Identifying new-found Complexities in Chimpanzee Communication

Chimpanzees produce diverse vocal sequences with ordered and recombinatorial properties

Cédric Girard-Buttoz, Emiliano Zaccarella, Tatiana Bortolato, Angela D. Friederici, Roman M. Wittig, and Catherine Crockford

Summarized by Michael Hallinan

What data were used? This study uses 4826 recordings of 46 wild adult chimpanzees (Pan troglodytes) from Tai National Park located in Ivory Coast, West Africa. The chimpanzees recorded were fully accustomed to human observers, and comprised three different chimpanzee communities. These observations were performed by focusing on each of the 46 chimpanzees individually and continuously recording throughout January and February of 2019 as well as December 2019 to March 2020.

Methods: These recordings were classified by trained listeners into Grunts, Hoos, Barks, and Screams with variation on the sounds being panted or unpanted, meaning that they were either emitted singly (unpanted) or released as a string of sounds with audible breaths in between (panted). In addition to type classification, other information such as frequency level, noisiness, and occurrence were also collected. Each call was examined to identify which sounds occurred in association with other sounds or by itself. The data was analyzed to answer the following research questions: First, is there flexibility, can most sounds or calls used be combined with most others? (Can A, B, C, be AB, CB, BA, etc.?) Second, is there a specific ordering? (Does AB mean the same as BA?) Lastly, can short sequences be combined  into longer sequences? (Can AB occur as ABC, or can AB and CD occur as ABCD?)

Results: Initially, there were slightly over 400 sound units identified suggesting nearly 400 “words” within their vocabulary. It was found that chimpanzees could flexibly combine as well as recombine single units across those identified, with 11 out of the 12 single unit sounds also appearing in sequences together with 4-9 other sounds. Many of the single-unit calls could be emitted within two-unit calls and two-unit calls could be added to another unit to produce three-unit calls. Additionally, 52.6% of all two unit calls were produced reversed and forwards at least once (BA and AB) with 57% of the tested two unit calls showing bias for appearing in a certain order. Although some of the single units could appear at the beginning or end of calls, there generally was bias suggesting a more complex grammatical structure and ordering pattern. That being said though, to identify a more complex grammar structure and how that could be used to understand meaning, more research needs to be done. 

A bar chart with length of speech units displayed across the x-axis and number of recordings the units appeared in on the y-axis in a log scale. The frequency of sounds recorded decreases as the unit length of the sounds increases, with the exception of single-unit panted sounds, which appears slightly less frequent then two-unit sounds. Once the units reach a length of 7 units, only females are recorded making sounds of these lengths up till 10 units. Generally, 1 through 4 unit length sounds appear very frequently with over 40 appearing, until it drops off to 32 at 5 units, 20 at 6 units, 10 at 7 units, and finally 4 at 8 units, and 2 for 9 and 10 units.
This bar graph shows the number of different utterances, with the number of units in each utterance displayed across the x axis, while the y axis shows the number of recordings on a log scale. The number of individuals using the unit at each length appears on top of each individual bar. The dark blue bars represent male and female generated sounds and the light blue bars represent only female-generated sounds. Last, the red box indicates the data used for two-unit analysis and the yellow box for three-unit analysis.

Why is this study important? Language is one of the biggest challenges in evolutionary science. Although different species such as certain types of birds, non-human primates and bats have been identified to have specific sound sets used to communicate, there is a huge limitation to our understanding of its development since what we know as language can not be fossilized. Comparative studies like this are therefore our main resource for understanding language and how it has evolved throughout time. This study reveals that primate language is more complex than we previously thought it was, showing more complex structures, and therefore has the potential to convey more meaning and more intricate speech. 

The Big Picture: Chimpanzees have been identified to produce and order sounds using a more complex grammatical structure than previously thought. This was observed through multiple sound units being combined together in patterns beyond random chance, in multiple fashions with bias towards certain structures. Although we don’t understand the content of their sounds, this reveals a more complex nature to their communication and with more research we might be able to potentially decipher their meaning. 

Citation: Girard-Buttoz, C., Zaccarella, E., Bortolato, T. et al. Chimpanzees produce diverse vocal sequences with ordered and recombinatorial properties. Commun Biol 5, 410 (2022).

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