How animals extract information about the ambient environment: Passive listening

By listening to sounds, much can be learned about the environment. As sound travels underwater from a source to a receiver, the features of the sound change. These features can be used by receivers as cues for useful information such as location, distance, and direction of a sound source, and characteristics of habitats, objects, or other animals (https://dosits.org/science/sounds-in-the-sea/people-animals-use/).

Significant cues for estimating the distance of a sound source include received level, spectral characteristics, and temporal structure of the sound. Sound level can provide information about the distance to the sound source and whether the distance is changing. Sound from sources that are farther are generally less loud than sounds from sources that are closer, due to sound spreading.

Frequency components of a sound (spectral characteristics) change with distance because of absorption (attenuation). As the sound travels through a medium, some of the acoustic energy is changed to heat energy which means the acoustic energy is absorbed by the medium. High frequency sound does not travel as far as low frequency sound because the wavelengths of higher frequency sounds are shorter and absorption is frequency dependent. Therefore, the loss of higher frequencies may be perceived as greater distance to/from the sound source. These higher frequencies may be harmonic frequencies, which are integer multiples of the lowest frequency (fundamental frequency) of a signal. Therefore, the fundamental frequency of the signal remains, but the harmonics will disappear as the range increases.

Finally, as a sound propagates, it may encounter changes in density and sound speed that alter the temporal structure of the sound. This could result in multipath propagation, which occurs when there is more than one propagation path between a sound source and receiver. There is often a direct path from the source to the receiver, but with distance, the sound may encounter the sea surface, the seafloor, or changes in sound speed that result in reflection, refraction, and/or scattering. Reverberation is the total of all scattering received at a particular location. A typical characteristic of reverberation is a long, slowly decaying sound with several spectral peaks caused by backscattering from stronger scatterers. As the distance a sound travels increases, there is increased potential for interactions with scatterers, therefore, an increased potential for reverberation. An example of reverberation can be heard in the Audio Gallery’s airgun sound. The sounds of airguns operating off Sable Island, Nova Scotia, Canada were detected by hydrophones located thousands of kilometers away on the Mid-Atlantic Ridge in the North Atlantic Ocean. Rather than being short, loud, impulsive sounds, as one would hear closer to the airguns, the airgun sounds were longer, less intense sounds. 

Studies have documented the ability of bottlenose dolphins to detect distance. In one study (Mulsow et al. 2018), dolphins were able to differentiate between two sounds: one that was “undegraded,” in that it mimicked a signal near the source, and a “degraded” signal with attenuated high-frequencies and reverberations as though the source was 30 km away. The figure below demonstrates the amplitude and frequency differences among the two test signals. The results showed the dolphins primarily used high-frequency attenuation as a distance cue and were able to differentiate amongst signals with different levels of degradation consistent with multiple distances.

The figure demonstrates the change in amplitude and frequency composition among the various test signals. Panel (a) shows the undegraded signal. Panel (b) shows the 30-km degraded signals. Panel (c) shows the 20-km signal degraded with reverberation. Panel (d) shows the 20-km signal degraded with high-frequency attenuation. Figure provided with permission from Mulsow et al. 2018 The Journal of the Acoustical Society of America.

 

Other studies have investigated how some fishes and invertebrates use acoustic cues to locate suitable habitat for multiple purposes such as recruitment and settlement (Underwater Sound and Coral Reef Restoration). There is some evidence that underwater reef sounds may guide the larvae of some coral reef fishes and invertebrates to suitable reef habitats and coastal areas (Kennedy et al. 2010; Mann et al. 2007; Simpson et al. 2004, 2010; Vermeij et al. 2010). Different coastal habitat types have been found to produce different ambient sounds over short distances.

 

The larvae of some coral reef fish species like these damselfish, may use sound to locate suitable settlement areas. Image credit: NOAA.

