Meaning, intention, and inference in primate vocal communication

Forms, Mechanisms, and Roles of Iconicity in Spoken Language: A Review

Historically, debates over relationships between spoken lexical form and meaning have been dominated by views of arbitrariness. However more recent research revealed a different perspective, in which non-arbitrary mappings play an important role in the makeup of a lexicon. It is now clear that phoneme-sound symbolism - along with other types of form-to-meaning mappings - contributes to non-arbitrariness (iconicity) of spoken words, which is present in many forms and degrees in different languages. Attempts have been made to provide a mechanistic explanation of the phenomenon, and these theories largely centre around cross-modal correspondences. We build on these views to explore iconicity within the evolutionary context and the neurobiological framework for human language processing. We argue that the multimodal bihemsipheric communicative system, to which iconicity is integral, has important phylogenetic and ontogenetic advantages, facilitating language learning, comprehension, and processing. Despite its numerous advantages however, iconicity must compete with arbitrariness, forcing language systems to balance the competing needs of perceptual grounding of the linguistic form and ensuring an effective signal. We conclude that, on balance, iconicity should be viewed as integral to language, and not merely a marginal phenomenon.

Crows "count" the number of self-generated vocalizations

Producing a specific number of vocalizations with purpose requires a sophisticated combination of numerical abilities and vocal control. Whether this capacity exists in animals other than humans is yet unknown. We show that crows can flexibly produce variable numbers of one to four vocalizations in response to arbitrary cues associated with numerical values. The acoustic features of the first vocalization of a sequence were predictive of the total number of vocalizations, indicating a planning process. Moreover, the acoustic features of vocal units predicted their order in the sequence and could be used to read out counting errors during vocal production.

A multifaceted framework to establish the presence of meaning in non-human communication

Does non-human communication, like language, involve meaning? This question guides our focus through an interdisciplinary review of the theories and terminology used to study meaning across disciplines and species. Until now, it has been difficult to apply the concept of meaning to communication in non-humans. This is partly because of the varied approaches to the study of meaning. Additionally, while there is a scholarly acknowledgement of potential meaning in non-human cognition, there is also scepticism when the topic of communication arises. We organise some of the key literature into a coherent framework that can bridge disciplines and species, to ensure that aspects of meaning are accurately and fairly compared. We clarify the growing view in the literature that, rather than requiring multiple definitions or being split into different types, meaning is a multifaceted yet still unified concept. In so doing, we propose that meaning is an umbrella term. Meaning cannot be summed up with a short definition or list of features, but involves multiple complexities that are outlined in our framework. Specifically, three global facets are needed to describe meaning: a Signal Meaning Facet, an Interactant Meaning Facet, and a Resultant Meaning Facet. Most importantly, we show that such analyses are possible to apply as much to non-humans as to humans. We also emphasise that meaning nuances differ among non-human species, making a dichotomous approach to meaning questionable. Instead, we show that a multifaceted approach to meaning establishes how meaning appears within highly diverse examples of non-human communication, in ways consistent with the phenomenon's presence in human non-verbal communication and language(s). Therefore, without further recourse to 'functional' approaches that circumvent the critical question of whether any non-human meaning exists, we show that the concept of meaning is suitable for evolutionary biologists, behavioural ecologists, and others to study, to establish exactly which species exhibit meaning in their communication and in what ways.

Higher social tolerance is associated with more complex facial behavior in macaques

The social complexity hypothesis for communicative complexity posits that animal societies with more complex social systems require more complex communication systems. We tested the social complexity hypothesis on three macaque species that vary in their degree of social tolerance and complexity. We coded facial behavior in >3000 social interactions across three social contexts (aggressive, submissive, affiliative) in 389 animals, using the Facial Action Coding System for macaques (MaqFACS). We quantified communicative complexity using three measures of uncertainty: entropy, specificity, and prediction error. We found that the relative entropy of facial behavior was higher for the more tolerant crested macaques as compared to the less tolerant Barbary and rhesus macaques across all social contexts, indicating that crested macaques more frequently use a higher diversity of facial behavior. The context specificity of facial behavior was higher in rhesus as compared to Barbary and crested macaques, demonstrating that Barbary and crested macaques used facial behavior more flexibly across different social contexts. Finally, a random forest classifier predicted social context from facial behavior with highest accuracy for rhesus and lowest for crested, indicating there is higher uncertainty and complexity in the facial behavior of crested macaques. Overall, our results support the social complexity hypothesis.

Comprehension of own and other species' alarm calls in sooty mangabey vocal development

Abstract

Primates understand the meaning of their own and other species' alarm calls, but little is known about how they acquire such knowledge. Here, we combined direct behavioural observations with playback experiments to investigate two key processes underlying vocal development: comprehension and usage. Especifically, we studied the development of con- and heterospecific alarm call recognition in free-ranging sooty mangabeys, Cercocebus atys, across three age groups: young juveniles (1-2y), old juveniles (3-4y) and adults (> 5y). We observed that, during natural predator encounters, juveniles alarm called to a significantly wider range of species than adults, with evidence of refinement during the first four years of life. In the experiments, we exposed subjects to leopard, eagle and snake alarm calls given by other group members or sympatric Diana monkeys. We found that young juveniles' locomotor and vocal responses were least appropriate and that they engaged in more social referencing (look at adults when hearing an alarm call) than older individuals, suggesting that vocal competence is obtained via social learning. In conclusion, our results suggest that alarm call comprehension is socially learned during the juvenile stage, with comprehension preceding appropriate usage but no difference between learning their own or other species' alarm calls.

