Helium-huffing gibbons 'sing with soprano technique'
When the apes made calls while in an atmosphere rich in helium, the team analysed the calls' frequencies.
|The gibbons were tested using an atmosphere containing 50% helium with oxygen and nitrogen|
As the team report in the American Journal of Physical Anthropology, the apes were able to control the natural frequencies of their "vocal tracts".
Such control, exemplified by sopranos, was thought to be unique to humans.
We share a great deal of the biological equipment of sound production with apes. That includes first of all the "source" - the vocal folds that humans and many animals share.
There is also the "vocal tract" - the upper oesophagus and trachea and the mouth, which are well known in humans to shape sung notes and subtle vowel sounds.
In humans the vocal tract acts as a filter on the sound from the source, and the "source-filter theory" held that the separate, fine control of the vocal tract to be the product of a long evolution in the development of the subtleties of speech.
Singing too has evolved, and soprano singers reach their piercing high notes by precisely controlling the shape of their vocal tract to match its natural, resonant frequency with multiples of the one being produced by their vocal folds.
Now Takeshi Nishimura of Kyoto University's Primate Research Institute and his colleagues have tested whether lar gibbons (also known as white-handed gibbons, Hylobates lar) have this same separate control - by using helium.
As anyone who has breathed helium knows, its presence raises the pitch of the voice. It increases the natural resonant frequency in the vocal tract because the speed of sound in helium is very different from that in air.
That shift allowed the team to record calls in helium and examine separately the sounds of gibbons' "pure-tone" vocalisations from the vocal folds as well as how they were modified in the vocal tract.
Detailed analyses of the frequencies produced showed that the gibbons modified their vocal tracts to match multiples of the vocal folds' frequencies - just like soprano singers.
Dr Nishimura told BBC News the findings were significant - not only that the "source-filter theory" was not the preserve of human physiology, but also that the gibbons had mastered techniques that in humans were only found in professional singers.
He explained that it upended a long history of research suggesting the control humans enjoy is the product of a long line of physiological and anatomical changes under the influence of evolution.
"The present study challenges that concept and throws new insight into the studies on biological foundations producing the diversifications in primate vocalisations, including human speech," he said.
"It is hoped that this study will encourage researchers in various research fields to conduct further investigations of primate vocalisations and that such empirical evidence will lead to a deeper understanding of the evolution of speech and language."