When asked what we know about gibbons, most of us think of a long-armed non-human primate that makes a lot of noise. That’s pretty accurate. Gibbons are well known for their loud calls, which ring out across their forest homes in South-East Asia. Those of us who study them refer to these calls as ‘songs’, and a quick trip to the zoo at the right time of day explains why; these songs are not only loud, but elaborate, melodious and repetitive, giving them all the hallmarks of other animal song, including birdsong.
Although gibbons use song to communicate with distant individuals, they typically live in small family groups. Their short-range, quieter vocal communication between close relatives is something we know almost nothing about.
Our team are the first to empirically investigate the quieter intra-group calls of wild white-handed gibbons (Hylobates lar) in Thailand. While anyone who studies gibbons knows that these quiet calls are ubiquitous, we know very little about what they mean and why they are used.
Quieter calls are context-specific and vary by sex
Our study shows that one type, the ‘hoo’ call, is context-specific as revealed by acoustic analysis. This is surprising, since to the untrained human ear, hoos in all contexts sound pretty much alike. This specificity means that hoos may be a candidate for the meaningful transfer of information between gibbons in close proximity to each other.
Hoos are produced in several contexts including feeding, encountering a neighbouring group or a predator, and as a prelude to their well-known songs. Our study reveals there are subtle acoustic differences between all of these.
Of particular note, hoos given in response to raptor predators are acoustically distinct from all other hoos, including those produced to tigers and clouded leopards, making the raptor hoo a predator-specific hoo variant. Raptor hoos are the lowest frequency, lowest duration and most spread out temporally, making them the least audible.
This is in stark contrasts with gibbon responses to cat predators, which, although they begin with hoos, always escalate into loud song (see an earlier study of ours here). Raptors have poor hearing at low frequency, and one possibility is that gibbons produce very quiet, low frequency calls in order to avoid detection.
Interestingly, we also found that male gibbon voices are pitched significantly higher than female voices. This contradicts most of the evidence from other mammals, including humans, and is all the more perplexing since gibbons show little to no sexual dimorphism in body size. One explanation is that female gibbons prefer males with higher pitched voices, and a recent study by Claudia Barelli and colleagues has suggested that male gibbon voice pitch increases with androgens.
Finding such subtle variation in just one call type – the hoo – makes the gibbons’ potential vocal repertoire much larger than previously thought. Playbacks are now needed to determine whether these differences are salient to conspecifics and therefore ‘referential’.
Helping us understand the evolution of communication
Gibbon songs are long, loud and complex, but we are only just beginning to see the extent of their communicative repertoire and hint at the underlying cognitive processes (see our 2006 study here).
A recent article in the New Scientist showcases the payoff of hours of diligent time spent listening to gibbon calls by Angela Dassow and Michael Coen. They found that white-handed gibbon song contains at least 26 different units (compared with the originally described 7) and that’s only in a subset of songs so far.
Our study shows that gibbon quiet calls are also complex, and vary in very subtle ways: we’ve been able to split the hoo unit into six sub-units, and there are probably more. Subtle variations in frequency-related parameters like pitch can also be critical carriers of meaning in some human languages such as Thai which can, rather uncomfortably, make them tricky for Westerners like me to get right.
Gibbons are extraordinarily vocal creatures, and because they are represented by 16 extant species, they give us the rare opportunity to study the evolution of complex vocal communication in a non-human primate. In the future, gibbon vocalisations may reveal much about the processes that shape vocal communication. And, because they are an ape species, they may be one of our best hopes at tracing the evolution of human communication as well.