Significance statement

Under natural conditions, animals do not just interact with members of their own species, but usually operate in a network of associated species. However, ontogenetic research on primate communication frequently ignores this significant element. We studied the development of con- and heterospecific alarm call recognition in wild sooty mangabeys. We found that communicative competence was acquired during the juvenile stages, with alarm call comprehension learning preceding appropriate vocal usage and with no clear difference in learning of con- and heterospecific signals. We also found that, during early stages of life, social referencing, a proactive form of social learning, was key in the acquisition of competent alarm call behaviour. Our results show that primates equally learn to interpret alarm calls from their own and other species during their early stages of life and that this learning process is refined as the animals mature.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00265-023-03318-6.

The acoustic structure of spider monkey (Ateles geoffroyi) calls is related both to caller goal and arousal

Analyzing the factors related to the acoustic structure of primate calls is fundamental to identifying the potential information these signals convey, and therefore, their potential function. It is well-accepted that caller arousal is an important driver of acoustic variation in call structure. However, evidence suggests that arousal alone cannot explain the production of calls with different acoustic structures. The "caller goal" framework posits that a call type (e.g., bark and chitter) is a signal which evolved to elicit specific changes on receiver behavior, and a caller's goal, not its arousal state, determines the basic acoustic structure of animal calls. To get a better insight into the effects of caller goal and arousal on the acoustic structure of primate calls, we analyzed the acoustic variation of 382 calls produced by 27 free-ranging spider monkeys (Ateles geoffroyi) in six call contexts representing three caller goals (establishing contact, engage in aggression, and predator presence) across two arousal states (high and low). Discriminant function analyses and linear mixed models showed a strong effect of caller goal on calls' acoustic structure and supported the effect of arousal state on acoustic variation; misclassified calls mainly occurred between the same caller goal categories (e.g., alarm low arousal and alarm high arousal), and acoustic differences between arousal contexts showed the same pattern across the three caller goals (e.g., low center frequency [kHz] values in high arousal contexts). Our results supported the view that caller goal is the primary driver of acoustic difference and showed the effects of caller arousal on other structural differences.

Context-dependent alarm responses in wild vervet monkeys

The alarm calls of nonhuman primates are occasionally cited as functionally equivalent to lexical word meaning in human language. Recently, however, it has become increasingly unlikely that one-to-one relations between alarm call structures and predator categories are the default, mainly because many call types are produced in multiple contexts, requiring more complex notions of meaning. For example, male vervet monkeys produce the same alarm calls during encounters with terrestrial predators and neighbouring groups, suggesting that recipients require additional information to attribute meaning to the calls. We empirically tested the hypothesis that vervet monkeys take contextual information into account when responding to each other's alarm calls. In playback experiments, we exposed subjects to recordings of male alarm barks during actual intergroup encounters (predator unlikely) or when there was no intergroup encounter (predator likely). Subjects responded more strongly in the no intergroup encounter situations, typically associated with discovering a hiding predator, measured in terms of startle responses, vigilance behaviour and gazing towards the presumed caller. We discuss the significance of using contextual information for meaning attribution in nonhuman primate communication.

Intentional gestural communication amongst red-capped mangabeys (Cercocebus torquatus)

Apes, human's closest living relatives, are renowned for their intentional and highly flexible use of gestural communication. In stark contrast, evidence for flexible and intentional gestural communication in monkeys is scarce. Here, we investigated the intentionality and flexibility of spontaneous gesture use in red-capped mangabeys (Cercocebus torquatus). We applied established methods used in ape gesture research to analyse whether the body acts produced by a total of 17 individuals living in three different groups in captivity qualified as intentionally produced gesture instances. Results showed that signallers showed all hallmarks of intentionality during the production of 20 out of a total of 21 different types of body acts. These were only produced in the presence of other individuals, and the monkeys showed audience checking, sensitivity to the attentional states of recipients, adjustment of signal modality, and response waiting relative to their production. Moreover, in case of communication failure, the monkeys showed goal persistence, and regarding the production contexts they showed some signs of means-ends dissociation. Therefore, these monkeys are capable of flexible and intentional gestural communication and use this to communicate with conspecifics. Our results corroborate recent findings showing that intentional gestural communication was already present in the monkey lineage of catarrhine primates. We discuss our results in light of the comparative approach towards human language evolution and highlight our finding that these monkeys also showed flexible and intentional use of four 'free' manual gesture types.

Language as a tool for social bonding: evidence from wild chimpanzee gestural, vocal and bimodal signals

The evolution of language has fascinated anthropologists, psychologists and biologists for centuries, seeking to infer language origins from the communication of primates, our closest living relatives. Capacity for intentional signalling is a key feature of transition to language in our hominin ancestors, facilitating complex social dynamics in complex social groups. However whether vocal, gestural and bimodal signals are differentiated according to intentional use and hence complex sociality has not been studied, making unclear the modality of language evolution. We addressed this question in wild chimpanzees. We found that larger social network size was associated with a larger network of gestural but not vocal or bimodal signals. Response waiting was more common in association with gestures than vocalizations, but elaborations were more common in vocal than gestural or bimodal signals. Overall, chimpanzees were more likely to manage weak social bonds through vocalizations, whereas strong social bonds were managed through gestures and bimodal signals. However, when social bonds were weak, gestures accompanied by response waiting were more likely to elicit approaches than vocalizations accompanied by elaboration, which elicited avoidance. This suggests that gestures were the primary modality of language evolution and that the use of more sophisticated gestural signalling led to evolution of complex social groups of hominin ancestors. This article is part of the theme issue 'Cognition, communication and social bonds in primates'.

