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.

Although the key features of human face-to-face communication have been studied for decades and the range of empirical approaches is becoming ever wider satisfactory answers concerning the evolutionary building blocks of language are yet to be found. As our closest living relatives, great apes are a model of choice to address these questions with comparative research. A critical feature of human communication is its extraordinary plasticity, reflected in the flexible production and interpretation of signals based on the common ground shared by interaction partners. This is partly manifested in linguistic politeness and other forms of audience design, through which interactants adjust their signalling to one another in order to reduce uncertainty, build and maintain relationships, and adhere to social norms. This flexibility might seem redundant and cognitively demanding, but it actually helps reinforce mutual understanding. Great apes' communication systems are among the most flexible in the animal kingdom: similarly to humans, they are able to combine multiple signals at the same time and adjust their signal use to the social context, the attention state of the interlocutor and to past social interactions (gorillas (Gorilla gorilla); orang-utans (Pongo spp.); chimpanzees (Pan troglodytes); bonobos (Pan paniscus). This is also reflected in their capacity to persist and elaborate their signalling when their social goal is not achieved and to flexibly dissociate signals and outcomes: a single (e.g. gestural) signal can be used to achieve several purposes, and multiple signals can lead to the same outcome.

Language is intrinsically multimodal, that is, communicative signals are simultaneously produced across different articulators (multicomponent) and understood across different sensory modalities (multisensory). Although multimodality serves as a key source of language flexibility, one major shortcoming of the comparative study of primate communication is that, for decades, it has been approached in a unimodal way. For a long time, researchers have been divided into two major camps: proponents of either the gesture-first or the vocal-first theory of language origin both argue that language evolved from one or the other of these two modalities of communication. However, multimodality is essential in language expression and integration, as is evidenced by the McGurk effect. That is, our understanding of speech comes from the simultaneous integration of vocal (speech itself) and visual (in this case, facial) components, and different combinations (e.g. unmatched vocal and facial components) result in different speech understandings. Considering the pervasiveness of multimodality in interactive language use (where speech is usually accompanied by facial expressions and gestures, the multimodal production of signals by many non-human species and their importance in social negotiation a unimodal approach most likely results in the loss of important information concerning language origins. While many human and non-human multimodal signals are fixed (vocalizations, including most speech sounds, cannot be expressed without their complementary facial signal), multimodality in signal combination (e.g. combinatory use of vocal and gestural signals) can be adapted to specific environments and enables highly flexible communication. In addition, the unimodal approach of communication hinders reliable comparisons between the different modalities. Thus, to gain a more holistic understanding of communicative interactions and enable better comparisons with human communication, a growing number of studies have focused on the combination of signal modalities. However, although multimodal approaches are spreading in studies on animal primate communication, they do not investigate multimodal communicative behaviour in all its diversity: studies often focus either on the co-occurrence of signal types (e.g) or of sensory modalities (e.g.. Within this framework, one key goal of this study is to investigate and compare the flexibility of specific multimodal communicative strategies in chimpanzees through a holistic approach.

Although multimodal approaches have become more common, another critical problem with current comparative research is that variation in communicative behaviour is mostly studied on the population or species level. These approaches are of great importance to investigate communication systems, but have for consequence the treatment of individual variation as 'noise', which is assumed to disappear once sufficient data have been collected. However, between-individual variation in behavioural type, or individual variation in the average expression of behaviours (i.e. personality: consistent individual differences in social behaviour, Fig. 1a), has been shown to have biological and ecological importance, whether for population dynamics and survival or for social evolution including in great ape species. Thus, variation in behavioural and communicative patterns found between populations may often be due to extreme behavioural expression in only a few individuals. Moreover, natural selection acts directly on individual differences, potentially leading to adaptive evolution. Variation in genetics and epigenetics between individuals might not only impact the expression of innate behavioural traits, but also the way these traits are inherited through time in a given population. Additionally, some behavioural traits might be propagated through social learning, with individuals adopting strategies to learn adaptive behaviours from others providing a second, social level for selection to act. As a consequence, individual variation is highly biologically relevant and must be taken into account in studies of human behavioural evolution.

Importantly, individuals may not only differ consistently in their average expression of behaviour, but they may also modify their behaviour when the social or environmental conditions change. This flexibility in behavioural expression (often called behavioural plasticity in behavioural ecology, Fig. 1b) is particularly characteristic of the primate order. For instance, maternal style was found to be altered when new individuals were added to the social group of vervet monkeys (Cercopithecus aethiops). In addition to this component of within-individual variation, individuals may differ in the strength of such responses to environmental changes, that is, in the expression of behavioural plasticity (i.e. individual plasticity, or between-individual variation in the adjustment to environmental conditions, Fig. 1c). For instance, some individuals may adjust their behaviour to each environment they encounter, while others may not. Although they have largely been studied separately, behavioural type (the average behavioural expression of an individual) and individual plasticity tend to be linked, and flexibility can only be fully understood when the individual level of variation is explicitly addressed. Dingemanse et al. (2010) argue that studying these two phenomena together is essential to accurately interpret individual variation and to gain the most comprehensive insight into variability. Such an approach, which is usually specific to behavioural ecology, has already been applied to social and spatial behaviour in several species. For example Hertel et al. (2020) used Generalized Linear Mixed Models to assess reversible and intrinsic individual variation in the spatial movements of African elephants, and Revathe et al. (2025) used a similar method to investigate individual differences and plasticity in the maternal behaviour of orangutans. Very little is known about between-individual variation in behavioural type and individual plasticity in primate communicative behaviour but a recent multi-level study (including individuals of two different species in two different research settings) using this method on the infant-directed communication of orangutan mothers provided the first evidence for individual plasticity in primate communication. This proof-of-concept study showed significant differences in the modification of mothers' behaviour across contexts, with some showing greater responsiveness to their infants than others, suggesting that the degree of behavioural plasticity varies among individuals and paving the way for more investigation of this type.

