Koala bears have been bearing an untold secret for centuries, they have a unique sex organ in their throat that they use to woo females during mating season. The call emitted is uncharacteristically low for their size and researchers now believe they have found out why.
Koalas are known for their small, cute size and now Benjamin Charlton from the University of Sussex has reported that they have incredible vocal range, despite their size. The discovery was published on the 2nd of December in Current Biology.
According to the recent report, koalas have an extra vocal organ that provides the male with a depth in his vocal range and assists the males in calling a mate. The koalas’ size is in stark contrast to the depth of its vocal range- the male koala is able to emit sounds that almost 20 times the depth of the average animal its size. The male koala’s vocal range is closer to the range of an elephant, an animal known for its deep and often inaudible communication.
The published report states that the male koala emits a bellow-like sound during the breeding season. This sound carries characteristics of continuous sections of inhalations and exhalations. The call’s fundamental frequency (known as the Fo) is very low. The inhalation section of the bellow has an average fundamental frequency of 27.1 Hz, although it ranges between 9.8 Hz and 61.5 Hz. This fundamental frequency is 20 times less than the expected frequency for an animal that weighs less than or equal to eight kg’s. The newly discovered vocal organ is what is used to create this uniquely low frequency mating call.
The newly discovered vocal “sex” organ, found by Charlton, is found exterior to the koala bear’s larynx in the soft palate where the oral and nasal cavities meet. This unique placement, coupled with the fact that it is an extra pair of vocal folds, produces the surprising depth of the male’s mating call.
What is more is that there are no other known terrestrial mammal, or animal, that has such a vocal organ that is exterior to its larynx. Varying whale species such as the toothed whales use phonic lips to emit echolocation clicks and these whales are the only other known animals to have this exterior vocal organ. The toothed whales So, what does it sound like? The mating call’s inhalation has been likened to a human’s loud snore and the exhalation to the bray of a donkey. While this may not sound ver romantic, it is the impressiveness of the call’s frequency that is more likely to attract and impressive the female.
The koala is currently the only animal to display such unique vocal traits and Charlton is planning to extend his field of research so that he may determine whether or not there are other animals with this type of vocal adaptation.
The authors of the study have suggested that the “new” velar vocal folds found within the males are developed so that the he is able to produce the resultant low Fo. The authors go on to suggest that the reason for this is so that the vocal tract resonance salience is enhanced, which in turn enables the transference of bodily and identity cues such as size and weight. The other speculation put forth by the authors is that the the Fo is a more direct clue as to the quality of the male. The authors did not however, expand on these qualities- would be different to the purely physical aspects or could the Fo communicate aspects of personality to the listening female? And how much would it matter? These are some of the questions the researchers have been introduced to with this new vocal fold discovery and their work is to now further investigate.
The female koala also has a low bellow but they only use it only on rare occasions. The investigation into the anatomy of female koalas is now needed so that the researchers may see whether or not the female has the same type of velar vocal folds. There are also other qualities that could be the reason for such a disproportionate Fo in animals; certain vocal fold specialisations, such as hypertrophied larynges, could increase the vibrating mass which would lead to the low Fo despite an animal’s size. The data presented in the Current Biology paper details the very first mammal that has developed an organ that is specifically for sound production. The next step? New studies will soon follow to discover whether this incredible sound and “sex” organ is unique to the koala or if there are other mammals that share this remarkable bit of vocal anatomy.
University of Sussex – Dr. Benjamin Charlton