Communicative roots of complex sociality and cognition: neuropsychological mechanisms underpinning the processing of social information

Primate social bonds are described as being especially complex in their nature, and primates have unusually large brains for their body size compared to other mammals. Communication in primates has attracted considerable attention because of the important role it plays in social bonding. It has been proposed that differentiated social relationships are cognitively complex because primates need to continuously update their knowledge about different types of social bonds. Therefore, primates infer whether an opportunity for social interaction is rewarding (valuable to individual goals) based on their knowledge of the social relationships of the interactants. However, exposure to distraction and stress has detrimental effects on the dopaminergic system, suggesting that understanding social relationships as rewarding is affected in these conditions. This paper proposes that complex communication evolved to augment the capacity to form social relationships during stress through flexibly modifying intentionality in communication (audience checking, response waiting and elaboration). Intentional communication may upregulate dopamine dynamics to allow recognition that an interaction is rewarding during stress. By examining these associations between complexity of communication and stress, we provide new insights into the cognitive skills involved in forming social bonds in primates and the evolution of communication systems in both primates and humans. This article is part of the theme issue 'Cognition, communication and social bonds in primates'.

The Biological Roots of Music and Dance : Extending the Credible Signaling Hypothesis to Predator Deterrence

After they diverged from panins, hominins evolved an increasingly committed terrestrial lifestyle in open habitats that exposed them to increased predation pressure from Africa's formidable predator guild. In the Pleistocene, Homo transitioned to a more carnivorous lifestyle that would have further increased predation pressure. An effective defense against predators would have required a high degree of cooperation by the smaller and slower hominins. It is in the interest of predator and potential prey to avoid encounters that will be costly for both. A wide variety of species, including carnivores and apes and other primates, have therefore evolved visual and auditory signals that deter predators by credibly signaling detection and/or the ability to effectively defend themselves. In some cooperative species, these predator deterrent signals involve highly synchronized visual and auditory displays among group members. Hagen and Bryant (Human Nature, 14(1), 21-51, 2003) proposed that synchronized visual and auditory displays credibly signal coalition quality. Here, this hypothesis is extended to include credible signals to predators that they have been detected and would be met with a highly coordinated defensive response, thereby deterring an attack. Within-group signaling functions are also proposed. The evolved cognitive abilities underlying these behaviors were foundations for the evolution of fully human music and dance.

The Ontogeny of Vocal Sequences: Insights from a Newborn Wild Chimpanzee (Pan troglodytes schweinfurthii)

Observations of early vocal behaviours in non-human primates (hereafter primates) are important for direct comparisons between human and primate vocal development. However, direct observations of births and perinatal behaviour in wild primates are rare, and the initial stages of behavioural ontogeny usually remain undocumented. Here, we report direct observations of the birth of a wild chimpanzee (Pan troglodytes schweinfurthii) in Budongo Forest, Uganda, including the behaviour of the mother and other group members. We monitored the newborn’s vocal behaviour for approximately 2 hours and recorded 70 calls. We categorised the vocalisations both qualitatively, using conventional call descriptions, and quantitatively, using cluster and discriminant acoustic analyses. We found evidence for acoustically distinct vocal units, produced both in isolation and in combination, including sequences akin to adult pant hoots, a vocal utterance regarded as the most complex vocal signal produced by this species. We concluded that chimpanzees possess the capacity to produce vocal sequences composed of different call types from birth, albeit in rudimentary forms. Our observations are in line with the idea that primate vocal repertoires are largely present from birth, with fine acoustic structures undergoing ontogenetic processes. Our study provides rare and valuable empirical data on perinatal behaviours in wild primates.

Preliminary evidence for one-trial social learning of vervet monkey alarm calling

How do non-human primates learn to use their alarm calls? Social learning is a promising candidate, but its role in the acquisition of meaning and call usage has not been studied systematically, neither during ontogeny nor in adulthood. To investigate the role of social learning in alarm call comprehension and use, we exposed groups of wild vervet monkeys to two unfamiliar animal models in the presence or absence of conspecific alarm calls. To assess the learning outcome of these experiences, we then presented the models for a second time to the same monkeys, but now without additional alarm call information. In subjects previously exposed in conjunction with alarm calls, we found heightened predator inspection compared to control subjects exposed without alarm calls, indicating one-trial social learning of 'meaning'. Moreover, some juveniles (but not adults) produced the same alarm calls they heard during the initial exposure whereas the authenticity of the models had an additional effect. Our experiment provides preliminary evidence that, in non-human primates, call meaning can be acquired by one-trail social learning but that subject age and core knowledge about predators additionally moderate the acquisition of novel call-referent associations.

Preliminary evidence for one-trial social learning of vervet monkey alarm calling

How do non-human primates learn to use their alarm calls? Social learning is a promising candidate, but its role in the acquisition of meaning and call usage has not been studied systematically, neither during ontogeny nor in adulthood. To investigate the role of social learning in alarm call comprehension and use, we exposed groups of wild vervet monkeys to two unfamiliar animal models in the presence or absence of conspecific alarm calls. To assess the learning outcome of these experiences, we then presented the models for a second time to the same monkeys, but now without additional alarm call information. In subjects previously exposed in conjunction with alarm calls, we found heightened predator inspection compared to control subjects exposed without alarm calls, indicating one-trial social learning of 'meaning'. Moreover, some juveniles (but not adults) produced the same alarm calls they heard during the initial exposure whereas the authenticity of the models had an additional effect. Our experiment provides preliminary evidence that, in non-human primates, call meaning can be acquired by one-trail social learning but that subject age and core knowledge about predators additionally moderate the acquisition of novel call-referent associations.

Bonobos assign meaning to food calls based on caller food preferences

Human communication relies heavily on pragmatic competence. Speech utterances are often ambiguous requiring listeners to use interaction history, shared knowledge, presumed intention and other contextual variables to make inferences about a speaker’s meaning. To probe the evolutionary origins of pragmatic competence we tested whether bonobos (Pan paniscus) can make inferences about the type of food available from listening to other group members’ food calls. We trained two group members to either prefer blue or pink chow and demonstrated these preferences to observers. A third group member served as an untrained control. In playback experiments, we broadcast the food calls of a trained demonstrator and the untrained group member to investigate whether subjects were able to infer which coloured chow was most likely available, based on the callers’ trained food preferences or lack thereof. As predicted, when hearing the untrained group member’s calls, subjects did not exhibit a bias, whereas they responded with a significant foraging bias when hearing a trained group member’s calls. These findings suggest that bonobos may take into account the idiosyncratic food preferences of others, although subjects probably differed in what they remembered.

