# how whales click

How do sperm whales vocalize? This is...[apparently](https://www.lesswrong.com/posts/eZaDucBYmWgSrQot4/how-articulate-are-the-whales)...a topic that LessWrong readers are interested in, and someone asked me to write a quick post on it.

The clicks they make originate from blowing air through "phonic lips" that look [like this](https://anatomypubs.onlinelibrary.wiley.com/cms/asset/fdb88a63-bf42-4920-91fd-319f6a921a16/mfig008.jpg); picture is from [this paper](https://doi.org/10.1002/ar.20652). This works basically like you closing your lips and blowing air through them. By blowing air between your lips with different amounts of tension and flow rate, you can vary the sound produced somewhat, and sperm whales can do the same thing but on a larger scale at higher pressure. As [this convenient open-access paper](https://anatomypubs.onlinelibrary.wiley.com/doi/full/10.1002/ar.23168) notes:

> Muscles appear capable of tensing and separating the solitary pair of phonic lips, which would control echolocation click frequencies. ... When pressurized air is forced between these opposing phonic lips, they vibrate at a frequency that may be governed by airflow rate, muscular tension on the lips, and/or the dimensions of the lips (Prestwich, 1994; Cranford et al., 1996, 2011).

After the phonic lips, sound passes through the vocal cap. The same paper notes:

> The phonic lips are enveloped by the “vocal cap,” a morphologically complex, connective tissue structure unique to kogiids. Extensive facial muscles appear to control the position of this structure and its spatial relationship to the phonic lips. The vocal cap's numerous air crypts suggest that it may reflect sounds.

I suspect that the vocal cap is actually used to direct vocalizations. Sound travels much faster in water than air, so varying the ratio of air to fluid across the vocal cap (eg by squeezing air pockets) could be used to refract sound at varying angles. (You could minimize reflection by having lots of small air pockets at sharp angles to the sound waves.) It's also possible that it acts as a variable frequency filter using [periodic structures](https://en.wikipedia.org/wiki/Acoustic_metamaterial) that match sound wavelengths.

The phonic lips are at the front of the skull. Sound from them passes through the skull, gets reflected, and passes through the skull again. Kind of like how many dish antennas have a feed horn out in front, and signals get reflected back towards the feed horn. Here's a diagram comparing echolocation in dolphins and sperm whales:

![](https://39669.cdn.cke-cs.com/rQvD3VnunXZu34m86e5f/images/d2703dbf3869000b4cfa97f19a1ebbb599b3ebc2bbb2faf4.jpg)

(picture from "Sound production and propagation in cetaceans" by Chong Wei)

As the generated sound passes through the organs inside a sperm whale skull, it gets refracted, focusing it. Muscles can change the shape of those organs to adjust that focusing. The same paper notes:

> The melon is formed by specialized “acoustic lipids” that are heterogeneously arranged within this structure. Sound travels at lower velocities through the lipids found in the melon's central core than they do through the lipids in the outer melon cortex (Litchfield et al., 1973; Norris and Harvey, 1974; reviewed in Cranford et al., 1996; Koopman et al., 2003). This sound velocity disparity causes refraction of the acoustic energy towards the melon core, resulting in a focused beam of sound. Facial muscles likely act to change the shape of the fatty sound transmission pathway, which may affect the direction in which the sound beam is emitted and/or change the frequency of the emitted sounds before they exit the melon and are transmitted into the environment (Norris and Harvey, 1974; Mead, 1975; Harper et al., 2008).

Wikipedia of course has a [long page](https://en.wikipedia.org/wiki/Sperm_whale) about sperm whales, but I wanted to note something it gets wrong:

> Some of the sound will reflect back into the spermaceti organ and back towards the front of the whale's nose, where it will be reflected through the spermaceti organ a third time. This back and forth reflection which happens on the scale of a few milliseconds creates a multi-pulse click structure.

About that, I think [this paper](https://pubs.aip.org/asa/jasa/article-abstract/114/2/1143/547670/The-monopulsed-nature-of-sperm-whale-clicks) is correct that:

> Traditionally, sperm whale clicks have been described as multipulsed, long duration, nondirectional signals of moderate intensity and with a spectrum peaking below 10 kHz. Such properties are counterindicative of a sonar function, and quite different from the properties of dolphin sonar clicks. Here, data are presented suggesting that the traditional view of sperm whale clicks is incomplete and derived from off-axis recordings of a highly directional source. A limited number of assumed on-axis clicks were recorded and found to be essentially monopulsed clicks

That's a paper from 22 years ago but outdated information about things like this hangs around for a long long time; Wikipedia's citation there is from 1966.

All toothed whales use the same basic system for echolocation. As [this paper](https://www.cambridge.org/core/journals/journal-of-the-marine-biological-association-of-the-united-kingdom/article/abs/nose-of-the-sperm-whale-overviews-of-functional-design-structural-homologies-and-evolution/559739BEF0369E7D715CEB09DDCE7A21) notes:

> Comparison of nasal structures in sperm whales and other toothed whales reveals that the existing air sac system as well as the fat bodies and the musculature have the same topographical relations and thus may be homologous in all toothed whales (Odontoceti). This implies that the nasal sound generating system evolved only once during toothed whale evolution and, more specifically, that the unique hypertrophied nasal complex was a main driving force in the evolution of the sperm whale taxon.

Systems for echolocation have evolved in ocean mammals multiple times; the reason why toothed whale echolocation only evolved once might be the extra complexity needed to handle pressure changes from deep diving. Increased pressure makes air shrink, which requires compensation using blood to replace air volume, to prevent organs from changing shape and thus breaking echolocation functions. While I do have enough baseline biology/physics knowledge to go on a bit here, if you want to read more about that [here's a related open access paper](https://anatomypubs.onlinelibrary.wiley.com/doi/10.1002/ar.20652). And [here's a recording](https://www.youtube.com/watch?v=HS9OIBifQVw) of what sperm whale clicks sound like.