The function and mechanism of vocal accommodation in humans and other primates

Sex differences in the impact of social relationships on individual vocal signatures in grey mouse lemurs (Microcebus murinus)

Vocalizations coordinate social interactions between conspecifics by conveying information concerning the individual or group identity of the sender. Social accommodation is a form of vocal learning where social affinity is signalled by converging or diverging vocalizations with those of conspecifics. To investigate whether social accommodation is linked to the social lifestyle of the sender, we investigated sex-specific differences in social accommodation in a dispersed living primate, the grey mouse lemur (Microcebus murinus), where females form stable sleeping groups whereas males live solitarily. We used 482 trill calls of 36 individuals from our captive breeding colony to compare acoustic dissimilarity between individuals with genetic relatedness, social contact time and body weight. Our results showed that female trills become more similar the more time females spend with each other, independent of genetic relationship, suggesting vocal convergence. In contrast, male trills were affected more by genetic than social factors. However, focusing only on socialized males, increasing time as cage partners caused greater divergence in males' trills. Thus, grey mouse lemurs show the capacity for social accommodation, with females converging their trills to signal social closeness to sleeping group partners, whereas males do not adapt or diverge their trills to signal individual distinctiveness. This article is part of the theme issue 'The power of sound: unravelling how acoustic communication shapes group dynamics'.

Marmosets mutually compensate for differences in rhythms when coordinating vigilance

Synchronization is widespread in animals, and studies have often emphasized how this seemingly complex phenomenon can emerge from very simple rules. However, the amount of flexibility and control that animals might have over synchronization properties, such as the strength of coupling, remains underexplored. Here, we studied how pairs of marmoset monkeys coordinated vigilance while feeding. By modeling them as coupled oscillators, we noted that (1) individual marmosets do not show perfect periodicity in vigilance behaviors, (2) nevertheless, marmoset pairs started to take turns being vigilant over time, a case of anti-phase synchrony, (3) marmosets could couple flexibly; the coupling strength varied with every new joint feeding bout, and (4) marmosets could control the coupling strength; dyads showed increased coupling if they began in a more desynchronized state. Such flexibility and control over synchronization require more than simple interaction rules. Minimally, animals must estimate the current degree of asynchrony and adjust their behavior accordingly. Moreover, the fact that each marmoset is inherently non-periodic adds to the cognitive demand. Overall, our study provides a mathematical framework to investigate the cognitive demands involved in coordinating behaviors in animals, regardless of whether individual behaviors are rhythmic or not.

Vocal learning-associated convergent evolution in mammalian proteins and regulatory elements

Vocal production learning ("vocal learning") is a convergently evolved trait in vertebrates. To identify brain genomic elements associated with mammalian vocal learning, we integrated genomic, anatomical, and neurophysiological data from the Egyptian fruit bat (Rousettus aegyptiacus) with analyses of the genomes of 215 placental mammals. First, we identified a set of proteins evolving more slowly in vocal learners. Then, we discovered a vocal motor cortical region in the Egyptian fruit bat, an emergent vocal learner, and leveraged that knowledge to identify active cis-regulatory elements in the motor cortex of vocal learners. Machine learning methods applied to motor cortex open chromatin revealed 50 enhancers robustly associated with vocal learning whose activity tended to be lower in vocal learners. Our research implicates convergent losses of motor cortex regulatory elements in mammalian vocal learning evolution.

Active Sampling in Primate Vocal Interactions

Active sensing is a behavioral strategy for exploring the environment. In this study, we show that contact vocal behaviors can be an active sensing mechanism that uses sampling to gain information about the social environment, in particular, the vocal behavior of others. With a focus on the realtime vocal interactions of marmoset monkeys, we contrast active sampling to a vocal accommodation framework in which vocalizations are adjusted simply to maximize responses. We conducted simulations of a vocal accommodation and an active sampling policy and compared them with real vocal exchange data. Our findings support active sampling as the best model for marmoset monkey vocal exchanges. In some cases, the active sampling model was even able to predict the distribution of vocal durations for individuals. These results suggest a new function for primate vocal interactions in which they are used by animals to seek information from social environments.