Sentence vs. Word Perception by Young Healthy Females: Toward a Better Understanding of Emotion in Spoken Language

Expression and perception of emotions by voice are fundamental for basic mental health stability. Since different languages interpret results differently, studies should be guided by the relationship between speech complexity and the emotional perception. The aim of our study was therefore to analyze the efficiency of speech stimuli, word vs. sentence, as it relates to the accuracy of four different categories of emotions: anger, sadness, happiness, and neutrality. To this end, a total of 2,235 audio clips were presented to 49 females, native Hebrew speakers, aged 20–30 years (M = 23.7; SD = 2.13). Participants were asked to judge audio utterances according to one of four emotional categories: anger, sadness, happiness, and neutrality. Simulated voice samples were consisting of words and meaningful sentences, provided by 15 healthy young females Hebrew native speakers. Generally, word vs. sentence was not originally accepted as a means of emotional recognition of voice; However, introducing a variety of speech utterances revealed a different perception. Thus, the emotional conveyance provided new, even higher precision to our findings: Anger emotions produced a higher impact to the single word (χ2 = 10.21, p < 0.01) as opposed to the sentence, while sadness was identified more accurately with a sentence (χ2 = 3.83, p = 0.05). Our findings resulted in a better understanding of how speech types can interpret perception, as a part of mental health.

Context-dependent and seasonal fluctuation in bottlenose dolphin (Tursiops truncatus) vocalizations

A fundamental question in animal behaviour is the role of vocal communication in the regulation of social interactions in species that organise themselves into social groups. Context dependence and seasonality in vocalizations are present in the communication of many species, although very little research has addressed this dependence in marine mammals. The study presented here examined variations in the rate at which free-ranging dyads of bottlenose dolphins emit social-signals in an effort to better understand the relationship between vocal communication and social context. The results demonstrate that changes in the social-signal production in bottlenose dolphins are related to the sex of the partner, mating season and social affiliation between the components of the dyad. In a context of foraging behaviour on the same feeding ground, mixed (male-female) dyads were found to emit more pulsed burst sounds during the mating season. Another relevant aspect of the study seems to be the greater production of agonistic social-signals in the dyads formed by individuals with a lower degree of social affiliation. Overall, this study confirms a clear relationship between dyad composition and context-specific social-signals that could reflect the motivational state of individuals linked to seasonal changes in vocal behaviour.

Inferential Communication: Bridging the Gap Between Intentional and Ostensive Communication in Non-human Primates

Communication, when defined as an act intended to affect the psychological state of another individual, demands the use of inference. Either the signaler, the recipient, or both must make leaps of understanding which surpass the semantic information available and draw from pragmatic clues to fully imbue and interpret meaning. While research into human communication and the evolution of language has long been comfortable with mentalistic interpretations of communicative exchanges, including rich attributions of mental state, research into animal communication has balked at theoretical models which describe mentalized cognitive mechanisms. We submit a new theoretical perspective on animal communication: the model of inferential communication. For use when existing proximate models of animal communication are not sufficient to fully explain the complex, flexible, and intentional communication documented in certain species, specifically non-human primates, we present our model as a bridge between shallower, less cognitive descriptions of communicative behavior and the perhaps otherwise inaccessible mentalistic interpretations of communication found in theoretical considerations of human language. Inferential communication is a framework that builds on existing evidence of referentiality, intentionality, and social inference in primates. It allows that they might be capable of applying social inferences to a communicative setting, which could explain some of the cognitive processes that enable the complexity and flexibility of primate communication systems. While historical models of animal communication focus on the means-ends process of behavior and apparent cognitive outcomes, inferential communication invites consideration of the mentalistic processes that must underlie those outcomes. We propose a mentalized approach to questions, investigations, and interpretations of non-human primate communication. We include an overview of both ultimate and proximate models of animal communication, which contextualize the role and utility of our inferential communication model, and provide a detailed breakdown of the possible levels of cognitive complexity which could be investigated using this framework. Finally, we present some possible applications of inferential communication in the field of non-human primate communication and highlight the role it could play in advancing progress toward an increasingly precise understanding of the cognitive capabilities of our closest living relatives.

Roars, groans and moans: Anatomical correlates of vocal diversity in polygynous deer

Eurasian deer are characterized by the extraordinary diversity of their vocal repertoires. Male sexual calls range from roars with relatively low fundamental frequency (hereafter fo ) in red deer Cervus elaphus, to moans with extremely high fo in sika deer Cervus nippon, and almost infrasonic groans with exceptionally low fo in fallow deer Dama dama. Moreover, while both red and fallow males are capable of lowering their formant frequencies during their calls, sika males appear to lack this ability. Female contact calls are also characterized by relatively less pronounced, yet strong interspecific differences. The aim of this study is to examine the anatomical bases of these inter-specific and inter-sexual differences by identifying if the acoustic variation is reflected in corresponding anatomical variation. To do this, we investigated the vocal anatomy of male and female specimens of each of these three species. Across species and sexes, we find that the observed acoustic variability is indeed related to expected corresponding anatomical differences, based on the source-filter theory of vocal production. At the source level, low fo is associated with larger vocal folds, whereas high fo is associated with smaller vocal folds: sika deer have the smallest vocal folds and male fallow deer the largest. Red and sika deer vocal folds do not appear to be sexually dimorphic, while fallow deer exhibit strong sexual dimorphism (after correcting for body size differences). At the filter level, the variability in formants is related to the configuration of the vocal tract: in fallow and red deer, both sexes have evolved a permanently descended larynx (with a resting position of the larynx much lower in males than in females). Both sexes also have the potential for momentary, call-synchronous vocal tract elongation, again more pronounced in males than in females. In contrast, the resting position of the larynx is high in both sexes of sika deer and the potential for further active vocal tract elongation is virtually absent in both sexes. Anatomical evidence suggests an evolutionary reversal in larynx position within sika deer, that is, a secondary larynx ascent. Together, our observations confirm that the observed diversity of vocal behaviour in polygynous deer is supported by strong anatomical differences, highlighting the importance of anatomical specializations in shaping mammalian vocal repertoires. Sexual selection is discussed as a potential evolutionary driver of the observed vocal diversity and sexual dimorphisms.

