Abstract All primate species produce vocalizations for complex communication, conveying a wide range of information, regulating social interactions, coordinating activities, and serving as antipredator strategies. Together, these vocalizations form a species' vocal repertoire. However, despite their widespread presence and ecological significance, the vocal repertoires of less than 15% of primate taxa have been documented to date. The purpose of this study was to quantitatively characterize the vocal repertoire and behavior of a wild population of Ucayali bald or red uakaris ( Cacajao ucayalii ), a rare Amazonian platyrrhine primate. We analyzed and categorized 1,155 red uakari vocalizations using a combination of machine-learning-based random forest analyses, bioacoustic analysis, and field observations. We identified and described 12 acoustically distinct call types, two of which were not previously reported ( High-chick and Shriek ). The Hic was the most common vocalization, both in context and frequency of use. Some calls were specific to age, sex, or context, while others occurred across a range of situations. The vocal repertoire exhibited properties of both graded and discrete calls, depending on the call type and its apparent function. The uakaris’ tail plays a key role in the species’ acoustic-visual multimodal communication, as evidenced by the frequent combination of different call types and tail wagging, probably to draw attention from other group members and emphasize the transmission of information. The bioacoustic characterization of the Ucayali bald uakari's vocal repertoire provides a groundwork for potential acoustic monitoring of this species and the potential use of uakari monkeys as models for studying multimodal communication in primates.

Humans and other primates communicate in multiple sensory modalities, and language itself is usually a multimodal form of communication. Discussion of the multimodal nature of primate communication goes back at least as far as the publications of Charles Darwin and has recently seen renewed interest. Here, I review key topics in the study of multimodal communication in nonhuman primates and humans, including issues of definitional complexity as well as classification systems and empirical approaches. I argue that multimodal communication is ubiquitous, ingrained, and advantageous. I discuss sensory aging and how multimodal communication can offer alternative routes to comprehension when sensory systems become impaired. To conclude, I consider future avenues of research that seem likely to prove productive.

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.Supplementary InformationThe online version contains supplementary material available at 10.1038/s41598-025-14835-x.

Human cognition is often claimed to be unique within the animal kingdom. Comparing infant development across primate species to examine maternal interactions is a promising approach to understanding potential cognitive differences between humans and other primates. In this review, we focus on the communicative interactions between mother-infant dyads in nonhuman primate species and humans from different cultural backgrounds, as these interactions are crucial for infant survival, the formation of emotional bonds, and cognitive development. We explore different aspects of mother-infant communication, including the dynamics of these interactions, intra-species variation in mother-infant communication across groups and individuals, and the potential role of other caregivers. Additionally, we address current methodological challenges, such as the underrepresentation of multimodal approaches in studying mother-infant communication and the need for standardized methods to facilitate systematic comparisons across species and study populations. We conclude that it is currently premature to answer the question if human mother-infant communicative interactions are fundamentally different as compared to those of other primates, as critical knowledge gaps must be filled first. We address some of these gaps and propose key aspects that need to be examined to develop a more comprehensive understanding of mother-infant communication across primates and its potential influence on infants' developmental trajectories.

Understanding the neural basis of social communication and vocal perception in primates is a key challenge in systems neuroscience. Vocalizations are fundamental for communication, and several cortical areas, known as "voice patches," have been identified as being sensitive to conspecific vocalizations in primates, vital for distinguishing species-specific calls. While the dorsal stream's role in complex auditory-motor functions and human speech processing is established, its specific contribution to processing species-specific vocalizations in non-human primates remains unclear. Using high-resolution fMRI (0.6 mm isotropic voxels), we investigated brain regions involved in processing vocalizations in awake rhesus monkeys exposed to coos, screams, and aggressive calls, among control sounds. Our analyses revealed a widespread network involved in vocalization processing, encompassing auditory-associated areas including the core and belt auditory cortices, as well as premotor, and somatosensory-related areas. Moreover, we found selective activation in the caudal part of the lateral sulcus (area Tpt) and the dorsal premotor cortex (area 6DR/F2) in response to vocalizations. Also, the population responses in these areas could discriminate between different vocalizations. Our results enhance our understanding of the neural basis of vocal communication in primates. Specifically, they highlight the involvement of a voice-related dorsal network in macaques, including Tpt and part of 6DR/F2, in processing the acoustic features of salient vocal stimuli, potentially linking them to motor representations. These findings provide insights into potential evolutionary precursors of auditory-motor pathways that support complex auditory communication systems in primates, including human speech.

Studies of communication in primates, birds and fish show that signalling behaviour is often flexibly adjusted depending on the presence, characteristics and behavioural states of others. Such flexibility is likely important in competitive social contexts such as sexual signalling, where attractiveness is directly affected by rival behaviour. Although sexual displays are often sensitive to the presence and proximity of rivals, less is known about the effect of changes in rival signalling. In this study, we used a biomimetic robot to manipulate rival morphology and waving behaviour in a wild population of fiddler crabs (Afruca tangeri) and investigated whether males responded flexibly by adjusting their own activity and waving. Males were less likely to enter their burrow when the robotic rival was waving, particularly if that rival had a small claw, and spent less time in their burrow if they did enter. While outside, males produced more waves when the robotic rival was waving fast, not by changing their own rate but by prolonging their bouts. These results reveal the subtle ways in which an invertebrate flexibly adjusts behaviour to remain competitive in a dynamic environment, investing more in signalling when it is likely most profitable.