 

Studies comparing the acoustic features of marine protected areas vs. non-protected areas found that protected sites with greater biological diversity produced significantly higher ambient sound pressure levels and were more acoustically complex than those that are not protected (https://dosits.org/people-and-sound/investigate-marine-animals/how-is-sound-used-to-study-coral-reefs/). Variations in the acoustic signature of different reef habitats may provide information about habitat quality and help larvae and juveniles differentiate amongst reefs. 

 

A healthy coral reef. Image credit: NOAA.

 

Several studies suggest that the larvae of coral reef fishes and invertebrates may use a combination of visual, chemical, and acoustic cues to recognize and orient to suitable reef habitat on which they can settle and develop into adults. Studies have shown that larvae of some species may be attracted to certain reef sounds and avoid others. In a field-based, playback study in Australia, sounds from degraded reefs were found to be less attractive to larvae than sounds from healthy reefs (Gordon et al. 2018). To enhance and/or accelerate community development at restored coral reef sites, scientists are using “acoustic enrichment,” underwater playbacks of sounds of healthy reefs, to attract fishes and other organisms to restored reefs. In a laboratory study, scientists found flat oyster larvae to have a higher settlement response in the presence of restored reef sound playbacks when compared to sounds of degraded reefs or a “no sound” control treatments (Williams et al. 2022). https://dosits.org/underwater-sound-and-coral-reef-restoration/ 

 

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References

  • Gordon, T. A. C., Harding, H. R., Wong, K. E., Merchant, N. D., Meekan, M. G., McCormick, M. I., Radford, A. N., & Simpson, S. D. (2018). Habitat degradation negatively affects auditory settlement behavior of coral reef fishes. Proceedings of the National Academy of Sciences, 115(20), 5193–5198. https://doi.org/10.1073/pnas.1719291115
  • Kennedy, E.V., Holderied, M.W., Mair, J.M., Guzman, H.M. and Simpson, S.D. (2010) Spatial Patterns in Reef-Generated Noise Relate to Habitats and Communities: Evidence from a Panamanian Case Study. Journal of Experimental Marine Biology and Ecology, 395, 85–92. https://doi.org/10.1016/j.jembe.2010.08.017.
  • Mann, D., Casper, B., Boyle, K. and Tricas, T. (2007) On the Attraction of Larval Fishes to Reef Sounds. Marine Ecology Progress Series, 338, 307–310. https://doi.org/10.3354/meps338307.
  • Mulsow, J., Finneran, J. J., Schlundt, C. E., & Jones, R. (2018). Bottlenose dolphin (Tursiops truncatus) discrimination of harmonic stimuli with range-dependent signal degradation. The Journal of the Acoustical Society of America, 143(6), 3434–3443. https://doi.org/10.1121/1.5040498.
  • Simpson, S., Meekan, M., McCauley, R. and Jeffs, A. (2004) Attraction of Settlement-Stage Coral Reef Fishes to Reef Noise. Marine Ecology Progress Series, 276, 263–268. https://doi.org/10.3354/meps276263.
  • Simpson, S.D., Meekan, M.G., Larsen, N.J., McCauley, R.D. and Jeffs, A. (2010) Behavioral Plasticity in Larval Reef Fish: Orientation Is Influenced by Recent Acoustic Experiences. Behavioral Ecology, 21, 1098–1105. https://doi.org/10.1093/beheco/arq117.
  • Vermeij, M.J.A., Marhaver, K.L., Huijbers, C.M., Nagelkerken, I. and Simpson, S.D. (2010) Coral Larvae Move toward Reef Sounds. Vollmer, S., Ed., PLoS ONE, 5, e10660. https://doi.org/10.1371/journal.pone.0010660.

  • Williams, B., Lamont, T. A. C., Chapuis, L., Harding, H. R., May, E. B., Prasetya, M. E., Seraphim, M. J., Jompa, J., Smith, D. J., Janetski, N., Radford, A. N., & Simpson, S. D. (2022). Enhancing automated analysis of marine soundscapes using ecoacoustic indices and machine learning. Ecological Indicators, 140, 108986. https://doi.org/10.1016/j.ecolind.2022.108986.