Effects of deafening on vocal production learning in the Egyptian fruit-bat

Some species have evolved the ability to use the sense of hearing to modify existing vocalizations, or even create new ones. This ability corresponds to various forms of vocal production learning that are all possessed by humans, and independently displayed by distantly related species. Among mammals, a few species, including the Egyptian fruit-bat, would possess such vocal production learning abilities. Yet the necessity of an intact auditory system for the development of the Egyptian fruit-bat typical vocal repertoire has not been tested. Here we addressed this gap by eliminating pups' sense of hearing at birth and assessing its effects on vocal production in adulthood. The deafening treatment enabled us to both causally test these bats vocal learning ability and discern learned from innate aspects of their vocalizations. Leveraging wireless individual audio recordings from freely interacting adults, we show that a subset of the Egyptian fruit-bat vocal repertoire necessitates auditory feedback. Intriguingly, these affected vocalizations belong to different acoustic groups in the vocal repertoire of males and females. These findings open the possibilities for targeted studies of the mammalian neural circuits that enable sexually dimorphic forms of vocal learning.

Significance

Vocal production learning is the rare capacity amongst animals where hearing is used to modify or create new vocalizations. The Egyptian fruit-bat is believed to possess this capacity, yet whether they need audition to achieve a mature vocal repertoire is unknown. Furthermore, a systematic causal examination of learned and innate aspects of the entire repertoire has never been performed in a vocal learner. Here, we addressed these major gaps directly by abolishing hearing in Egyptian fruit-bats at birth and investigating its effects on the vocal production in adulthood. Leveraging simultaneous individual wireless audio-recordings from freely interacting adult bats, we identify the subset of learned vocalizations and provide evidence that vocal learning is sexually dimorphic in that species.

A convergent interaction engine: vocal communication among marmoset monkeys

To understand the primate origins of the human interaction engine, it is worthwhile to focus not only on great apes but also on callitrichid monkeys (marmosets and tamarins). Like humans, but unlike great apes, callitrichids are cooperative breeders, and thus habitually engage in coordinated joint actions, for instance when an infant is handed over from one group member to another. We first explore the hypothesis that these habitual cooperative interactions, the marmoset interactional ethology, are supported by the same key elements as found in the human interaction engine: mutual gaze (during joint action), turn-taking, volubility, as well as group-wide prosociality and trust. Marmosets show clear evidence of these features. We next examine the prediction that, if such an interaction engine can indeed give rise to more flexible communication, callitrichids may also possess elaborate communicative skills. A review of marmoset vocal communication confirms unusual abilities in these small primates: high volubility and large vocal repertoires, vocal learning and babbling in immatures, and voluntary usage and control. We end by discussing how the adoption of cooperative breeding during human evolution may have catalysed language evolution by adding these convergent consequences to the great ape-like cognitive system of our hominin ancestors. This article is part of the theme issue 'Revisiting the human 'interaction engine': comparative approaches to social action coordination'.

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.

