Oh, to live a koala's life. Via cafuego/Flickr
They're cute, they're fuzzy, and their surprisingly deep, dulcet tones are key to gettin' it on. I'm talking about male koalas, whose mating calls are far deeper than would be expected considering their diminutive size. How's that possible? As it turns out, male koalas have a separate organ to help produce their deep mating calls—which sounds rather flatulent, I must admit—a feature that's extremely rare in mammals.
Mammals vary widely in shape and size, but they generally play by the same physiological rules. So a mouse's heart is proportionally similar in size to a gorilla's, and the same constraints apply to vocal cords. But, as new research published in Current Biology points out, male koala's breeding bellows are some 20 times lower than would be expected for marsupials weighing in at 10 pounds or less. And the call is truly deep: With an average frequency of around 27.1 Hz, koala bellows are "more typical of an animal the size of an elephant," according to the paper.
To sort out exactly how this happens, the team led by Benjamin Charlton of the University of Sussex examined the larynges of 10 male koalas. Observations of the specimens' vocal folds were unrevealing; they're simply not long enough to vibrate at such a low frequency.
The team then moved outward, and found something novel: a pair of much larger vocal folds located, not in the larynx where you'd expect them to be, but outside it where vocal and nasal cavities meet. As described by the authors, these velar vocal folds look like a pair of long, thick lips located near the animal's soft palette, a location that helps explain why male koalas' bellows can be made on both inhalation and exhalation.
This diagram from the paper shows the velar vocal folds (VVFs) in relation to the larynx and intra-pharyngeal ostium. Via Charlton et. al
Of course, the authors had to test out whether the folds, which they write have not previously been described, can actually make sound. I'll let them explain how they did so:
Because koalas produce the low F0 sections of bellows on inhalation, we reproduced natural sound production in three male koala cadavers by sucking air through the pharynx and the larynx via the trachea, mimicking inhalation of air using the lungs. This allowed us to investigate whether an ingressive ﬂow of air can induce self-sustained oscillation of the velar vocal folds and produce low frequency sounds.
And yes, the team recorded their setup on video:
As the authors note, other mammals are capable of producing calls far lower than would be expected. Male hammer-headed bats, for example, have extremely enlarged larynges, while howler monkeys' distinctly shaped hyoid bones help produce their guttural calls.
But having an entirely new vocal structure, as koalas do, is rare. It's known that toothed whales have a similar structure, called phonic lips, that helps them make their loud and distinctive clicks, but such organs haven't been described in terrestrial mammals. Naturally, more research is needed to find out if any other deep-voiced mammals have similar structures, but for now, koalas' extra crooning organ appears to be unique.