December 3, 2013
Mating ‘Belch’ Of The Male Koala Created By Novel Organ
[ Watch the Video: Belching For The Ladies, Koala-Style ]
Koalas have the appearance of a living teddy bear, inviting many of us to assume they are cute and placid. When mating season starts, however, the males produce a bellow that defies their size and cuteness.
As the males inhale, they make a sound somewhat like a creaky door, according to National Geographic reporter Ed Yong. Exhalations sound more like someone belching, with gusto. Yong says that together these noises create a continuous racket that sounds like an angry Wookie. Scientists and tourists alike are perplexed by the depth of these sounds. Koalas, with their tiny bodies, shouldn't be able to produce such a sound.
A new study, published in Current Biology, has discovered the secret to the koala's deep bellow: koalas have a specialized sound-producing organ that has never before been seen in any other land-dwelling mammal. The newly described organ is outside the voice box, or larynx.
"We have discovered that koalas possess an extra pair of vocal folds that are located outside the larynx, where the oral and nasal cavities connect," says Benjamin Charlton of the University of Sussex. "We also demonstrated that koalas use these additional vocal folds to produce their extremely low-pitched mating calls."
Generally, mammals create vocalizations using the larynx in their throats. As air passes through the larynx, a pair of membranes called the vocal folds are vibrated—creating sound waves in the nose and mouth. The pitch of those sound waves can be controlled by using muscles in the larynx to change the tension in the vocal folds, also known as the vocal cords.
[ Watch the Video: Understanding A Koala’s Voice ]
The size of a mammals vocal cords matters as well by setting the lowest possible noise that each animal can make. This explains why small mammals can only produce high-pitched squeaks, while large species can produce rumbling bass.
The koala, however, is an exception to this rule. The mating bellow of the male has a lowest pitch frequency of 27 Hertz—at least three octaves below middle C. This is 20 times lower than would be expected for a mammal of their size. In fact, it is more in tune with what you would expect from an elephant.
Charlton's team discovered the secret: a completely new organ. Located in the koala's throat, the organ Charlton describes for the first time, allows the rumbling bellows. To date, scientists know of no other mammal with such an organ.
The new organ is a second set of vocal folds that look and work very much like the pair in the larynx. They are found, however, at the velum—the junction where the koala's windpipe branches into its nose and mouth.
These velar vocal folds, or VVFs, are three times longer than the folds in the larynx, 15 times wider, 14 times deeper and almost 700 times heavier. Their huge size allows them to produce the extremely deep pitches of the mating bellow, according to Charlton. These pitches range as low as 10 Hertz and the koalas can belt them out with tremendous energy.
Charlton placed suction pumps inside the cadavers of three male koalas. This allowed him to suck air in through their noses, vibrating the VVFs. This produced deep sounds that are remarkably like those of living, bellowing males.
Charlton suggests that the koalas evolved this special organ to help enhance information about a male's breeding quality. The bellows might be an exaggerated signal, given the size ratio of the animal to the sound produced, but they are still honest signals. Bigger males are probably capable of producing deeper, louder bellows in a way that smaller males would not be able to duplicate. This would give females a way to judge a male's quality as a potential mate.
Charlton and his team would like to investigate whether other mammals have developed these same VVFs, but at the moment, it appears to be 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.
"To our knowledge, the only other example of a specialized sound-producing organ in mammals that is independent of the larynx are the phonic lips that toothed whales use to generate echolocation clicks," Charlton says.