Thresholds



Marine-Mammal-Threshold-Chart-595x406

Marine Mammal Threshold Chart

At this juncture establishing what marine animals actually can hear is a challenge. There has been some detailed work on a few captive marine mammals, and a legacy of testing on fish. But audiological tests on any animal are difficult because heretofore most testing uses what is termed “operant conditioning” which involves training or “conditioning” an animal to associate a sound with either a reward or an aggravation. Once the animal perfects the response, the sound parameters are changed in some manner to bracket the limits and thresholds of how it hears sounds which it associates with the reward or aggravation.

So for example a dolphin is trained to expect a reward if they correctly identify a tone, theoretically if the pitch or amplitude of the tone is modified the dolphin will seek a reward if they can hear the new tone, and won’t seek a reward if it can’t hear a tone. Through this very time consuming process the limits and extents of the dolphin’s hearing can be ascertained. And while this would be a captive, trained, and thus “domesticated” animal, there will be some correlation between this dolphin’s hearing and the hearing of its wild kin.

Aside from the training and time-consuming procedure, some other drawbacks include:

  • Captive animals become habituated to testing and serve as a poor proxy for wild animals
  • Some animals such as baleen whales are not easily captured, let alone trained, so we can only speculate about their hearing
  • Test signals are most often calibrated laboratory signals that may not correlate to actual exposure signals
  • Test signals in captive enclosures mostly occur in the near field (close proximity) and don’t accurately reflect time and spatial domain sound characteristics found in marine habitats.
  • Testing individual animals that are usually schooling animals does not reflect aggregate signal amplification and behavioral adaptations that occur in group settings.
  • Most testing is done using “pressure gradient” acoustical energy at a specific receiver point and does not reflect other characteristics of acoustical energy perception.
  • Auditory testing typically focuses on sensitivity to how loud a signal is, and the extents of the perception of frequency or “pitch.” Other sound characteristics are rarely considered.

We have accumulated, evaluated, and plotted audiograms for marine mammals and fish. It is clear from these graphs that there is a disparity in the tested hearing of the various animals. There is more coherence among marine mammals because there are many similarities in their hearing systems. For example all mammals have a pitch discrimination organ in their inner ear called a “cochlea.” There is no analogous organ in fishes, so it is quite possible that pitch discrimination is not a useful adaptation for this class of animal.

Nonetheless audiograms for fish still represent amplitude/pitch sensitivity partially because amplitude/pitch sensitivity reflects our own auditory priorities. The lack of coherence among fish audiograms also suggests the diversity in their adaptations to their particular habitats.

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