Abstract 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 have known 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, only for the use of multisensory acts did individuals vary in how they adjusted to context. These findings reveal profound flexibility in chimpanzee communication, highlighting individual-specific patterns and supporting a gradual evolutionary pathway toward the complexity of human multimodal communication.

Facial expression is a key component of primate communication, and primates (including humans) have a complex system of facial musculature underpinning this behavior. Human facial musculature is highly variable across individuals, but to date, whether other primate species exhibit a similar level of inter‐individual variation is unknown. Whether individual‐level variation in facial musculature covaries with significant differences in facial movement within the same individual is also unknown. Here, we use facial dissection data from 31 adult rhesus macaques, the largest sample to date, to quantify inter‐individual variation in facial muscle presence. We used a subsample of eight individuals to measure covariation between facial muscle presence and the presence of external facial movements (action units in the Facial Action Coding System, or FACS). We found, in contrast to humans, limited inter‐individual variation in muscle presence, but the zygomatic region exhibited more gross anatomical variation in muscle presence and morphology than any other region of the macaque face. We also found a good correspondence between facial muscle presence and the presence of the associated action units. Our results indicate that the observed variation in rhesus macaque facial expressivity is not likely driven primarily by variation in facial muscle presence but may instead be due to other factors such as learned behavior and/or physiological differences. These findings provide insight into the anatomical basis of inter‐individual variation in facial behavior in primates and suggest potential differences in variation between humans and other primate species.

Discrimination between the facial gestures of vocalising and non-vocalising lemurs and small apes using deep learning

Speculations on the evolution of language have invoked comparisons across human and non-human primate communication. While there is widespread support for the claim that gesture plays a central, perhaps a predominant role in early language development and that gesture played the foundational role in language evolution, much empirical information does not accord with the gestural claims. The present study follows up on our prior work that challenged the gestural theory of language development with longitudinal data showing early speech-like vocalizations occurred more than 5 times as often as gestures in the first year of life. Now we bring longitudinal data on the second year (13, 16 and 20 mo), showing again that vocalizations predominated, and especially in conventional (learned) communication; > 9 times more spoken words were observed than gestures that could be viewed as functionally equivalent to words (i.e., signs). Our observations also showed that about ¾ of gestures across these second-year data were deictics (primarily pointing and reaching), acts that while significant in supporting the establishment of referential vocabulary in both spoken and signed languages, are not signs, but have single universal deictic functions in the here and now. In contrast, words and signs, the primary semantic components of spoken and signed languages, are functionally flexible, making possible reference to abstractions that are not bound to any particular illocutionary force nor to the here and now.

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.

Human languages use complex, structured signals whose meanings are compositional. Recent empirical research has claimed to demonstrate compositionality in bird and primate communication (Berthet et al. 2025; Engesser et al. 2016; Girard-Buttoz et al. 2025; Leroux et al. 2023; Suzuki et al. 2017). While the compositionality of human languages seems beyond doubt, it can be demonstrated powerfully and immediately because of several other important features of language. Clarifying the arguments for compositionality in human languages reveals open questions and alternative hypotheses about the evidence from other species, and directions for further research and possible limitations.

While spoken language is unique to humans, many features of human communication are shared with great apes, including the use of signals in multiple modalities such as vocalizations, gestures, and facial expressions. Communication signals can be unimodal (involving a single modality) or multimodal (combining multiple modalities simultaneously). Here, we examined age-related differences in bonobo (Pan paniscus) unimodal and multimodal communication signals. We assessed all vocalizations, gestures, facial expressions, and multimodal combinations produced by captive bonobos across a variety of behavioral contexts. All occurrences of communication signals were collected via focal observations from 12 individuals ranging from 6 months to 44 years of age. All individuals produced multimodal communication signals but all bonobos, regardless of age, produced multimodal signals at lower frequencies than unimodal signals. Age had a significant effect with younger bonobos producing more multimodal signals than older individuals (p < 0.001). The infant and juveniles produced the most multimodal signals and there was an approximately 6% increase in unimodal signals per age year increase. These findings indicate a developmental shift toward unimodal signals as bonobos age. Behavioral context was predictive of signal type usage with an increase of multimodal signals in agonistic (p < 0.001), play (p < 0.001), and sexual contexts (p = 0.001). This indicates that context is important for bonobo modality with multimodal signaling occurring more in “high-risk/high-reward” contexts where proper signal comprehension is vital. This study represents an overview of multimodal communication across bonobo life stages, offering further insights into primate communication patterns and developmental trajectories.