Here, we aim to expand upon this preliminary work by examining individual variation and plasticity in the communicative behaviour of chimpanzees in two different research settings. Chimpanzees, our closest living relatives along with bonobos, have been extensively studied for their communicative repertoire across modalities (e.g., providing a solid basis for more in-depth research on the communicative strategies of great apes. Vocal, gestural and facial signals all play major roles in chimpanzee communication, but they are often not used for the same social goals, and particular signals can be more or less tightly linked to specific outcomes. This has been identified in gestural expression: the gesture "leaf-clip" is used only to acquire sexual attention, while "grab" and "pull" are used toward two or more outcomes. The multimodal use of facial signals and vocalizations can also be expressed flexibly by chimpanzees, with facial signals not necessarily being paired with their typically associated vocalisations, which can also lead to altered meaning and ten different context-specific vocalizations being expressed in the wild. Furthermore, chimpanzees adapt their communicative strategies to the attentional state of the recipient: silent gestures are preferably chosen when the signaller is in the recipient's visual field, whereas physical contact is preferred when the signaller is out of sight. This directed, purposeful use of gestures reflects their intentional natureas well as their behavioural plasticity.

The principal goal of this study was to examine the sources of variation in the communicative behaviour of adult zoo- and sanctuary-housed chimpanzees living in multiple social groups through a multimodal approach. To evaluate both the production and perception of multimodal communication, this study distinguished two different facets of it, building on Fröhlich et al. . Multicomponent (MC) communicative acts involve multiple signals (of the same or different signal types, i.e. vocal, bodily, manual, facial components) produced simultaneously by the sender. Multisensory (MS) communicative acts are those that are received and processed through several sensory channels (e.g. auditory, visual, tactile). Both communicative strategies might be expressed in the same communicative act (e.g. the combination "touch other + bared-teeth display" is multicomponent with the combination of a limb gesture and a facial signal and multisensory with the combination of the tactile and visual sensory channels) or not (e.g. a "big loud scratch" only includes one gestural component but involves both the visual and auditory sensory channels, while "bipedal stance + reach" involves a body and a limb signal but only the visual sensory channel) and are categorised independently for each communicative act.

This study had four core objectives. The first aim was to explore the specific types of MC and MS acts used by the study subjects in order to better visualise which specific combinations of signals and sensory channels are expressed and at which rates, and to add precision to our current knowledge of the role of multimodality in the communicative system of chimpanzees (but see).

Second, we examined the extent to which individual (e.g. sex, age) and environmental (e.g. research setting, social group) factors influence both MC and MS signal use. We hypothesized that socio-demographic parameters, especially sex and research setting, determine whether these two communicative strategies are expressed. We predicted that sanctuary-housed and female individuals likely combined signals and sensory channels more often than their zoo-housed and male counterparts. Indeed, the more restricted space available in the zoos compared to sanctuaries induces higher levels of proximity. As a consequence, zoo-housed chimpanzees experience reduced social uncertainty and have less need to resort to complex communicative strategies. Similarly, females were expected to use more MC and MS acts than males, reflecting the differential sociality level and social challenges between the sexes. Chimpanzee females are typically lower ranking than males, and might, as a consequence, more often face unpredictable interactions in which they would benefit from communicating unambiguously and conspicuously. Individuals from different social groups might also express variation in their communicative signal use, as differences in size and composition of groups (e.g. sex ratio, presence and availability of juveniles) might impose different social constraints on their members (e.g. affiliation rates or feeding tolerance.

Third, we investigated whether individual chimpanzees differed in the expression of these strategies on average, regardless of possible socio-demographic differences and other confounding factors (e.g. sex, social context), given that consistent and repeatable differences in social behaviour over time have already been identified in both captive and wild chimpanzees. These individual differences in sociality are likely reflected in the chimpanzees' communicative behaviour as well, we thus expected individuals to consistently express varying levels of MC and MS acts, irrespective of individual (sex, age) and environmental (social context, research setting) parameters.

Finally, we explored the possibility that chimpanzees might shift their communicative strategies between behavioural contexts, and also differ individually in how they do so, using the behavioural reaction norm framework introduced above. In chimpanzees, a single signal can be used to meet different purposes and different signals can lead to the same outcome at the population levelit can be assumed that individuals flexibly adjust their communicative strategy to the context in which the signaller was involved prior to the initiation of the interaction (i.e. social vs. non-social contexts). Specifically, some individuals may combine signals more often if they are not already engaged in a social activity prior to signalling, e.g. due to different prior interactional experiences. Alternatively, it is also possible that we do not find any evidence for individual plasticity in the studied contexts, as shifts of communicative behaviour in the same direction may be highly adaptive across all individuals in a certain group or setting. In this case, we would expect socio-environmental (e.g. setting) or demographic (e.g. sex) effects to override individual-level effects.

Our results showed that the signaller's research setting and sex, but not social group, affected the multimodal use of the communicative repertoire, but also that chimpanzees expressed consistent differences in behavioural type for both MC and MS communicative strategies. Moreover, plasticity in relation to the behavioural context was identified for both communicative strategies, but individuals differed in their expressed plasticity only in their use of MS acts. These findings demonstrate the importance of considering individual-level variation in the investigation of great ape communicative behaviour.