NetFACS: Using network science to understand facial communication systems

Understanding facial signals in humans and other species is crucial for understanding the evolution, complexity, and function of the face as a communication tool. The Facial Action Coding System (FACS) enables researchers to measure facial movements accurately, but we currently lack tools to reliably analyse data and efficiently communicate results. Network analysis can provide a way to use the information encoded in FACS datasets: by treating individual AUs (the smallest units of facial movements) as nodes in a network and their co-occurrence as connections, we can analyse and visualise differences in the use of combinations of AUs in different conditions. Here, we present ‘NetFACS’, a statistical package that uses occurrence probabilities and resampling methods to answer questions about the use of AUs, AU combinations, and the facial communication system as a whole in humans and non-human animals. Using highly stereotyped facial signals as an example, we illustrate some of the current functionalities of NetFACS. We show that very few AUs are specific to certain stereotypical contexts; that AUs are not used independently from each other; that graph-level properties of stereotypical signals differ; and that clusters of AUs allow us to reconstruct facial signals, even when blind to the underlying conditions. The flexibility and widespread use of network analysis allows us to move away from studying facial signals as stereotyped expressions, and towards a dynamic and differentiated approach to facial communication.

Aping Language: Historical Perspectives on the Quest for Semantics, Syntax, and Other Rarefied Properties of Human Language in the Communication of Primates and Other Animals

In 1980, Robert Seyfarth, Dorothy Cheney and Peter Marler published a landmark paper in Science claiming language-like semantic communication in the alarm calls of vervet monkeys. This article and the career research program it spawned for its authors catalyzed countless other studies searching for semantics, and then also syntax and other rarefied properties of language, in the communication systems of non-human primates and other animals. It also helped bolster a parallel tradition of teaching symbolism and syntax in artificial language systems to great apes. Although the search for language rudiments in the communications of primates long predates the vervet alarm call story, it is difficult to overstate the impact of the vervet research, for it fueled field and laboratory research programs for several generations of primatologists and kept busy an equal number of philosophers, linguists, and cognitive scientists debating possible implications for the origins and evolution of language and other vaunted elements of the human condition. Now 40-years on, the original vervet alarm call findings have been revised and claims of semanticity recanted; while other evidence for semantics and syntax in the natural communications of non-humans is sparse and weak. Ultimately, we are forced to conclude that there are simply few substantive precedents in the natural communications of animals for the high-level informational and representational properties of language, nor its complex syntax. This conclusion does not mean primates cannot be taught some version of these elements of language in artificial language systems - in fact, they can. Nor does it mean there is no continuity between the natural communications of animals and humans that could inform the evolution of language - in fact, there is such continuity. It just does not lie in the specialized semantic and syntactic properties of language. In reviewing these matters, I consider why it is that primates do not evince high-level properties of language in their natural communications but why we so readily accepted that they did or should; and what lessons we might draw from that experience. In the process, I also consider why accounts of human-like characteristics in animals can be so irresistibly appealing.

Distinct neural networks for the volitional control of vocal and manual actions in the monkey homologue of Broca's area

The ventrolateral frontal lobe (Broca's area) of the human brain is crucial in speech production. In macaques, neurons in the ventrolateral prefrontal cortex, the suggested monkey homologue of Broca's area, signal the volitional initiation of vocalizations. We explored whether this brain area became specialized for vocal initiation during primate evolution and trained macaques to alternate between a vocal and manual action in response to arbitrary cues. During task performance, single neurons recorded from the ventrolateral prefrontal cortex and the rostroventral premotor cortex of the inferior frontal cortex predominantly signaled the impending vocal or, to a lesser extent, manual action, but not both. Neuronal activity was specific for volitional action plans and differed during spontaneous movement preparations. This implies that the primate inferior frontal cortex controls the initiation of volitional utterances via a dedicated network of vocal selective neurons that might have been exploited during the evolution of Broca's area.

Intentional communication: solving methodological issues to assigning first-order intentional signalling

Intentional signalling plays a fundamental role in human communication. Mapping the taxonomic distribution of comparable capacities may thus shed light on the selective pressures that enabled the evolution of human communication. Nonetheless, severe methodological issues undermine comparisons among studies, species and communicative modalities. Here, we discuss three main obstacles that hinder comparative research of 'first-order' intentional signalling (i.e. voluntary signalling in pursuit of a cognitively represented goal): (i) inconsistency in how behavioural hallmarks are defined and operationalised, (ii) testing of behavioural hallmarks without statistical comparison to control conditions, and (iii) bias against the publication of negative results. To address these obstacles, we present a four-step scheme with 20 statistical operational criteria to distinguish between non-intentional and first-order intentional signalling. Our unified scheme applies to visual and audible signals, thereby validating comparison across communicative modalities and species. This, in turn, promotes the generation and testing of hypotheses about the evolution of intentional communication.

