What do whales and dolphins hear?

a data-driven article

The title of this article asks a deceptively simple question. The answer, however, is not that straightforward. Many researchers around the globe are working to figure out what sounds whales and dolphins (collectively referred to as cetaceans) can hear, the mechanisms by which they can hear those sounds, and the effects of those sounds on the animals' behavior and health. Here I will quantitatively summarize the data about what is currently known about hearing in these species. I will then touch on why that might matter in the context of human-made noise.

When we discuss what sounds any living being can hear, we are referring to the frequencies of sound that they can perceive. This is largely dictated by auditory anatomy. There are a variety of sizes and shapes of cetaceans, ranging from the massive baleen whales (mysticetes) to smaller toothed whales (odontocetes). The morphological differences between individuals in these two suborders could mean huge differences in the frequencies that these animals can hear.

Many dolphins emit high-frequency echolocation. It would be logical to assume that they can perceive these high frequency sounds that they produce. Likewise, most mysticetes produce low-frequency vocalizations, so we might assume that mysticetes are low-frequency species. But the key word here is assume. Maybe these animals can hear outside of their vocalization range as well. Obtaining the empirical data and numbers necessary to prove what frequencies they can hear has been very tricky. But, we do have some data, which is a good start.

Audiograms

Hearing is not only about the frequencies that can be heard, but also the intensities of sound necessary to hear at those frequencies. The standard measurement of hearing ability is an audiogram, or hearing threshold curve. Specifically, an audiogram is a graph of hearing threshold as a function of frequency. There are two major methods of measuring audiograms in cetaceans. The first is the behavioral method, in which a trained subject will visibly respond to auditory stimuli of varying intensity. The researcher will determine the intensity for each frequency that is just barely audible at each tested frequency, and together those data points will make up the audiogram curve. The other major method is the auditory evoked potential (AEP) method. Unlike the behavioral method, this method does not require a trained subject. Electrodes are used to measure brain activity in response to auditory stimuli. Both of these tests are non-invasive and safe for the subject.

This graph shows (almost) every cetacean audiogram that was experimentally measured. Hover over the individual curves to discover more about the experimental subject and source. Use the filter beside the graph to limit the data by species. For each species, the hearing range is extrapolated from the original data.

Filter by Species

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[?] More information about audiogram shapes

[?] More information on data sources

Ranges of best hearing

For each species in the above figure, a frequency range of best hearing was calculated as the range of frequencies within 20 dB of the lowest threshold value (the peak frequency). In other words, sounds are easiest to perceive in this range of frequencies.

You will notice that only 20 cetacean species have experimentally-measured hearing threshold data. The number of total cetacean species varies depending on the source, but is generally around 90 species. Furthermore, all of these species are odontocetes. Mysticetes are more elusive and much less manageable for experimentation, making them difficult to study. However, three computational studies were able to estimate the hearing ranges of three different species of mysticete by using models rather than experimentation.

By total
By suborder

In the next figure, we can look closer at and compare those frequency ranges calculated above. This graph also includes the ranges (determined by the same 20 dB method) for the three mysticetes from each modeling paper: humpback whale, minke whale, and fin whale.

These are the ranges of best hearing for each cetacean species represented in the literature through experimentally measured thresholds or biophysical model results. Use the buttons above the graph to sort the data by different properties.

Sort by:

Low frequency
High frequency
Peak frequency
Bandwidth

[?] More information about modeling data

Anthropogenic Noise

There are many sources of noise in the ocean, both natural and man-made (anthropogenic). Sources of anthropogenic noise include: shipping traffic, sonar, and airgun arrays used to detect oil and gas below the seafloor. Much of the man-made noise in the ocean is high-intensity and spans the gamut of frequencies, from infrasonic to ultrasonic. This can potentially interfere with hearing in all cetacean species.

Caveats

As with most cases of working with data, this is a simplification. I feel it is my duty to address the shortcomings of this data analysis so that future iterations may give better estimations of marine mammal hearing. Some of the thresholds for species are wildly different, showing a likely dependence on the instrumentation and experimental standards that each study used. Furthermore, some of the animals may have been in poor health or exhibited hearing loss. Age may also be a factor: do infant cetaceans have different thresholds than adults? Some of the animals may exhibit age-related hearing loss.

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