This literature review explores recent research on primate communication, social learning, and the presence of social norms in non-human animals. It highlights significant findings, including group-specific communication patterns, the adaptive benefits of social learning through imitation and rapid behavioural mimicry, and the existence of socially maintained behaviours akin to human norms. These studies collectively illustrate the complex social lives of primates and offer valuable insights into the evolutionary origins of human culture and social norms. The review also delves into the philosophical implications of these findings, challenging traditional views of human uniqueness and expanding our understanding of animal behaviour and cognition.

Female crested gibbons (genus Nomascus)perform conspicuous sequences of twitching movements involving the rump and extremities. However, these dances have attracted little scientific attention and their structure and meaning remain largely obscure. Here we analyse close-range video recordings of captive crested gibbons, extracting descriptions of dance in four species (N. annamensis, N. gabriellae, N. leucogenys and N. siki). In addition, we report results from a survey amongst relevant professionals clarifying behavioural contexts of dance in captive and wild crested gibbons. Our results demonstrate that dances in Nomascus represent a common and intentional form of visual communication restricted to sexually mature females. Whilst primarily used as a proceptive signal to solicit copulation, dances occur in a wide range of contexts related to arousal and/or frustration in captivity. A linguistically informed view of this sequential behaviour demonstrates that movement withindances is organizedin groups andfollows an isochronous rhythm-patterns not described for visual displays in other non-human primates. We argue that applying the concept of dance to gibbons allows us to expand our understanding of communication in non-human primates and todevelop hypotheses on the rules and regularities characterising it. We propose that crestedgibbon dances likely evolved from less elaborate rhythmic proceptive signals, similar to those found in siamangs. Although dance displays in humans and crested gibbons share a number of key characteristics, they cannot be assumed to be homologous. Nevertheless, gibbon dances represent a strikingmodel behaviour to investigate the use of complex gestural signals in hominoid primates.

Social living affords primates (including humans) many benefits. Communication has been proposed to be the key mechanism used to bond social connections, which could explain why primates have evolved such expressive faces. We assessed whether the facial expressivity of the dominant male (quantified from the coding of anatomically based facial movement) was related to social network properties (based on social proximity and grooming) in nine groups of captive rhesus macaques (Macaca mulatta) housed in uniform physical and social environments. More facially expressive dominant male macaques were more socially connected and had more cohesive social groups. These findings show that inter-individual differences in facial expressivity are related to differential social outcomes at both an individual and group level. More expressive individuals occupy more beneficial social positions, which could help explain the selection for complex facial communication in primates.

Abstract Ideophones stand out as promising constructions to be considered as linguistic “fossils”. Allegedly, this is due to some of their distinctive features, including their sound-symbolic nature, ample use of reduplication, reliance on the simplest combinatorial processes, attachment to emotional content, and presumed bootstrapping effects on language acquisition. These features might exhibit some continuity with primate communication systems, including the co-occurrence with gestures. Because the nature and complexity of ideophones in modern languages can vary significantly from culture to culture, in this paper, we focus on their links with cross-modality, and ultimately, on how they are processed by the brain and how our brain evolved. We embed our analysis in the framework of the human self-domestication hypothesis, according to which human evolution was characterized by a gradual decrease in reactive aggression, which had an impact on our cognitive and behavioural features. Our framework implicates the cortico-striatal brain networks, whose enhanced connectivity is a mechanism for both the suppression of reactive aggression, and for cross-modality and language processing more generally. In conclusion, even though present-day ideophones can certainly show many new complexities, some of their most basic features can be reconstructed as approximations of early (but not archaic), creative uses of language.

Primate communication relies on multimodal cues, such as vision and audition, to facilitate the exchange of intentions, enable social interactions, avoid predators, and foster group cohesion during daily activities. Understanding the integration of facial and vocal signals is pivotal to comprehend social interaction. In this study, we acquire whole-brain ultra-high field (9.4 T) fMRI data from awake marmosets (Callithrix jacchus) to explore brain responses to unimodal and combined facial and vocal stimuli. Our findings reveal that the multisensory condition not only intensifies activations in the occipito-temporal face patches and auditory voice patches but also engages a more extensive network that includes additional parietal, prefrontal and cingulate areas, compared to the summed responses of the unimodal conditions. By uncovering the neural network underlying multisensory audiovisual integration in marmosets, this study highlights the efficiency and adaptability of the marmoset brain in processing facial and vocal social signals, providing significant insights into primate social communication.

The place of language in multimodal communication in humans and other primates

Vocal communication is essential for social behaviors in humans and non-human primates. While the frontal cortex is crucial to human speech production, its role in vocal production in non-human primates has long been questioned. It is unclear whether activities in the frontal cortex represent diverse vocal signals used in non-human primate communication. Here we studied single neuron activities and local field potentials (LFP) in the frontal cortex of male marmoset monkeys while the animal engaged in vocal exchanges with conspecifics in a social environment. We found that both single neuron activities and LFP were modulated by the production of each of the four major call types. Moreover, neural activities showed distinct patterns for different call types and theta-band LFP oscillations showed phase-locking to the phrases of twitter calls, suggesting a neural representation of vocalization features. Our results suggest important functions of the marmoset frontal cortex in supporting the production of diverse vocalizations in communication.