The secret to koalas' distinctive low-pitched vocalisations has been found, according to a study published today in the journal Current Biology.
Male koalas produce a bellow of a very low pitch, much lower than would be expected from an animal of their size, and more typical of an animal the size of an elephant.
But, unlike any other land-dwelling mammal, the Australian marsupial has been found to have an extra pair of laryngeal vocal folds.
What is unique about this extra organ is its location – rather than inside the voice box, it sits outside where the oral and nasal cavities connect.
Benjamin Charlton, from the University of Sussex and one of the authors of the study, said, “We found that koalas use these additional vocal folds to produce their extremely low-pitched mating calls. To our knowledge, this is the only example of a specialised sound-producing organ outside the larynx in any terrestrial mammal.”
According to Mathew Crowther from the University of Sydney, who was not involved the study, researchers initially dissected ten male koala larynges and found no specialisations that could produce this sound.
After further dissections of koala carcasses, they found a pair of larger and previously undocumented folds spanning the intra-pharyngeal ostium (an oval opening within the soft palate which connects the mouth and nose sections of the pharynx). The shape and position of these folds can produce a sound as air is sucked through the nostrils during inhalation.
The authors then replicated the sound on three koala cadavers by sucking air through the pharynx and larynx via the trachea, mimicking the way koalas would inhale.
By placing an endoscopic video camera to visualise the movement of the newly described folds, the researchers were able to confirm that koalas did use these folds to make their low-pitched bellows.
Dr Charlton explained that the koala’s bellows are produced as a continuous series of sounds on inhalation and exhalation, similar to a donkey’s braying. On inhalation, the bellows sound like snoring. As they exhale, the sound is similar to belching.
“Not only are they actually quite loud, the pitch of a male koala’s mating call is about 20 times lower than it should be, given its relatively small size,” he said.
Low-pitched calls are typical of an animal the size of an elephant because it is the size of the laryngeal vocal folds that limits the lowest frequency that an animal can generate. In other words, smaller animals will produce calls with higher frequencies than larger ones.
By having their vocal folds outside the voice box, koalas have bypassed this size constraint.
“Male koalas need to communicate to female koalas during the breeding season, and these females can be very far away, particularly in low density populations,” Dr Crowther explained.
“Producing the low-pitched bellows means the sound can travel a great distance. The males whose calls could be heard by the greatest number of females would have the selective advantage.'
Brandon Menzies, from the University of Melbourne, agreed, saying, “Even though this study has thoroughly demonstrated the biomechanical origin of low frequency vocalisations in the koala, scientists can still only speculate as to why koalas of either sex require such low frequency communication given that the biological selection for this trait has resulted in a completely new structure that is disassociated from the larynx.
“A likely explanation for this drive towards low frequency may be that these kinds of sounds travel further in the environment relative to high frequency sounds. Elephants use low frequency vocalisations to communicate with each other over many kilometres. Given that koalas may be spread over a large area of complex forest canopy, low frequency communication may provide an effective way of communicating size, gender, reproductive status, territory etc without having to obtain these cues visually.”
William Ellis, a wildlife researcher at the University of Queensland, noted that these low-pitched calls may play a different social role.
“Male koalas are able to avoid physical encounters with other males, and it seems they can do this by detecting body size in the bellow. Interactions between the males are very rare. It seems that the bellow is accurate in conveying body size and so there is little need for a physical fight.”
Suggesting other reasons for the koala’s unique mating calls, Dr Charlton proposed that the low pitch could enhance the salience of the vocal tract resonances, thereby facilitating the communication of cues to identity or body size.
“It could also be that the low pitch itself acts as a direct cue to male quality,” he added.
Dr Ellis agreed, saying that female koalas he studies seem to select mates based on the bellow.
It is likely that that these specialised vocal structures are unique to koalas. No structures for producing such sounds have been found in any other mammal, and no other marsupial communicates in quite the same way.
“The only other example of a specialised sound-producing organ, that is independent of the larynx, are the phonic lips used by toothed whales to generate echolocation clicks,” said Dr Crowther.
To find out if this vocal adaptation is truly unique to the koala, Dr Charlton has plans to conduct detailed anatomical investigations of other closely-related marsupial species.