Male Koala Bears Use Extra Vocal Chords To Woo Ladies
December 10, 2013
During the breeding season, male koalas produce ‘bellow’ vocalisations that are characterised by a continuous series of inhalation and exhalation sections, and an extremely low fundamental frequency (the main acoustic correlate of perceived pitch).
Remarkably, the fundamental frequency (F0) of bellow inhalation sections averages 27.1 Hz (range: 9.8–61.5 Hz ), which is 20 times lower than would be expected for an animal weighing 8 kg and more typical of an animal the size of an elephant.
Here, we demonstrate that koalas use a novel vocal organ to produce their unusually low-pitched mating calls.
Benjamin D. Charlton, Roland Frey, Allan J. McKinnon, Guido Fritsch, W. Tecumseh Fitch and David Reby. School of Psychology, University of Sussex, Brighton, BN1 9QH, U.K 2 Leibniz Institute for Zoo and Wildlife Research, Berlin, Germany 3 Moggill Koala Hospital, Department of Environment and Heritage Protection, Moggill, Queensland, Australia 4 Department of Cognitive Biology, University of Vienna, Vienna, Austria
HOW KOALAS BECAME THE BARRY WHITES OF THE MAMMAL WORLD
Oh, to live a koala’s life. Photo via cafuego/Flickr
They’re cute, they’re fuzzy, and their surprisingly deep, dulcet tones are key to gettin’ it on.
I’m talking about male koalas, whose mating calls are far deeper than would be expected considering their diminutive size.
How’s that possible? As it turns out, male koalas have a separate organ to help produce their deep mating calls—which sounds rather flatulent, I must admit—a feature that’s extremely rare in mammals.
Mammals vary widely in shape and size, but they generally play by the same physiological rules. So a mouse’s heart is proportionally similar in size to a gorilla’s, and the same constraints apply to vocal cords.
But, as new research published in Current Biology points out, male koala’s breeding bellows are some 20 times lower than would be expected for marsupials weighing in at 10 pounds or less.
And the call is truly deep: With an average frequency of around 27.1 Hz, koala bellows are “more typical of an animal the size of an elephant,” according to the paper.
To sort out exactly how this happens, the team led by Benjamin Charlton of the University of Sussex examined the larynges of 10 male koalas.
Observations of the specimens’ vocal folds were unrevealing; they’re simply not long enough to vibrate at such a low frequency. The team then moved outward, and found something novel: a pair of much larger vocal folds located, not in the larynx where you’d expect them to be, but outside it where vocal and nasal cavities meet.
As described by the authors, these velar vocal folds look like a pair of long, thick lips located near the animal’s soft palette, a location that helps explain why male koalas’ bellows can be made on both inhalation and exhalation.
This diagram from the paper shows the velar vocal folds (VVFs) in relation to the larynx and intra-pharyngeal ostium.
Via Charlton et. al
Of course, the authors had to test out whether the folds, which they write have not previously been described, can actually make sound. I’ll let them explain how they did so:
Because koalas produce the low F0 sections of bellows on inhalation, we reproduced natural sound production in three male koala cadavers by sucking air through the pharynx and the larynx via the trachea, mimicking inhalation of air using the lungs. This allowed us to investigate whether an ingressive ﬂow of air can induce self-sustained oscillation of the velar vocal folds and produce low frequency sounds.
And yes, the team recorded their setup on video:
As the authors note, other mammals are capable of producing calls far lower than would be expected.
Male hammer-headed bats, for example, have extremely enlarged larynges, while howler monkeys’ distinctly shaped hyoid bones help produce their guttural calls.
But having an entirely new vocal structure, as koalas do, is rare. It’s known that toothed whales have a similar structure, called phonic lips, that helps them make their loud and distinctive clicks, but such organs haven’t been described in terrestrial mammals.
Naturally, more research is needed to find out if any other deep-voiced mammals have similar structures, but for now, koalas’ extra crooning organ appears to be unique.
See a male koala making the extraordinary bellowing sounds (footage by Benjamin D Charlton at Lone Pine Koala Sanctuary, Brisbane)
It is a low, rumbling bellow that seems very incongruous coming from the mouth of a diminutive koala.
And now scientists have found that these famously sleepy marsupials have evolved a vocal organ that allows them to produce very low-pitched sound. Koalas, researchers discovered, have an “extra pair of vocal folds” outside the larynx, which they use to make their mating calls.
The findings are published in the journal Current Biology.
“The first time I heard a koala bellow I was genuinely amazed that an animal this small could produce such a sound,” said Benjamin D Charlton, of the University of Sussex, who led the research.
“Just think of a guitar string: as you shorten the string by placing a finger on the fret board you raise the frequency of the sound produced”
Benjamin Charlton University of Sussex
The pitch of the bellow, Dr Charlton said, was about “20 times lower than would be expected for an animal of its size”.
“[It is] more typical of an animal the size of an elephant,” he told BBC News.
The pitch of a call is generally associated with a mammal’s size, because vocalisations come mainly from the larynx – an organ we sometimes refer to as our “voice box”. This organ has a valve-like opening with two lips – or folds – running across it.
The vibration of these folds creates most of the sound we make when we speak. Smaller mammals, like koalas, which can weigh as little as 8kg (18lb), tend to have a smaller larynx with shorter, thinner vocal folds. And just like strings on a musical instrument, these make a higher pitched sound. But when the researchers dissected the koalas’ vocal tracts, they found a second, larger set of vocal folds.
“[They] are located outside the larynx, where the oral and nasal cavities connect,” said Dr Charlton.
These folds, called the velar vocal folds are “over three times longer and around 700 times heavier than the laryngeal vocal folds”, he added.
Koala (c) Joni Avenell
Very low pitch might help koalas transmit information more clearly in their calls “Larger structures can oscillate at lower frequencies.
“Just think of a guitar string – as you shorten the string by placing a finger on the fret board, you raise the frequency of the sound produced, and the thickest strings produce the lowest frequencies.”
Dr David Reby, from the University of Sussex, who was also involved in the research, explained that the animals’ “unique vocal folds” were part of the soft palate, and “much bigger than the laryngeal vocal folds”.
He said that humans created a similar effect when they snored, but added that our own soft palate was “not specialised for the production of sound”.
The researchers think the very low pitched calls might have evolved because it helped the koalas to transfer information more clearly in their vocalisations.
Dr Charlton added: “Another possibility is that low pitch acts as a direct cue [to females of the male's quality], but this remains a topic for future studies.