Chimpanzee play sequences are structured hierarchically as games

Abstract

Social play is ubiquitous in the development of many animal species and involves players adapting actions flexibly to their own previous actions and partner responses. Play differs from other behavioural contexts for which fine-scale analyses of action sequences are available, such as tool use and communication, in that its form is not defined by its function, making it potentially more unpredictable. In humans, play is often organised in games, where players know context-appropriate actions but string them together unpredictably. Here, we use the sequential nature of play elements to explore whether play elements in chimpanzees are structured hierarchically and follow predictable game-like patterns. Based on 5,711 play elements from 143 bouts, we extracted individual-level play sequences of 11 Western chimpanzees (Pan troglodytes verus) of different ages from the Bossou community. We detected transition probabilities between play elements that exceeded expected levels and show that play elements form hierarchically clustered and interchangeable groups, indicative of at least six games that can be identified from transition networks, some with different roles for different players. We also show that increased information about preceding play elements improved predictability of subsequent elements, further indicating that play elements are not strung together randomly but that flexible action rules underlie their usage. Thus, chimpanzee play is hierarchically structured in short games which limit acceptable play elements and allow players to predict and adapt to partners’ actions. This “grammar of action” approach to social interactions can be valuable in understanding cognitive and communicative abilities within and across species.

Nonadjacent Dependencies and Syntactic Structure of Chimpanzee Action During a Natural Tool-Use Task

“…It is possible that, within the domain of manual actions, tool use is unique behaviour of wild chimpanzees which leads to the production of nonadjacent sequential dependencies. In another study performed on the sequential play behaviours performed by chimpanzees at Bossou, the accuracy with which an action in a play sequence could be predicted was aided by considering the two most recent actions, but actions further back in the sequence offered no additional benefit (Mielke and Carvalho, 2022). This suggests that play behaviours do not produce long-range dependencies, such as those we have identified during tool use.…”

“…As previously mentioned, outside of tool use, the presence of hierarchical thinking in primates may also manifest within alternative behavioural contexts and social cognition, including in species other than chimpanzees (Bergman et al, 2003; Lameira et al, 2024; Mielke and Carvalho, 2022). In all of these instances, comparative research across domains of behaviour would be of interest, both within and between species.…”

“…those which span many nested levels of organization) to evolve when the benefits of high behavioural flexibility outweigh the costs of maintaining the underlying neural architecture for more sophisticated cognition. This has led to an explosion of research in recent decades, aiming to address how and why humans evolved the cognition to support the hierarchical organization across such an array of behavioural domains (Asano and Boeckx, 2015; Coopmans et al, 2021; Fitch and Hauser, 2004; Fitch and Martins, 2014; Friederici, 2020; Greenfield, 1991; Hauser et al, 2002; Kemmerer, 2022; Lerdahl, 2015; Mielke and Carvalho, 2022).…”

“…Despite the historic pervasiveness of chaining models for explaining the sequential structure of animal behaviours, mounting evidence suggests that a number of different animal species employ hierarchical sequencing to produce behaviours in the wild. For example, hierarchical organization is found the songs of whales (Miksis-Olds et al, 2008; Suzuki et al, 2006) and birds (Sainburg et al, 2019), as well as in the behaviours of several primate species, including grouped hierarchical perceptions of social relationships in baboons (Bergman et al, 2003); the nested temporal organization of vocal elements during orangutan long calls (Lameira et al, 2024); and the mental organization of social games in chimpanzees (Mielke and Carvalho, 2022). Nevertheless, hierarchical organization does not yet appear to be a universal aspect of animal behaviours.…”