During mating season, male koalas produce an extremely low fundamental frequency (F0) vocalization known as "bellows''. This vocalization is produced by a continuous inhalation and exhalation, Researchers were able to identify a novel vocal organ in koalas that create the low-pitched mating calls. Remarkably, the f0 of bellow inhalation had an average of 27.1 Hz (ranging from 9.8-61.5 Hz), which is twenty times lower than would be expected for a creature weighing roughly 8 kg and more appropriate for an animal the size of an elephant. 

 

Upon the completion of close analysis of the larynx from 10 male koalas, researchers failed  to identify any specific specializations for their uniquely low frequency production. Vocal folds behave like a simple string, F0 is linearly and inversely related to its length. In the case of koalas, their vocal folds are about 9.8 mm and according to the theoretical string theory, vocal folds of this length should produce frequencies over 389 Hz, and definitely nothing under 51.0 Hz. But like all things that are unique to the koala, the production of bellows around 27.1 Hz is uncomplicated.

 

Further dissections revealed undocumented folds spanning the intrapharyngeal ostium, an oval opening within the velum that connects the oral and nasal portions of the pharynx, now known as the "velar vocal folds". These folds can be used to produce sound as air is sucked in through the nostrils, causing the folds to become heavily wrinkled and tense. Male koalas utilize their sternothyroid muscle to pull the larynx back when they vocalize, and contractions of the palatopharyngeal muscles directly change the shape and length of the velar vocal folds. 

 

In order to demonstrate the role the velar vocal folds play in the production of bellows, researchers reproduced the natural sound using 3 male cadavers. The cadavers were used to mimic lung inhalation by vacuuming air through the pharynx and the larynx via the trachea. Utilizing this approach, researchers were able to replicate 250 independent phonation events. Along with sounding remarkably like those produced from living koalas, the recorded range of frequencies from the velar vocal folds of the cadavers (mean:22.7 Hz; range:6–133 Hz) matched that of data collected on the previous 10 male koalas. Acoustic data collected presents evidence of the first terrestrial mammal with an organ other than the larynx that is specialized for sound production.  

 

Female koala bellowing has been reported, but much less frequently than male koalas, but literature on the subject is lacking and more analysis of the female koala vocal anatomy is needed. Reflecting on this specialization, this unique production is hypothesized to either enhance the importance of vocal tract resonances, promoting cues of identity or body mass, or the F0 itself communicates the quality of the male koala. The results of this study now opens the doors for further analysis, whether this adaptation is unique to koalas or shared amongst other mammalian species remains unknown.