A novel attention-getting vocalization in zoo-housed western gorillas

As a critical aspect of language, vocal learning is extremely rare in animals, having only been described in a few distantly related species. New evidence, however, extends vocal learning/innovation to the primate order, with zoo-housed chimpanzees and orangutans producing novel vocal signals to attract the attention of familiar human caregivers. If the ability to produce novel vocalizations as a means of navigating evolutionarily novel circumstances spans the Hominidae family, then we can expect to find evidence for it in the family’s third genus, Gorilla. To explore this possibility, we conduct an experiment with eight gorillas from Zoo Atlanta to examine whether they use species-atypical vocalizations to get the attention of humans across three different conditions: just a human, just food, or a human holding food. Additionally, we survey gorilla keepers from other AZA-member zoos to compile a list of common attention-getting signals used by the gorillas in their care. Our experiment results indicated that Zoo Atlanta gorillas vocalized most often during the human-food condition, with the most frequently used vocal signal being a species-atypical sound somewhere between a sneeze and a cough (n = 28). This previously undescribed sound is acoustically different from other calls commonly produced during feeding (i.e., single grunts and food-associated calls). Our survey and analyses of recordings from other zoos confirmed that this novel attention-getting sound is not unique to Zoo Atlanta, although further work should be done to better determine the extent and patterns of transmission and/or potential independent innovation of this sound across captive gorilla populations. These findings represent one of the few pieces of evidence of spontaneous novel vocal production in non-enculturated individuals of this species, supporting the inclusion of great apes as moderate vocal learners and perhaps demonstrating an evolutionary function to a flexible vocal repertoire.

The mystery of emotional mimicry: multiple functions and processing levels in expression imitation

Mimicry of appearance or of facial, vocal, or gestural expressions emerges frequently among members of different species. When such mimicry directly relates to affective aspects of an interaction, researchers talk about "emotional mimicry". Emotional mimicry has been amply documented but its functionality is still debated. Why and when do people mimic the expressions of others, who benefits, the mimicker or the mimicked, and how do they benefit? Which processes underlie emotional mimicry? Is it completely automatic and unconscious or can it be deliberate and conscious? The current Theory Section addresses these questions from different theoretical perspectives. The invited article by Hess and Fischer focused on the role of mimicry in social regulation and social bonding. The invited comment by Krets and Akyüz highlights information gathering and prediction in social interaction. The invited comment by Bernhold and Giles emphasizes vocal communication and its role in interpersonal accommodation. In this editorial, I propose the different theoretical perspectives may be integrated by assuming a multilevel appraisal and response generation mechanism. I also suggest that emotional mimicry research may be broadened by including social learning, vocal imitation, interspecies comparisons, and affective computing approaches.

Vocal accommodation in penguins (Spheniscus demersus) as a result of social environment

The ability to vary the characteristics of one's voice is a critical feature of human communication. Understanding whether and how animals change their calls will provide insights into the evolution of language. We asked to what extent the vocalizations of penguins, a phylogenetically distant species from those capable of explicit vocal learning, are flexible and responsive to their social environment. Using a principal components (PCs) analysis, we reduced 14 vocal parameters of penguin's contact calls to four PCs, each comprising highly correlated parameters and which can be categorized as fundamental frequency, formant frequency, frequency modulation, and amplitude modulation rate and duration. We compared how these differed between individuals with varying degrees of social interactions: same-colony versus different-colony, same colony over 3 years and partners versus non-partners. Our analyses indicate that the more penguins experience each other's calls, the more similar their calls become over time, that vocal convergence requires a long time and relative stability in colony membership, and that partners' unique social bond may affect vocal convergence differently than non-partners. Our results suggest that this implicit form of vocal plasticity is perhaps more widespread across the animal kingdom than previously thought and may be a fundamental capacity of vertebrate vocalization.

The function and evolution of child-directed communication

Humans communicate with small children in unusual and highly conspicuous ways (child-directed communication (CDC)), which enhance social bonding and facilitate language acquisition. CDC-like inputs are also reported for some vocally learning animals, suggesting similar functions in facilitating communicative competence. However, adult great apes, our closest living relatives, rarely signal to their infants, implicating communication surrounding the infant as the main input for infant great apes and early humans. Given cross-cultural variation in the amount and structure of CDC, we suggest that child-surrounding communication (CSC) provides essential compensatory input when CDC is less prevalent-a paramount topic for future studies.

