Simon W. Townsend’s research while affiliated with University of Zurich and other places

Recent research has produced evidence for basic combinatorial abilities in the vocal systems of different animal species. Here, we investigate the structure of gesture sequences in Eastern chimpanzees ( Pan troglodytes schweinfurthii ) to detect whether gestural communication shows non-random combinations and how combinatorial rules influence predictability. Gesture, as compared to vocalization, offers greater flexibility in how signals are combined—for example overlapping in time — and as the parsing of signals into sequences is dependent on researcher decisions, we employ a multiverse approach, considering four different definitions of what constitutes a ‘sequence’ based on varying time thresholds. Our results indicate that sequences tend to be short (even with the most liberal time-window) and that transitions between some gesture types occur more frequently than expected by chance, with some transitions showing significant association across all time-windows. These transitions often involve repetition, suggesting persistence as a key aspect of chimpanzee gestural sequences. Information about previous gestures reduced uncertainty in predicting subsequent gestures. The order of gestures within sequences appears to be less critical than their cooccurrence, challenging assumptions based on the linear patterning derived from vocal communication. Our findings highlight the importance of methodological choices in sequence definition and suggest that chimpanzee gestural communication is characterised by a mix of predictability and flexibility, with implications for understanding the evolution of complex communication systems.

Over the last two decades, an emerging body of research has demonstrated that non-human animals exhibit the ability to combine context-specific calls into larger sequences. These structures have frequently been compared with language’s syntax, whereby linguistic units are combined to form larger structures, and leveraged to argue that syntax might not be unique to language. Currently, however, the overwhelming majority of examples of call combinations are limited to simple sequences comprising just two calls which differ dramatically from the open-ended hierarchical structuring of the syntax found in language. We revisit this issue by taking a whole-repertoire approach to investigate combinatoriality in common marmosets (Callithrix jacchus). We use Markov chain models to quantify the vocal sequences produced by marmosets providing evidence for structures beyond the bigram, including three-call and even combinations of up to eight or nine calls. Our analyses of these longer vocal sequences are suggestive of potential further internal organization, including some amount of recombination, nestedness and non-adjacent dependencies. We argue that data-driven, whole-repertoire analyses are fundamental to uncovering the combinatorial complexity of non-human animals and will further facilitate meaningful comparisons with language’s combinatoriality.

Pre-babbling infants can track nonadjacent dependencies (NADs) in the auditory domain. While this forms a crucial prerequisite for language acquisition, the neurodevelopmental origins of this ability remain unknown. We applied functional near-infrared spectroscopy in neonates and 6- to 7-month-old infants to investigate the neural substrate supporting NAD learning and detection using tone sequences in an artificial grammar learning paradigm. Detection of NADs was indicated by left prefrontal activation in neonates while by left supramarginal gyrus (SMG), superior temporal gyrus (STG), and inferior frontal gyrus activation in 6- to 7-month-olds. Functional connectivity analyses further indicated that the neonate activation pattern during the test phase benefited from a brain network consisting of prefrontal regions, left SMG and STG during the rest and learning phases. These findings suggest a left-hemispheric learning-related functional brain network may emerge at birth and serve as the foundation for the later engagement of these regions for NAD detection, thus, providing a neural basis for language acquisition.

Language is unbounded in its generativity, enabling the flexible combination of words into novel sentences. Critically, these constructions are intelligible to others due to our ability to derive a sentence’s compositional meaning from the semantic relationships among its components. Some animals also concatenate meaningful calls into compositional-like combinations to communicate more complex information. However, these combinations are structurally highly stereotyped, suggesting a bounded system of holistically perceived signals that impedes the processing of novel variants. Using long-term data and playback experiments on pied babblers, we demonstrate that, despite production stereotypy, they can nevertheless process structurally modified and novel combinations of their calls, demonstrating a capacity for deriving meaning compositionally. Furthermore, differential responses to artificial combinations by fledglings suggest that this compositional sensitivity is acquired ontogenetically. Our findings demonstrate animal combinatorial systems can be flexible at the perceptual level and that such perceptual flexibility may represent a precursor of language-like generativity.

