Koalas may look cute and placid but come the mating season, the males produce a bellow that… well… is not the sound you expect them to make. As they inhale, they sound like a loud, creaky door. As they exhale, they sound like someone belching vigorously. Put these together, and you get a continuous racket that sounds like an angry Wookiee.
The bellows are surprising to passers-by, but they perplex scientists too. Koalas just shouldn’t be able to make a sound that low.
Mammals make calls using an organ in our throats called the larynx, or voicebox. When air passes through the larynx, it vibrates a pair of membranes called the vocal folds (or vocal cords). These create sound waves in our nose and mouth. We can control the pitch of those waves by using muscles in the larynx to change the tension in the vocal cords. The size of the cords also matters—it sets the lowest possible noise that we can make. This is why small mammals can only manage high-pitched squeaks, while big species can produce rumbling bass.
The koala is an exception. The lowest pitch of its bellows has a frequency of 27 Hertz—at least three octaves below middle C, and 20 times lower than you’d expect for an animal of their size. It’s the pitch you’d expect from an elephant.
Now, Benjamin Charlton from the University of Sussex has discovered their secret. He found a completely new organ in the koala’s throat that allows them to make their rumbling bellows. No one had seen it before and, as far as we know, no other mammal has evolved something similar.
The new organ is a pair of vocal folds that look and work very much like the ones in the larynx. But these are found at the velum—the junction where the koala’s windpipe branches into it nose and its mouth. Charlton calls them the velar vocal folds, or VVFs.
The VVFs are 3 times longer than the vocal folds in the larynx, as well as 15 times wider, 14 times deeper, and almost 700 times heavier. Charlton calculated that their huge size allows them to produce extremely deep pitches, as low as 10 Hertz, and to belt out these frequencies with tremendous power.
To test this idea, he placed suction pumps inside the cadavers of three male koalas and sucked air in through their noses. Sure enough, the velar vocal folds vibrated, and produced deep sounds that are remarkably like those of living, bellowing males.
Why has the koala evolved this special organ? It’s not clear, but Charlton suspects that it helps to enhance information about a male’s quality. The bellows may be an exaggerated signal but they’re still honest ones. Bigger males probably have bigger vocal velar folds and produce deeper, louder bellows, in a way that a smaller male just can’t duplicate. Females could use these cues to judge the quality of potential mates.
The team wants to check if other related mammals have the same folds but for now, it looks like they’re a koala innovation. “It appears that this remarkable adaptation has evolved independently in the koala specifically to produce their exceptionally low-pitched mating calls,” says Charlton.
This isn’t the only new body part that’s been discovered recently. In 2012, Nicholas Pyenson found a volleyball-shaped organ in the mouths of the biggest whales, which helps them to coordinate their titanic mouthfuls. And in 2011, John Hutchinson discovered a sixth toe in the feet of elephants—a stiletto heel in the world’s biggest platform shoes.
I love discoveries like these. They’re testament to the continuing importance of old-fashioned disciplines like anatomy and dissections. Koalas, whales and elephants are familiar and charismatic creatures and it’s wonderful to think that their bodies could hold secrets that only a careful scalpel will reveal.
Reference: Charlton, Frey, McKinnon, Fritsch, Fitch & Reby. 2013. Koalas use a novel vocal organ to produce unusually low-pitched mating calls. Current Biology http://dx.doi.org/10.1016/j.cub.2013.10.069.