Marlen Fröhlich

Human communication is remarkable for its flexibility, a trait largely reflected in its multimodal nature and shared to some extent with nonhuman primates. Although individual differences in social behaviour are known to have evolutionary implications, their role in shaping primate communication remains largely unexplored. This study adopts a multimodal framework to partition variation in chimpanzees’ use of multicomponent and multisensory communicative strategies into socio-environmental, between-individual, and within-individual sources. Results showed that research setting and signaller’s sex affected communicative expression. Importantly, we also detected consistent between-individual differences in both strategies, independent of age, sex, or setting. While only multicomponent signal use was predicted by behavioural context at the population level, individuals varied in how they adjusted to context only in their use of multisensory acts. These findings reveal substantial flexibility in chimpanzee communication, highlighting individual-specific patterns and supporting a gradual evolutionary pathway toward the complexity of human multimodal communication.

The comparative study of communicative behaviour in non‐human animals, especially primates, has yielded crucial insights into the evolution of human language. This research, mostly focused on the species and population level, has improved our understanding of the various socio‐ecological factors that shape communication systems. However, despite the inherent flexibility of human communication, the impact of individual variation on non‐human communication systems has often been overlooked, along with its potential to shed light on the roots of human language. While the eco‐evolutionary relevance of genetic and phenotypic differences between individuals is well established, animal communication studies have traditionally focused on group averages and treated outliers as noise. In this review, we address this gap by providing a comprehensive overview of the sources of individual variation in animal communicative behaviour (e.g. physiological, sociodemographic, or personality traits), across parameters such as signal forms, repertoires, and usage strategies. In particular, recent evidence from comparative work underscores the potential evolutionary significance of individual plasticity in communicative behaviour. We argue for an explicit focus on within‐individual variation and propose advancing the study of animal communication through multi‐level approaches that integrate intrinsic and environmental factors, as well as between‐ and within‐individual variation. Such approaches not only refine our view of complexity in animal communication systems and their implications for social evolution, but also help trace the evolutionary trajectory of human language through comparative studies.

The concept of flexibility in communication is central to reconstructing the evolutionary history of language, and grappling with “contextual flexibility” in particular is pivotal to address implications for pragmatics‐first accounts of language evolution. Despite significant advances in the field, research is hindered by definitional inconsistencies and methodological gaps across modalities. We build on recent frameworks to propose further, detailed methods for studying contextual and functional variability, incorporating modality‐agnostic and standardised terminology to facilitate cross‐species comparisons. Our approach includes a detailed classification of communicative contexts and outcomes, offering practical methods to disentangle context from function and meaning. By integrating insights across behavioural ecology and comparative psychology, we aim to enhance the comparability of findings and provide a robust foundation for exploring the evolutionary trajectory of communicative flexibility and pragmatics.

The concept of flexibility in communication is central to reconstructing the evolutionary history of language, and grappling with ‘contextual flexibility’ in particular is pivotal to address implications for pragmatics-first accounts of language evolution. Despite significant advances in the field, research is hindered by definitional inconsistencies and methodological gaps across modalities. We build on recent frameworks to propose further, detailed methods for studying contextual and functional variability, incorporating modality-agnostic and standardised terminology to facilitate cross-species comparisons. Our approach includes a detailed classification of communicative contexts and outcomes, offering practical methods to disentangle context from function and meaning. By integrating insights across behavioural ecology and comparative psychology, we aim to enhance the comparability of findings and provide a robust foundation for exploring the evolutionary trajectory of communicative flexibility and pragmatics.

Mothers play a crucial role in the early development and survival of mammalian offspring, and differences in maternal care may affect offspring’s development. Whereas previous research has primarily focused on biological and socioecological factors to understand population-level variation in maternal behaviour, the individual as a source of variation remains understudied. We investigated between-individual variation in the average expression of, and plasticity in, six maternal behaviours in Sumatran orangutans ( Pongo abelii ), using 15 years of behavioural data. We found that mothers differed substantially in the average expression of four maternal behaviours, even after controlling for socioecological conditions, biological state characteristics and the offspring’s influence on these behaviours. Furthermore, not controlling for these confounding effects exaggerated or masked between-individual variation. Mothers also substantially differed in how they adjusted three of the maternal behaviours during offspring development, meaning that mothers differed in behavioural plasticity. Our results suggest that Sumatran orangutan mothers are constrained in the average expression of maternal behaviours and their plastic responses, potentially resulting in consistent differences among mothers, otherwise called maternal personality. Our findings highlight that there is biologically meaningful variation around the population mean in maternal behaviour and present novel opportunities to study evolutionary processes that shape maternal behaviour.