Emotion recognition in nonhuman primates: How experimental research can contribute to a better understanding of underlying mechanisms

Recognising conspecifics' emotional expressions is important for nonhuman primates to navigate their physical and social environment. We address two possible mechanisms underlying emotion recognition: emotional contagion, the automatic matching of the observer's emotions to the emotional state of the observed individual, and cognitive empathy, the ability to understand the meaning and cause of emotional expressions while maintaining a distinction between own and others' emotions. We review experimental research in nonhuman primates to gain insight into the evolution of emotion recognition. Importantly, we focus on how emotional contagion and cognitive empathy can be studied experimentally. Evidence for aspects of cognitive empathy in different nonhuman primate lineages suggests that a wider range of primates than commonly assumed can infer emotional meaning from emotional expressions. Possibly, analogous rather than homologous evolution underlies emotion recognition. However, conclusions regarding its exact evolutionary course require more research in different modalities and species.

Emotional Voice Intonation: A Communication Code at the Origins of Speech Processing and Word-Meaning Associations?

The aim of the present work is to investigate the facilitating effect of vocal emotional intonation on the evolution of the following processes involved in language: (a) identifying and producing phonemes, (b) processing compositional rules underlying vocal utterances, and (c) associating vocal utterances with meanings. To this end, firstly, I examine research on the presence of these abilities in animals, and the biologically ancient nature of emotional vocalizations. Secondly, I review research attesting to the facilitating effect of emotional voice intonation on these abilities in humans. Thirdly, building on these studies in animals and humans, and through taking an evolutionary perspective, I provide insights for future empirical work on the facilitating effect of emotional intonation on these three processes in animals and preverbal humans. In this work, I highlight the importance of a comparative approach to investigate language evolution empirically. This review supports Darwin’s hypothesis, according to which the ability to express emotions through voice modulation was a key step in the evolution of spoken language.

Information transfer efficiency differs in wild chimpanzees and bonobos, but not social cognition

Several theories have been generated to understand the socio-cognitive mechanisms underlying the unique cooperative abilities of humans. The ‘interdependence hypothesis' posits first, that the cognitive dimension of human cooperation evolved in contexts when several individuals needed to act together to achieve a common goal, like when hunting large prey. Second, the more interdependent individuals are, the more likely they are to provide services to conspecifics in other contexts. Alternatively, the ‘social tolerance hypothesis' proposes that higher social tolerance allows conspecifics to cooperate more efficiently and with a wider range of partners. We conducted the first field experimental evaluation of both hypotheses in our closest living relatives by contrasting chimpanzees to the less interdependent but more tolerant bonobos. We compared each species' performance during a cooperative task: informing conspecifics about a danger. We presented Gaboon viper models to 82 individuals from five wild communities. Chimpanzees arriving late at the snake were significantly more likely to have heard a call and less likely to startle, indicating that chimpanzees were better informed about the presence of the threat than bonobos. This stems from clear species differences in how individuals adjusted their calling decisions to the level of information already available. Chimpanzees were more likely to call and produced more alarm calls when they had not yet heard a call, whereas bonobos did so when they already heard a call. Our results confirm the link between interdependence and cooperation performance. These species differences were most likely driven by differences in motivation rather than in cognitive capacities because both species tended to consider audience knowledge in their decision to call. Our results inform theories on the evolution of human cooperation by linking inter-group competition pressure and in-group cooperative motivation and/or capability.

The evolutionary origins of natural pedagogy: Rhesus monkeys show sustained attention following nonsocial cues versus social communicative signals

The natural pedagogy hypothesis proposes that human infants preferentially attend to communicative signals from others, facilitating rapid cultural learning. In this view, sensitivity to such signals is a uniquely human adaptation and as such nonhuman animals should not produce or utilize these communicative signals. We test these evolutionary predictions by examining sensitivity to communicative cues in 206 rhesus monkeys (Macaca mulatta) using an expectancy looking time task modeled on prior work with infants. Monkeys observed a human actor who either made eye contact and vocalized to the monkey (social cue), or waved a fruit in front of her face and produced a tapping sound (nonsocial cue). The actor then either looked at an object (referential look) or looked toward empty space (look away). We found that, unlike human infants in analogous situations, rhesus monkeys looked longer at events following nonsocial cues, regardless of the demonstrator's subsequent looking behavior. Moreover younger and older monkeys showed similar patterns of responses across development. These results provide support for the natural pedagogy hypothesis, while also highlighting evolutionary changes in human sensitivity to communicative signals.

Communicative roots of complex sociality and cognition

Mammals living in more complex social groups typically have large brains for their body size and many researchers have proposed that the primary driver of the increase in brain size through primate and hominin evolution was the selection pressures associated with sociality. Many mammals, and especially primates, use flexible signals that show a high degree of voluntary control and these signals may play an important role in forming and maintaining social relationships between group members. However, the specific role that cognitive skills play in this complex communication, and how in turn this relates to sociality, is still unclear. The hypothesis for the communicative roots of complex sociality and cognition posits that cognitive demands behind the communication needed to form and maintain bonded social relationships in complex social settings drives the link between brain size and sociality. We review the evidence in support of this hypothesis and why key features of cognitively complex communication such as intentionality and referentiality should be more effective in forming and maintaining bonded relationships as compared with less cognitively complex communication. Exploring the link between cognition, communication and sociality provides insights into how increasing flexibility in communication can facilitate the emergence of social systems characterised by bonded social relationships, such as those found in non-human primates and humans. To move the field forward and carry out both within- and among-species comparisons, we advocate the use of social network analysis, which provides a novel way to describe and compare social structure. Using this approach can lead to a new, systematic way of examining social and communicative complexity across species, something that is lacking in current comparative studies of social structure.

A Hierarchy of Autonomous Systems for Vocal Production

Vocal production is hierarchical in the time domain. These hierarchies build upon biomechanical and neural dynamics across various timescales. We review studies in marmoset monkeys, songbirds, and other vertebrates. To organize these data in an accessible and across-species framework, we interpret the different timescales of vocal production as belonging to different levels of an autonomous systems hierarchy. The first level accounts for vocal acoustics produced on short timescales; subsequent levels account for longer timescales of vocal output. The hierarchy of autonomous systems that we put forth accounts for vocal patterning, sequence generation, dyadic interactions, and context dependence by sequentially incorporating central pattern generators, intrinsic drives, and sensory signals from the environment. We then show the framework's utility by providing an integrative explanation of infant vocal production learning in which social feedback modulates infant vocal acoustics through the tuning of a drive signal.

