There is some interesting news on gibbons. But first, what are gibbons? They are apes, called lesser apes but definitely in our group with chimps, gorillas, and orangs and not with monkeys. The Chinese used to call them “gentlemen of the forest” to separate them from troublesome monkeys. Our lineage split from theirs about 18 million years ago. For context, the separation with orangs was 14, gorillas 7, and chimps 5 mya.

They are the fastest travelers through forest canopy, clocked at 55 km/hr, swinging from branch to branch. They have ball and socket wrists on their long and powerful arms. When they are forced to the ground they walk upright (more upright than chimps can manage). Gibbons are social, territorial and pair bond for life. And they sing with very powerful voices due to reverberating throat sacs. They sing duets, and family choir performances. But they also whisper or “hoo”. There have been some studies of their song but the Clarke paper (citation below) is the first study of the softer hoos.

This is important to the inquiry into the history of human language. There are two approaches to looking at our language: one is to look at what is unique and separates us from our nearest cousins; the other is to look a what is similar and forms a continuum with our relatives. We can read many articles on the uniqueness but only recently have there been articles on the similarities.

“Although language is a uniquely human behaviour, it is likely to have evolved from precursors in the primate lineage, some of which may still be detectable in the vocal behaviour of extant primates. One important candidate for such a precursor is the ability to produce context-specific calls, a prerequisite to referential communication during which an actor refers a recipient’s attention to an external event. … More recently, functionally referential calling behaviour also has been described for other species of monkeys, apes, dogs, dolphins, and birds such as fowl, jays and chickadees.

Overall, context-specific calling behaviour appears to be widespread in animal communication, presumably because the selection pressure to attend to and understand context-specific calls is very strong, especially in evolutionarily urgent situations. In addition, there is good evidence for call comprehension between different species of primates, between primates and birds and between primates and other mammals, suggesting that such phenomena are driven by a generalised cognitive mechanism that is widely available to animals. Whether or not such abilities are relevant for understanding language evolution has triggered much debate with no real consensus. Nevertheless, the comparative study of animal communication, especially across non-human primates, is one of the most useful tools to make progress and address open questions about human language evolution.”

Although gibbon hoos sound much the same to human observers, when they are recorded and analyzed for highest pitch, lowest pitch, pitch delta, duration, volume, interval between calls, it is possible to see the difference between hoo calls in different situations. The distinct situations noted included: tiger, leopard, raptor, encounter with another group, feeding, and introduction to duet song.

This communication based calling, that is fairly common in non-solitary animals, differs from human language to the extent that the calls are relatively fixed to particular situations and are small in number for most animals (dolphins and whales may have a surprising number and it is not known whether they are fixed). Some would say that animal calls are automatic and do not involve any decision to call; it is difficult to measure this and in any case does not seem to apply to the more intelligent animals. The exchange of information is clearly involved in animal communication – communication and exchange of information are almost synonymous. The idea that our communication is based on affecting one another’s attention, metaphorically pointing at concepts, objects, actions etc. fits nicely with animal referential communication.

Here is the paper’s abstract:

“Background: Close range calls are produced by many animals during intra-specific interactions, such as during home range defence, playing, begging for food, and directing others. In this study, we investigated the most common close range vocalisation of lar gibbons (Hylobates lar), the ‘hoo’ call. Gibbons and siamangs (family Hylobatidae) are known for their conspicuous and elaborate songs, while quieter, close range vocalisations have received almost no empirical attention, perhaps due to the difficult observation conditions in their natural forest habitats.

Results: We found that ‘hoo’ calls were emitted by both sexes in a variety of contexts, including feeding, separation from group members, encountering predators, interacting with neighbours, or as part of duet songs by the mated pair. Acoustic analyses revealed that ‘hoo’ calls varied in a number of spectral parameters as a function of the different contexts. Males’ and females’ ‘hoo’ calls showed similar variation in these context-specific parameter differences, although there were also consistent sex differences in frequency across contexts.

Conclusions: Our study provides evidence that lar gibbons are able to generate significant, context-dependent acoustic variation within their main social call, which potentially allows recipients to make inferences about the external events experienced by the caller. Communicating about different events by producing subtle acoustic variation within some call types appears to be a general feature of primate communication, which can increase the expressive power of vocal signals within the constraints of limited vocal tract flexibility that is typical for all non-human primates. In this sense, this study is of direct relevance for the on-going debate about the nature and origins of vocally-based referential communication and the evolution of human speech.”



Clarke, E., Reichard, U., & Zuberbühler, K. (2015). Context-specific close-range “hoo” calls in wild gibbons (Hylobates lar) BMC Evolutionary Biology, 15 (1) DOI: 10.1186/s12862-015-0332-2