Animal Mystery: The Elefantine Cry of Horny Koalas Finally Explained

Koalas have a well-earned reputation for being dopey. Sleeping 19 h out of every 24 h, and feeding for 3 h of the remaining 5 h, there doesn’t seem to be much time for anything else in their lethargic lifestyle: that is until the mating season. Then the males begin bellowing. To do so, koalas use a novel vocal organ to produce unusually low-pitched mating calls.

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Mating Koalas. Photo: outnow.ch

The love song of the koala is not very glamorous. The thunderous burps punctuatedwith snoring are as deep as the trumpeting of an elephant. The main vibration, called “fundamental” in acoustics, of this weird buzz is around twenty hertz (Hz) – six times deeper than a standard human voice.

However, the vocal cords of the koala are no longer than one to two centimeters. With such organs, it is theoretically impossible to produce such low-pitched noises. So what is behind this biological mystery?

Benjamin Charlton, a researcher in bio-acoustics at the School of Psychology of Sussex has launched a long investigation to unravel this mystery. In 2011, he demonstrated that the koala had a descended larynx, situated, as in humans, in the throat and not in the jaw. Deer, lion, elephant and even some birds share this characteristic (which had long thought to be the preserve of humans). They probably allow the deepness of koalas’ secondary vibrations, but still does not explain their low-pitched fundamental note.

It is through the dissection of dead koala bodies that they discovered the koala’s vocalisation organ,  located between the nasal and oral cavities was formed by two lips, which vibrate when air passes – a kind of second mouth at the bottom of the palate (velum). These two additional vocal cords (lips) are longer and thicker, and explain the formation of such deep sounds. According to the researcher, only toothed whales are known to have a vocal organ independent of the larynx (these are the phonic lips they use for echolocation).

A detailed description of this “canopy” has been published this week in the journal Current Biology.

Why these deep sounds

Benjamin Charlton explains that koalas probably bellow to attract females and to intimidate other males. But what messages could these rumbling bellows communicate about their senders. Doing so, koala males are able to communicate their size, with the largest animals producing the richest baritone bellows. What about evolution? Individuals that could elongate their vocal tracts by lowering the larynx may have gained advantages during sexual competition by sounding larger, and this would drive the evolution of laryngeal descent. A deep sound scares other males and attracts females. And as a bonus, it repels humans. What more?

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