Is language only used by humans? Most linguists would say “yes”, especially given the complexity of human language. Whether it can be dubbed “language” or not, research is uncovering that many animals have far more sophisticated communication protocols than thus far assumed. Scientists have determined that prairie dogs are able to construct information-rich “sentences” with subtle variations in their squeaks and chirps. Now, a newly published study1 shows that gibbons too have their own “language”, consisting of many calls, each with a specific meaning.
Study lead Esther Clarke had established in past studies that gibbons produce melodic “song” vocalizations,2 which are used to repel gibbons from other groups, attract mates, and show off pair bonds. However, her latest study found that there are various categories of other vocalizations, known as “hoos”. These hoos are very soft in volume, and can be very hard to pick up with recording devices or hear with human ears. The categories of hoos relate to foraging, predator sightings, interactions with neighboring gibbons, and more. When examining acoustic variations within the categories, they break down into even more complex sub-categories, indicating that the gibbons can be very specific in what they communicate.
Given the specific meanings associated with the different hoos, it could be said that they function almost like words. “Are they words as we know them? No. Does that mean that only human words are words? No, I don’t think so.”3 said Professor Michael Coen, regarding similar gibbon language research he is participating in.
Clarke and her colleagues were able to determine that the gibbons would use particular calls when sighting different types of predators. For example, all tigers and leopards were signaled with a call that seemed to pertain only to big cats, while eagles and hawks were signaled with a distinct call that pertained only to raptors. Slight variations could be detected between some of these predatory category calls as well, showing that the gibbons could sometimes communicate what particular species was spotted. This use of context-specific calls in identifying predators has been found in both prairie dogs and vervet monkeys, among other species.4,5 What’s also interesting is that the gibbons are able to keep their predator alert calls below the hearing ranges of the predators by using low sound frequencies. This allows the gibbons to sound the alarm without giving predators auditory clues about their location.
There is also evidence that general alarm calls of one animal species can be understood by others.6,7 Some primates have been found to respond when they hear the alarm calls of other primates. Birds and other mammals, too, have done the same.
Clarke is optimistic about what doors can be opened through her research of gibbons. “These animals are extraordinarily vocal creatures and 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.” She believes that her findings have strong implications for what has yet to be discovered in the domain of animal language. “It could be that, to give a hypothetical example, we think of a cat as having, say, five calls – well maybe it actually has 25 calls and we’ve been lumping lots of distinct calls together when they are actually graded.”
The new paper’s abstract:
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.
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.
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.
This article was written by Amanda Pachniewska, founder & editor of AnimalCognition.org
1 – Esther Clarke, Ulrich H Reichard, Klaus Zuberbühler
Context-specific close-range “hoo” calls in wild gibbons (Hylobates lar)
BMC Evolutionary Biology
2 – Esther Clarke, Ulrich H Reichard, Klaus Zuberbühler
The syntax and meaning of wild gibbon songs
3 – Alison Bauter
Listening to the zoo’s gibbons: They’re part of study that suggests language not just for humans
The Journal Times
4 – Conn Slobodchikoff, William Briggs, Patricia Dennis
Decoding the information contained in the alarm calls of Gunnison prairie dogs.
The Journal of the Acoustical Society of America
5 – Struhsaker TT
Auditory communication among vervet monkeys (cercopithecus aethiops)
Social communication among primates
6 – Klaus Zuberbühler
Interspecies semantic communication in two forest primates
The Royal Society
7 – Hugo J. Rainey, Klaus ZuberbühlerPeter J. B. Slater
Hornbills can distinguish between primate alarm calls
The Royal Society