Towards a new taxonomy of primate vocal production learning

The extent to which vocal learning can be found in nonhuman primates is key to reconstructing the evolution of speech. Regarding the adjustment of vocal output in relation to auditory experience (vocal production learning in the narrow sense), effects on the ontogenetic trajectory of vocal development as well as adjustment to group-specific call features have been found. Yet, a comparison of the vocalizations of different primate genera revealed striking similarities in the structure of calls and repertoires in different species of the same genus, indicating that the structure of nonhuman primate vocalizations is highly conserved. Thus, modifications in relation to experience only appear to be possible within relatively tight species-specific constraints. By contrast, comprehension learning may be extremely rapid and open-ended. In conjunction, these findings corroborate the idea of an ancestral independence of vocal production and auditory comprehension learning. To overcome the futile debate about whether or not vocal production learning can be found in nonhuman primates, we suggest putting the focus on the different mechanisms that may mediate the adjustment of vocal output in response to experience; these mechanisms may include auditory facilitation and learning from success. This article is part of the theme issue 'What can animal communication teach us about human language?'

The neurobiology of innate, volitional and learned vocalizations in mammals and birds

Vocalization is an ancient vertebrate trait essential to many forms of communication, ranging from courtship calls to free verse. Vocalizations may be entirely innate and evoked by sexual cues or emotional state, as with many types of calls made in primates, rodents and birds; volitional, as with innate calls that, following extensive training, can be evoked by arbitrary sensory cues in non-human primates and corvid songbirds; or learned, acoustically flexible and complex, as with human speech and the courtship songs of oscine songbirds. This review compares and contrasts the neural mechanisms underlying innate, volitional and learned vocalizations, with an emphasis on functional studies in primates, rodents and songbirds. This comparison reveals both highly conserved and convergent mechanisms of vocal production in these different groups, despite their often vast phylogenetic separation. This similarity of central mechanisms for different forms of vocal production presents experimentalists with useful avenues for gaining detailed mechanistic insight into how vocalizations are employed for social and sexual signalling, and how they can be modified through experience to yield new vocal repertoires customized to the individual's social group. This article is part of the theme issue 'What can animal communication teach us about human language?'

Coevolution of language and symbolic meaning: Co-opting meaning underlying the initial arts in early human culture

Many of language's components, including communicating symbolic meaning, have neurobiological roots that go back millions of years in evolutionary time. The intersection with the human social survival strategy spawned additional adaptive meaning systems. Under conditions threatening survival in socially oriented human groups, extra-language meaning systems co-opted and adapted to facilitate unity, including the early formats of the arts. They would have percolated into cultural practice for this social purpose and ultimately survival. With evolutionary pressures tapping into biologically inherited, physiologically functioning sensory-motor pathways, anchored specifically in rhythm cognition and motor synchrony output, initial art practice conveyed symbolic group cohesion through communal, all-inclusive synchronously moving dance formations and rhythmically produced vocal or percussion sounds. As with the sounds of language in the deep past, and numerous other cultural behaviors, such nonmaterial early art formats would not have left marks in the archeological record but their evolutionary driven practice would have contributed to adaptive genetic factors woven into brain-behavior evolution. Their practice is likely to have well predated unearthed art-related objects. Consolidation of evidence and notions from language evolution, genetics, human physiology, comparative animal communication, archeology, and climate history in the distant past of early humans in Africa supports the evolutionary driven practice of initial nonmaterial art formats conveying symbolic expressions optimizing group survival. This article is categorized under: Cognitive Biology > Evolutionary Roots of Cognition Linguistics > Evolution of Language Psychology > Comparative Psychology.

Conserved alarm calls but rapid auditory learning in monkey responses to novel flying objects

To evaluate the flexibility in vocal production and comprehension in a non-human primate, we presented a drone to West African green monkeys, Chlorocebus sabaeus. Calls given in response to the drone were clearly distinct from those given to other predators, but highly similar to the aerial alarm calls of the East African vervet monkey, Chlorocebus pygerythrus, suggesting that call structure is conserved. To probe how rapidly the animals attached meaning to the sound of the drone, we played back the drone sound after one to three exposures. Subjects immediately scanned the sky and ran for cover. In contrast to vocal production, comprehension learning was rapid and open-ended.

Acoustic variation of spider monkey (Ateles geoffroyi) contact calls is related to caller isolation and affects listeners' responses

Group living animals produce vocalizations denominated "contact calls" to maintain contact with out-of-sight group members. These calls have been shown to vary with caller identity and distance to potential listeners. However, it is not clear whether the acoustic variation of contact calls is related to caller social isolation (e.g., inside or outside a subgroup) and listeners' responses that can be helpful to maintain contact. Here, we addressed these questions in spider monkeys (Ateles geoffroyi), a Neotropical primate that exchanges contact calls denominated "whinnies", which show graded variation related to caller immediate behavior and distance between callers. Using 566 whinnies produced by 35 free-ranging adult spider monkeys recorded at ≤ 20 m from microphones, we first analyzed whether the acoustic variation of spontaneous whinnies (i.e., whinnies that are not responses to previous whinnies) is related to caller social isolation or whether acoustic variation is related to the likelihood of eliciting a response whinny from another individual. Secondly, we assessed whether listeners' responses (i.e., time to respond vocally, acoustic characteristics of response whinnies, orienting behaviors) were related to the acoustic variation of previous whinnies. Our study revealed that callers that were outside a subgroup produced whinnies with a lower fundamental frequency (F0), which travels longer distances, and increases the likelihood of producing a response whinny. Moreover, listeners (i.e., responders) responded faster to lower F0 whinnies. However, the acoustic variation (i.e., F0 variation) in response whinnies was better explained by the separation distance between callers, than by the acoustic variation of the previous whinny. Overall, our results suggest that whinny variation facilitates vocal contact to callers that are outside a subgroup, and that context and whinny variation affect listeners' responses.

