control exerted through sensitivity adjustments in the harbour porpoise auditory system. Proceedings of the Royal Society B: Biological Sciences, 279(1736), 2237–2245. https://doi.org/10.1098/rspb.2011.2465
6, 11 Supin, A. Ya., Nachtigall, P. E., Au, W. W. L., & Breese, M. (2004). The interaction of outgoing echolocation pulses and echoes in the false killer whale’s auditory system: Evoked-potential study. The Journal of the Acoustical Society of America, 115(6), 3218–3225. https://doi.org/10.1121/1.1707088.
7 Supin, A. Ya., Nachtigall, P. E., & Breese, M. (2008). Hearing sensitivity during target presence and absence while a whale echolocates. The Journal of the Acoustical Society of America, 123(1), 534–541. https://doi.org/10.1121/1.2812593.
8, 14 Nachtigall, P. E., & Supin, A. Y. (2008). A false killer whale adjusts its hearing when it echolocates. Journal of Experimental Biology, 211(11), 1714–1718. https://doi.org/10.1242/jeb.013862.
9, 12 Supin, A. Ya., Nachtigall, P. E., & Breese, M. (2008). Hearing sensitivity during target presence and absence while a whale echolocates. The Journal of the Acoustical Society of America, 123(1), 534–541. https://doi.org/10.1121/1.2812593.
10 Nachtigall, P.E. & Supin, A.Y. (2008). A false killer whale adjusts its hearing when it echolocations.  Journal of Experimental Biology, 211(11), 1714-1718. https://doi.org/10.1242/jeb.013862.
15 Supin, A. Ya., Nachtigall, P. E., & Breese, M. (2006). Source-to-sensation level ratio of transmitted biosonar pulses in an echolocating false killer whale. The Journal of the Acoustical Society of America, 120(1), 518–526. https://doi.org/10.1121/1.2202862.
17, 25, 29 Nachtigall, P. E., Supin, A. Ya., Pacini, A. F., & Kastelein, R. A. (2018). Four odontocete species change hearing levels when warned of impending loud sound. Integrative Zoology, 13(2), 160–165. https://doi.org/10.1111/1749-4877.12286.
18, 22 Nachtigall, P. E., Supin, A. Ya., Pacini, A. F., & Kastelein, R. A. (2016). Conditioned hearing sensitivity change in the harbor porpoise (Phocoena phocoena). The Journal of the Acoustical Society of America, 140(2), 960–967. https://doi.org/10.1121/1.4960783
19 Nachtigall, P. E., Supin, A. Y., Estaban, J.-A., & Pacini, A. F. (2016). Learning and extinction of conditioned hearing sensation change in the beluga whale (Delphinapterus leucas). Journal of Comparative Physiology A, 202(2), 105–113. https://doi.org/10.1007/s00359-015-1056-x.
20 Nachtigall, P. E., & Supin, A. Y. (2015). Conditioned frequency-dependent hearing sensitivity reduction in the bottlenose dolphin (Tursiops truncatus). Journal of . Biology, 218(7), 999–1005. https://doi.org/10.1242/jeb.114066.
21 Nachtigall, P. E., & Supin, A. Y. (2013). A false killer whale reduces its hearing sensitivity when a loud sound is preceded by a warning. Journal of Experimental Biology, 216(16), 3062–3070. https://doi.org/10.1242/jeb.085068.
23 Nachtigall, P. E., Supin, A. Ya., Smith, A. B., & Pacini, A. F. (2016). Expectancy and conditioned hearing levels in the bottlenose dolphin (Tursiops truncatus). Journal of Experimental Biology, jeb.133777. https://doi.org/10.1242/jeb.133777.
24 Nachtigall, P. E., Supin, A. Y., Estaban, J.-A., & Pacini, A. F. (2016). Learning and extinction of conditioned hearing sensation change in the beluga whale (Delphinapterus leucas). Journal of Comparative Physiology A, 202(2), 105–113. https://doi.org/10.1007/s00359-015-1056-x.
26, 35 Nachtigall, P. E., & Supin, A. Y. (2015). Conditioned frequency-dependent hearing sensitivity reduction in the bottlenose dolphin (Tursiops truncatus). Journal of Experimental Biology, 218(7), 999–1005. https://doi.org/10.1242/jeb.114066.
27 Finneran, J. J. (2018). Conditioned attenuation of auditory brainstem responses in dolphins warned of an intense noise exposure: Temporal and spectral patterns. The Journal of the Acoustical Society of America, 143(2), 795–810. https://doi.org/10.1121/1.5022784.
28, 37 Finneran, J. J. (2020). Conditioned attenuation of dolphin monaural and binaural auditory evoked potentials after preferential stimulation of one ear. The Journal of the Acoustical Society of America, 147(4), 2302–2313. https://doi.org/10.1121/10.0001033.
30 Henson, O. W. (1965). The activity and function of the middle-ear muscles in echo-locating bats. The Journal of Physiology, 180(4), 871–887. https://doi.org/10.1113/jphysiol.1965.sp007737.
31 Suga, N., & Jen, P.H. (1975). Peripheral control of acoustic signals in the auditory system of echolocating bats. Journal of Experimental Biology, 62(2), 277-311. https://doi.org/10.1242/jeb.62.2.277.
32, 33 Kick, S., & Simmons, J. (1984). Automatic gain control in the bat’s sonar receiver and the neuroethology of echolocation. The Journal of Neuroscience, 4(11), 2725–2737. https://doi.org/10.1523/JNEUROSCI.04-11-02725.1984.
34 Hartley, D. J. (1992). Stabilization of perceived echo amplitudes in echolocating bats. II. The acoustic behavior of the big brown bat, Eptesicus fuscus, when tracking moving prey. The Journal of the Acoustical Society of America, 91(2), 1133–1149. https://doi.org/10.1121/1.402640.
36 Finneran, J. J. (2018). Conditioned attenuation of auditory brainstem responses in dolphins warned of an intense noise exposure: Temporal and spectral patterns. The Journal of the Acoustical Society of America, 143(2), 795–810. https://doi.org/10.1121/1.5022784
38 Kastelein, R. A., Helder-Hoek, L., Cornelisse, S. A., von Benda-Beckmann, A. M., Lam, F.-P. A., de Jong, C. A. F., & Ketten, D. R. (2020). Lack of reproducibility of temporary hearing threshold shifts in a harbor porpoise after exposure to repeated airgun sounds. The Journal of the Acoustical Society of America, 148(2), 556–565. https://doi.org/10.1121/10.0001668.
39 Galambos, R. (1956). Suppression of auditory nerve activity by stimulation of efferent fibers to cochlea. Journal of Neurophysiology, 19(5), 424–437. https://doi.org/10.1152/jn.1956.19.5.424.
40, 46 Ward, W. D. (1962). Studies on the Aural Reflex. III. Reflex Latency as Inferred from Reduction of Temporary Threshold Shift from Impulses. The Journal of the Acoustical Society of America, 34(8), 1132–1137. https://doi.org/10.1121/1.1918260.
41, 47 Hung, I. J., & Dallos, P. (1972). Study of the Acoustic Reflex in Human Beings. I. Dynamic Characteristics. The Journal of the Acoustical Society of America, 52(4B), 1168–1180. https://doi.org/10.1121/1.1913229.
42, 48 Borg, E., & Counter, S.A.(1989). The Middle-Ear Muscles. Scientific American, 261(2), 74-81. JSTOR.
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