Living organisms throughout the animal kingdom habitually communicate with multi-modal signals that use multiple sensory channels. Such composite signals vary in their communicative function, as well as the extent to which they are recombined freely. Humans typically display complex forms of multi-modal communication, yet the evolution of this capacity remains unknown. One of our two closest living relatives, chimpanzees, also produce multi-modal combinations and therefore may offer a valuable window into the evolutionary roots of human communication. However, a currently neglected step in describing multi-modal systems is to disentangle non-random combinations from those that occur simply by chance. Here we aimed to provide a systematic quantification of communicative behaviour in our closest living relatives, describing non-random combinations produced across auditory and visual modalities. Through recording the behaviour of wild chimpanzees from the Kibale forest, Uganda we generated the first repertoire of non-random combined vocal and visual components. Using collocation analysis, we identified more than 100 vocal-visual combinations which occurred more frequently than expected by chance. We also probed how multi-modal production varied in the population, finding no differences in the number of visual components produced with vocalisations as a function of age, sex or rank. As expected, chimpanzees produced more visual components alongside vocalizations during longer vocalization bouts, however, this was only the case for some vocalization types, not others. We demonstrate that chimpanzees produce a vast array of combined vocal and visual components, exhibiting a hitherto underappreciated level of multi-modal complexity.

The arbitrary relationship between signifier and signified is one of the features responsible for language’s extreme lability, adaptability, and expressiveness. Understanding this arbitrariness and its emergence is essential in any account of the evolution of language. To shed light on the phylogeny of the phenomenon, comparative data examining the relationship between signal form and function in the communication systems of non-humans is central. Here we report the results of a study on the production and usage the whistle-high hoot call combination (W + HH) from two distant populations of wild bonobos (Pan paniscus): Lui Kotale, DRC, and Kokolopori, DRC. We find that the context in which bonobos produce the W + HHs varies systematically between populations. Our results suggest that variation in W + HH production may represent an example of signal-adjustment optionality, a key component of arbitrariness.

It has recently become clear that some language-specific traits previously thought to be unique to humans (such as the capacity to combine sounds) are widespread in the animal kingdom. Despite the increase in studies documenting the presence of call combinations in non-human animals, factors promoting this vocal trait are unclear. One leading hypothesis proposes that communicative complexity co-evolved with social complexity owing to the need to transmit a diversity of information to a wider range of social partners. The Western Australian magpie (Gymnorhina tibicen dorsalis) provides a unique model to investigate this proposed link because it is a group-living, vocal learning species that is capable of multi-level combinatoriality (independently produced calls contain vocal segments and comprise combinations). Here, we compare variations in the production of call combinations across magpie groups ranging in size from 2 to 11 birds. We found that callers in larger groups give call combinations: (i) in greater diversity and (ii) more frequently than callers in smaller groups. Significantly, these observations support the hypothesis that combinatorial complexity may be related to social complexity in an open-ended vocal learner, providing an important step in understanding the role that sociality may have played in the development of vocal combinatorial complexity. This article is part of the theme issue ‘The power of sound: unravelling how acoustic communication shapes group dynamics’.

Pre-babbling infants can track nonadjacent dependencies (NADs) in the auditory domain. While this forms a crucial prerequisite for language acquisition, the neurodevelopmental origins of this ability remain unknown. We applied functional nearinfrared spectroscopy in neonates and 6-7-month-old infants to investigate the neural substrate supporting NAD learning using tone sequences in an artificial grammar learning paradigm. Detection of NADs was indicated by left prefrontal activation in neonates while by left supramarginal gyrus (SMG), superior temporal gyrus (STG), and inferior frontal gyrus activation in 6-7-month-olds. Functional connectivity analyses further indicated that the neonate activation pattern during the test phase benefited from a brain network consisting of prefrontal regions, left SMG and STG during the rest and learning phases. These findings suggest a left-hemispheric learning-related functional brain network may emerge at birth and be strengthened by complex auditory input across the first half year of life, providing a neural basis for language acquisition.

Human communication is strikingly multi-modal, relying on vocal utterances combined with visual gestures, facial expressions and more. Recent efforts to describe multi-modal signal production in our ape relatives have shed important light on the evolutionary trajectory of this core hallmark of human language. However, whilst promising, a systematic quantification of primate signal production which filters out random combinations produced across modalities is currently lacking. Here, through recording the communicative behaviour of wild chimpanzees from the Kibale forest, Uganda we address this issue and generate the first repertoire of non-random combined vocal and visual components. Using collocation analysis, we identify more than 100 vocal-visual combinations which occur more frequently than expected by chance. We also probe how multi-modal production varies in the population, finding no differences between individuals as a function of age, sex or rank. The number of visual components exhibited alongside vocalizations was, however, associated with vocalization type and duration. We demonstrate that chimpanzees produce a vast array of combined vocal and visual components, exhibiting a hitherto underappreciated level of combinatorial complexity. We conclude that a multi-modal approach is crucial to accurately representing the communicative abilities of non-human primates.