Transition to language: From agent perception to event representation

Spoken language, as we have it, requires specific capacities-at its most basic advanced vocal control and complex social cognition. In humans, vocal control is the basis for speech, achieved through coordinated interactions of larynx activity and rapid changes in vocal tract configurations. Most likely, speech evolved in response to early humans perceiving reality in increasingly complex ways, to the effect that primate-like signaling became unsustainable as a sole communication device. However, in what ways did and do humans see the world in more complex ways compared to other species? Although animal signals can refer to external events, in contrast to humans, they usually refer to the agents only, sometimes in compositional ways, but never together with patients. It may be difficult for animals to comprehend events as part of larger social scripts, with antecedent causes and future consequences, which are more typically tie the patient into the event. Human brain enlargement over the last million years probably has provided the cognitive resources to represent social interactions as part of bigger social scripts, which enabled humans to go beyond an agent-focus to refer to agent-patient relations, the likely foundation for the evolution of grammar. This article is categorized under: Cognitive Biology > Evolutionary Roots of Cognition Linguistics > Evolution of Language Psychology > Comparative.

Re-evaluating vocal production learning in non-oscine birds

The study of vocal production learning in birds is heavily biased towards oscine songbirds, making the songbird model the reference for comparative studies. However, as vocal learning was probably ancestral in songbirds, interspecific variations might all be variations on a single theme and need not be representative of the nature and characteristics of vocal learning in other bird groups. To assess the possible mechanisms of vocal learning and its evolution therefore requires knowledge about independently evolved incidences of vocal learning. This review examines the presence and nature of vocal production learning in non-songbirds. Using a broad definition of vocal learning and a comparative phylogenetic framework, I evaluate the evidence for vocal learning and its characteristics in non-oscine birds, including well-known vocal learners such as parrots and hummingbirds but also (putative) cases from other taxa. Despite the sometimes limited evidence, it is clear that vocal learning occurs in a range of different, non-related, taxa and can be caused by a variety of mechanisms. It is more widespread than often realized, calling for more systematic studies. Examining this variation may provide a window onto the evolution of vocal learning and increase the value of comparative research for understanding vocal learning in humans.

This article is part of the theme issue ‘Vocal learning in animals and humans’.

The multi-dimensional nature of vocal learning

How learning affects vocalizations is a key question in the study of animal communication and human language. Parallel efforts in birds and humans have taught us much about how vocal learning works on a behavioural and neurobiological level. Subsequent efforts have revealed a variety of cases among mammals in which experience also has a major influence on vocal repertoires. Janik and Slater (Anim. Behav.60, 1-11. (doi:10.1006/anbe.2000.1410)) introduced the distinction between vocal usage and production learning, providing a general framework to categorize how different types of learning influence vocalizations. This idea was built on by Petkov and Jarvis (Front. Evol. Neurosci.4, 12. (doi:10.3389/fnevo.2012.00012)) to emphasize a more continuous distribution between limited and more complex vocal production learners. Yet, with more studies providing empirical data, the limits of the initial frameworks become apparent. We build on these frameworks to refine the categorization of vocal learning in light of advances made since their publication and widespread agreement that vocal learning is not a binary trait. We propose a novel classification system, based on the definitions by Janik and Slater, that deconstructs vocal learning into key dimensions to aid in understanding the mechanisms involved in this complex behaviour. We consider how vocalizations can change without learning, and a usage learning framework that considers context specificity and timing. We identify dimensions of vocal production learning, including the copying of auditory models (convergence/divergence on model sounds, accuracy of copying), the degree of change (type and breadth of learning) and timing (when learning takes place, the length of time it takes and how long it is retained). We consider grey areas of classification and current mechanistic understanding of these behaviours. Our framework identifies research needs and will help to inform neurobiological and evolutionary studies endeavouring to uncover the multi-dimensional nature of vocal learning. This article is part of the theme issue 'Vocal learning in animals and humans'.