The comparative study of the communicative behaviour of non-human animals, especially primates, has yielded crucial insights into the evolution of human language. This research, mostly focused on the species and population level, has helped to understand the various socio-ecological factors that shape communication systems. However, despite the inherent flexibility of human communication, the impact of individual variation on non-human communication systems has often been overlooked, as have its potential insights into the roots of human language. While the eco-evolutionary relevance of genetic and phenotypic differences between individuals is well established, animal communication studies traditionally focus on group means and treat outliers as noise. In this review, we address this gap by providing a comprehensive overview of the sources of individual variation in animal communicative behaviour (e.g. physiological, sociodemographic or personality traits) in numerous parameters such as signal forms, repertoires, and strategies of use. In particular, recent evidence from comparative work underscores the potential evolutionary implications of individual plasticity in communicative behaviour. Thus, we argue for an explicit focus on within-individual variation and propose a way to advance the study of animal communication through multi-level approaches that consider intrinsic, environmental as well as between- and within-individual variation together. Such approaches not only refine our perception of complexity in animal communication systems and implications for social evolution, but also help to trace the evolutionary trajectory of human language through comparative studies.

The concepts of social learning and exploration have been central to debates in comparative cognition research. While their roles in the origins of human cumulative culture on the one hand and creativity on the other have been highlighted, the two concepts have mostly been studied separately. In this article, we examine the relationship between adopting similar or different behaviours within a group, focusing on how exploration and exploitation shape primate communication systems. Using a comparative approach, we discuss how similarity and differentiation of communicative behaviour can be viewed as two endpoints on a continuum, impacting both individual- and group-level behavioural variation. While group-level variation is evident in some ape behaviours (e.g. foraging traditions), individual variation in communicative behaviour appears to outweigh group-level differences, making a widespread communicative culture in apes unlikely. Drawing parallels to language acquisition in human infants, we propose that ape communication follows an exploration–exploitation trajectory, with initial exploration gradually giving way to focused exploitation of genetically predisposed and/or individually developed communicative repertoires. By integrating the individual and social learning processes underlying communicative behaviour, we can gain a deeper understanding of how exploration–exploitation tensions shape communication systems across species.

Studies of meaning in human and primate communication face, in principle, similar methodological problems. In both cases, meaning is not observable directly, but must be inferred from more indirect sources, such as directly observable behavior. Recent work in probabilistic cognitive modeling of language use has therefore developed methods of inferring latent se- mantic meaning through the lens of a probabilistic model of language use. In this paper, we explore how to adapt such an approach for insightful investigations of primate communication. Towards this end, we develop a suitable probabilistic model of processes that generate communicative behavior by making use of functionally specified latent meaning representations. As a proof of concept, we apply this model to a rich, annotated data set of orangutan communicative dyadic interaction and conclude that explicit probabilistic modeling can provide additional insights for the study of animal communication pertaining to the context-dependent nature of signals and the gradual evolution of human communication systems.

In many group-living species, individuals are required to flexibly modify their communicative behaviour in response to current social challenges. To unravel whether sociality and communication systems co-evolve, research efforts have often targeted the links between social organisation and communicative repertoires. However, it is still unclear which social or interactional factors directly predict communicative complexity. To address this issue, we studied wild and zoo-housed immature orangutans of two species to assess the impact of the socio-ecological setting on the production of non-vocal signal repertoires. Specifically, we compared repertoire size, dyadic repertoire similarity, and number of social goals (i.e. observer’s estimate of the signaller’s intended interaction outcome) for communicative interactions with mothers versus other conspecifics, controlling for critical individual and environmental factors. In this small sample of immature orangutans, wild-captive contrasts were statistically significant only for other-directed repertoires, but not for mother-directed repertoires, and not for the number of social goals that immatures communicated towards. While the repertoires of individuals living in the same research setting were more similar than those living in contrasting settings, this difference was most pronounced for other-directed repertoires of the less socially tolerant orangutan species. These results suggest that the boosted interactional opportunities in captivity rather than mere differences in environmental affordances or communicative needs drive the wild-captive contrast in orangutan communicative repertoires. Overall, this fine-grained analysis of repertoires further underscores that not only a species’ social organisation but also the targeted audience may have a profound impact on communicative behaviour. Significance statement: Navigating a dynamic social environment often requires flexible signal use. While it has repeatedly been shown that the social organisation and structure of species predict the complexity of their communication systems, the mechanisms underlying these relationships are largely unknown. Because targeted studies to assess this issue in great apes are difficult, we take an alternative approach here: we compare the same species living in the wild and in artificial habitats in captivity. This contrast allows a direct test of how repertoires respond to the relevant difference in socio-ecological conditions. Our results show that the diversity of interaction partners (i.e. social opportunities), but not the diversity of social goals (i.e. possible interaction outcomes) or the broader physical opportunities (i.e. safe ground use), predict the size and consistency of wild and captive signalling repertoires.