Gibbons and humans have more in common than might immediately seem apparent. Among many behavioural traits shared by our two species is singing. Not just that – the songs of gibbons have the potential to teach us about the origin of our own human capacities.
A recent study in this field, published by Japanese researcher Takeshi Nishimura and colleagues in the American Journal of Physical Anthropology (AJPA), enjoyed the mixed blessing of widespread publicity.
Sadly, the methods and implications of the paper itself were badly misrepresented, with publications focusing on the joke or sensation value of singing gibbons.
I’m more interested in viewing the Japanese research in the wider context of communication theory and voice acoustics.
Background
The Japanese research emerges from a framework in evolutionary studies (with papers such as this one, this one and this one and here) in which cross-species comparison permits speculation on the origin of human capacities.
Among the features of gibbon behaviour that correspond to humans are:
- monogamy, and the “nuclear family”
- voice development in adolescence, especially in the male
- the capacity for upright posture
- vocal duetting in which male and female have acoustically complementary ranges, the adult male’s voice being lower than the female’s.
Walking instruments
As Martin Braun reminds us, all mammals are walking wind instruments. Research in acoustic communication addresses several key questions, among them:
- how sound is produced anatomically
- whether it is learned or stereotypical (the aspect of acquisition)
- what social or intra-specific purpose it serves
- the nature of the neural networks responsible for its perception and production
Back to the study
The authors of the AJPA study are interested in all of these interrelated factors. But their paper focuses in particular on the way the distinct acoustic signal (or sound) of female gibbons is produced and what this tells us about the feedback loop between sound-production and self-perception in the subject.
At the core of the AJPA study is a series of experiments – first modelled on a computer and then carried out in the laboratory – designed to test a rather complex theory: that female gibbons have the ability to modify the configuration of their vocal tracts in order to amplify signals in a specific range.
In this range, their voices are capable of extraordinarily effective exploitation of limited anatomical means.
In other words, humans and gibbons have both evolved neural control over their respiratory tracts. It is therefore what the brain achieves that allows this level of performance: the vocal tracts themselves are not especially well-suited to their task!
Where musical instruments such as flutes and trumpets have relatively fixed properties of bore, length and shape, the respiratory tracts of mammals are capable of modification in a variety of ways, including:
- lowering of the larynx
- adaptation of the posture of the larynx
- lifting of the soft palate
- nasalisation
- adaptation of the shape of the mouth
- position and posture of the tongue, and
- adaptation of the opening of the lips.
Significantly, all of these are more-or-less involved in the production of the variety of sounds on which language tends to depend.
But the aim of the Japanese research was to explore whether gibbons “tune” the shaping of their throats, mouths and lips to the frequencies generated by their vocal folds.
It’s a gas
The researchers used helium in their experiments. The use of helium as the medium in which the gibbon vocalised is thus a means of ascertaining how the subject responded to the effect – which in humans tends to shift the resonant frequencies of the voice towards “squeaky” higher “formants” (the acoustic response within an instrument – or voice – that defines the quality of sound, or timbre).
As in humans, gibbon vocalisation in the helium condition resulted in a shift of resonance to higher formants. Significantly, this was more pronounced in the higher range of the gibbon calls. This higher range marks their characteristic achievement of high-intensity vocal production able to carry over distance.
If this suggests parallels with human singing, it’s worth spelling out that all female gibbons achieve this remarkable capacity for self-amplification instinctively. Gibbon song is not learned – the ability is hard-wired.
By contrast, the opera singers to which the study’s title refers achieve a parallel ability only after careful training, and relatively few humans attempt this lest they resemble the dread example of Florence Foster Jenkins, as per the video below.
To hear the more effortlessly competent performance of a variety of gibbon species, a visit to Thomas Geissmann’s impressive gibbon research website is warmly recommended.
What comes next?
The research done by Nishimura and team opens up some fascinating avenues for follow-up studies, including:
- replication with human subjects, both expert singers and non-singers
- replication with other primate species (this is, apparently, underway)
- play-back of the helium-condition calls to conspecifics (as well as being great vocalisers, gibbons are sophisticated listeners. The purpose of studying this area further would be to see whether other gibbons recognised in any way the humanly distorted recordings as gibbon-song)
One of the most interesting things about gibbon/human voice comparison is how easy it is for suitably uninhibited humans to elicit vocal responses from gibbons by imitating their song.
This doesn’t work for many species – try it out on most dogs, cats, horses, chimpanzees and you will encounter failure … or worse.
But gibbons safely contained behind barriers are remarkably tolerant of human attempts at their song. I have personal experience of this at Twycross Zoo in the UK and Perth Zoo in Australia. My youngest son, when a boy treble, was able to imitate female gibbon calls with considerable accuracy.
This seems consistent with his ability to perform the high-lying solos in works such as Allegri’s Miserere that require precisely the acoustic control explored in this study, and whose range conforms to that in which the gibbons sing most effectively.
So some investigation of similarities with the voices of pre-adolescents in both species might also be of interest, especially with a view to tracing the means by which the ability is acquired in gibbons.
While the gibbon call is clearly acquired as a means of high-intensity transmission that permits communication across distance in dense rainforest vegetation, I have heard gibbons “rehearsing” precisely the same songs quietly: a solitary female in an isolated pen at Twycross Zoo in the UK; and two juveniles in quarantine under the supervision of their curator at Perth Zoo in Australia.
This adds additional dimensions of the roles of energy and pragmatics to the findings of this study: it illustrates that, while gibbons are capable, much like human sopranos, of ear-splitting volume, they can also achieve the same calls quietly.
In “engineering” terms this means they can control breath-flow alongside the “tuning” effects already described.
Investigation of these features would be of interest in order to tease out implications of this study for the evolution of human language.