Grr, baby. Very grr.

by Katie Pratt

“Grr”. That’s about as close to impersonating a lion’s roar that you, or I, or even Austin Powers, will ever be able to get, thanks to the lack of fat in our vocal folds.

“What’s a vocal fold?”, I hear you cry! I was also mildly bemused until a quick Google search informed me that a vocal fold is “more commonly known as a vocal cord”. So, now that mystery is cleared up, why would one want a fatty vocal cord or the ability to roar like a lion?

Well, one would probably rarely want either of these things, but a recent study in PLoS ONE investigated the physiology of lion and tiger vocal folds and found that they differed from most other mammalian vocal chords in that they contained a significant number of fat cells.

Mammals produce sound, or “phonate”, by pushing air through the larynx to induce vibration in vocal folds. The amplitude and frequency of these vibrations are responsible for the sounds made by an animal, and these properties are controlled by the nature of the tissues that comprise the vocal fold as well as the length and context of the cord itself. Therefore it is to be expected, given the wide variety of mammalian phonation, that different animals have different kinds of vocal folds.

But typically mammalian vocal folds are made up of epithelium, connective tissue, and muscle. The precise architecture of these tissues is however specific to every species, allowing for high-pitched sounds (high frequency) or very loud sounds (high amplitude) depending on the needs of the animal.

Lions and tigers use their characteristic roars to mark their territory: Their low frequency high amplitude vocalizations can be heard over a five-mile radius. Producing this kind of sound puts a huge amount of strain on the vocal folds, therefore it was unclear until recently how these large cats were able to sustain their roars. And it turns out fat is the answer.

Fat is a wonderful biological cushion. It is malleable, deformable, and able to dissipate all manner of impacts and stresses. A prime example of fat at work is in the human buttocks, which are (often significantly) padded to protect the lower spine in the event of a fall. In my case, my generous booty is put to work fairly frequently, and I can report that fat is indeed a good shock absorber. In big cat vocal cords the same principle is at work. The fatty layer provides a cushion for the non-fatty region of the fold.

The authors also challenged the accepted argument for the presence of fat in the vocal folds of certain species. Previously researchers had thought that fat increased the volume of the vocal fold thus modulating oscillations in order to produce lower frequencies. However, since the regions of the fold that contain fat are not part of the vibrating tissue, this is highly unlikely. Instead, the current study suggests that the fatty layer can modulate pitch by changing the overall shape of the larynx.

Great, but the real question remains: Does an extra fatty buttock produces lower pitched and louder farts? This is currently unknown, but I will keep my eyes pealed for any publications related to this critical question.

Katie Pratt is a graduate student in Molecular Biology at Brown University. She has a passion for science communication, and in an attempt to bring hardcore biology and medicine to everyone, she blogs jargon-free at www.katiephd.com. Follow her escapades in the lab and online on Twitter.

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About the winner: Signe is a MSc student in molecular biology, working with plants, bacteria and cancer. Currently seeing isothiocyanates as the coolest of compounds. You can follow her on Twitter @BananaFurby.

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