Preconceptual Spectral and Temporal Cues as a Source of Meaning in Speech and Music

This paper explores the importance of preconceptual meaning in speech and music, stressing the role of affective vocalizations as a common ancestral instrument in communicative interactions. Speech and music are sensory rich stimuli, both at the level of production and perception, which involve different body channels, mainly the face and the voice. However, this bimodal approach has been challenged as being too restrictive. A broader conception argues for an action-oriented embodied approach that stresses the reciprocity between multisensory processing and articulatory-motor routines. There is, however, a distinction between language and music, with the latter being largely unable to function referentially. Contrary to the centrifugal tendency of language to direct the attention of the receiver away from the text or speech proper, music is centripetal in directing the listener's attention to the auditory material itself. Sound, therefore, can be considered as the meeting point between speech and music and the question can be raised as to the shared components between the interpretation of sound in the domain of speech and music. In order to answer these questions, this paper elaborates on the following topics: (i) The relationship between speech and music with a special focus on early vocalizations in humans and non-human primates; (ii) the transition from sound to meaning in speech and music; (iii) the role of emotion and affect in early sound processing; (iv) vocalizations and nonverbal affect burst in communicative sound comprehension; and (v) the acoustic features of affective sound with a special emphasis on temporal and spectrographic cues as parts of speech prosody and musical expressiveness.

Development of snake-directed antipredator behavior by wild white-faced capuchin monkeys: III. the signaling properties of alarm-call tonality

In many primates, the acoustic properties of alarm calls can provide information on the level of perceived predatory threat as well as influence the antipredator behavior of nearby conspecifics. The present study examined the harmonics-to-noise ratio (tonality of spectral structure) of alarm calls emitted by white-faced capuchin monkeys (Cebus capucinus) in trees directed at photographic models of a boa constrictor, neotropical rattlesnake, scorpion eater snake, and white snake-shaped control presented on the ground. The average and peak harmonics-to-noise ratios of initial alarm calls by infants, juveniles, and adults and those of nearby second callers were analyzed using PRAAT software. Averaged for age class, the peak harmonics-to-noise ratio of alarm calls directed at the boa constrictor model, characterizing a primary capuchin predator, was reliably higher than the peak harmonics-to-noise ratio of alarm calls directed at the harmless scorpion eater model. This effect was influenced by the higher harmonics-to-noise ratio of infant alarm calls and it disconfirmed our prediction, based on primate vocalization research, that snake perception would increase arousal and alarm-call noisiness. Levels of call tonality did not distinguish the boa and rattlesnake or rattlesnake and scorpion eater models for any age class. Higher alarm-call tonality appeared contagious to nearby perceivers, with focal alarm calling influencing the level of tonality of the first calls of second callers. Together, these findings suggest that the higher peak harmonics-to-noise ratio of capuchin alarm calling directed at snakes is contagious and possibly conveys information about the level of perceived predatory threat.

Call type signals caller goal: a new take on ultimate and proximate influences in vocal production

After 40 years of debate it remains unclear whether signallers produce vocalizations in order to provide receivers with information about call context or external stimuli. This has led some researchers to propose that call production is arousal- or affect-based. Although arousal influences certain acoustic parameters within a call type, we argue that it cannot explain why individuals across vertebrates produce different call types. Given emerging evidence that calls are goal-based, we argue that call type is a signal of a caller's goal to elicit a change in receiver behaviour. Using chimpanzees (Pan troglodytes) and vervet monkeys (Cercopithecus aethiops) as case studies, we demonstrate the two benefits of viewing call production as signalling both caller goal (which determines call type) and caller arousal (which affects within-call-type variation). Such a framework can explain first, why a single class of calls is apparently given in multiple contexts, and, second, why some species have larger call repertoires than others. Previous studies have noted links between sociality and repertoire size, but have not specified exactly why animals living in societies that are more complex might require a greater number of differentiated signals. The caller-goal framework potentially clarifies how social complexity might favour call diversification. As social complexity increases, callers may need to elicit a larger number of distinct behaviours from a wider range of distinct audiences.

Gender Differences in the Recognition of Vocal Emotions

The conflicting findings from the few studies conducted with regard to gender differences in the recognition of vocal expressions of emotion have left the exact nature of these differences unclear. Several investigators have argued that a comprehensive understanding of gender differences in vocal emotion recognition can only be achieved by replicating these studies while accounting for influential factors such as stimulus type, gender-balanced samples, number of encoders, decoders, and emotional categories. This study aimed to account for these factors by investigating whether emotion recognition from vocal expressions differs as a function of both listeners' and speakers' gender. A total of N = 290 participants were randomly and equally allocated to two groups. One group listened to words and pseudo-words, while the other group listened to sentences and affect bursts. Participants were asked to categorize the stimuli with respect to the expressed emotions in a fixed-choice response format. Overall, females were more accurate than males when decoding vocal emotions, however, when testing for specific emotions these differences were small in magnitude. Speakers' gender had a significant impact on how listeners' judged emotions from the voice. The group listening to words and pseudo-words had higher identification rates for emotions spoken by male than by female actors, whereas in the group listening to sentences and affect bursts the identification rates were higher when emotions were uttered by female than male actors. The mixed pattern for emotion-specific effects, however, indicates that, in the vocal channel, the reliability of emotion judgments is not systematically influenced by speakers' gender and the related stereotypes of emotional expressivity. Together, these results extend previous findings by showing effects of listeners' and speakers' gender on the recognition of vocal emotions. They stress the importance of distinguishing these factors to explain recognition ability in the processing of emotional prosody.