Factors affecting call usage in wild black-and-white ruffed lemurs (Varecia variegata) at Mangevo, Ranomafana National Park

Vocal communication is an important modality for group-living primates inhabiting dense forest habitats that can hinder visual and olfactory signals. Nevertheless, research on primate vocalizations has historically focused on a narrow subset of haplorhine taxa; comparatively few studies have been focused on strepsirrhines, despite facing similar ecological and social challenges. Ruffed lemurs (Varecia)-a taxon known for their raucous calls-are rainforest specialists that exhibit strong fission-fusion dynamics and communally rear large litters of young. However, surprisingly few studies have examined Varecia vocalizations in the wild, meaning virtually nothing is known about the call types or how they facilitate their unique social and reproductive strategies. Our goal for this study was to examine how various contextual factors such as weather, behavioral state, and subgroup size and composition affected vocal activity across call types in wild black-and-white ruffed lemurs. We conducted focal follows on 31 individuals (two communities) in Mangevo (Ranomafana National Park) from May-August 2019 to record behavioral and vocal activity. We distinguished 11 call types, although three (hum, roar-shriek, chatter) constituted the majority of vocal activity. Calling rates were consistent throughout the day, but decreased with high rainfall. We found sex- and subgroup-specific differences in call usage, likely related to female dominance and subgroup composition, respectively. We identified behavioral contexts that some call types were consistently given in; this can be used to help infer call function. This study provides some of the first quantitative analyses of ruffed lemur vocal communication and lays the groundwork for more systematic hypothesis testing in future studies.

Trade-offs between vocal accommodation and individual recognisability in common marmoset vocalizations

Recent studies find increasing evidence for vocal accommodation in nonhuman primates, indicating that this form of vocal learning is more prevalent than previously thought. Convergent vocal accommodation (i.e. becoming more similar to partners) indicates social closeness. At the same time, however, becoming too similar may compromise individual recognisability. This is especially problematic if individual recognisability is an important part of the call function, like in long-distance contact calls. In contrast, in calls with a different function, the trade-off between signalling social closeness and individual recognisability might be less severe. We therefore hypothesized that the extent and consequences of accommodation depend on the function of a given call, and expected (1) more accommodation in calls for which individual identity is less crucial and (2) that individual identity is less compromised in calls that serve mainly to transmit identity compared to calls where individual recognisability is less important. We quantified vocal accommodation in three call types over the process of pair formation in common marmoset monkeys (Callithrix jacchus, n = 20). These three call types have different functions and vary with the degree to which they refer to individual identity of the caller. In accordance with our predictions, we found that animals converged most in close contact calls (trill calls), but less in calls where individual identity is more essential (phee- and food calls). In two out of three call types, the amount of accommodation was predicted by the initial vocal distance. Moreover, accommodation led to a drop in statistical individual recognisability in trill calls, but not in phee calls and food calls. Overall, our study shows that patterns of vocal accommodation vary between call types with different functions, suggestive of trade-offs between signalling social closeness and individual recognisability in marmoset vocalizations.

Functionally Flexible Signaling and the Origin of Language

At the earliest break of ancient hominins from their primate relatives in vocal communication, we propose a selection pressure on vocal fitness signaling by hominin infants. Exploratory vocalizations, not tied to expression of distress or immediate need, could have helped persuade parents of the wellness and viability of the infants who produced them. We hypothesize that hominin parents invested more in infants who produced such signals of fitness plentifully, neglecting or abandoning them less often than infants who produced the sounds less frequently. Selection for such exploratory vocalization provided a critically important inclination and capability relevant to language, we reason, because the system that encouraged spontaneous vocalization also made vocalization functionally flexible to an extent that has not been observed in any other animal. Although this vocal flexibility did not by itself create language, it provided an essential foundation upon which language would evolve through a variety of additional steps. In evaluating this speculation, we consider presumable barriers to evolving language that are thought to be implications of Darwinian Theory. It has been claimed that communication always involves sender self-interest and that self-interest leads to deceit, which is countered through clever detection by receivers. The constant battle of senders and receivers has been thought to pose an insuperable challenge to honest communication, which has been viewed as a requirement of language. To make communication honest, it has been proposed that stable signaling requires costly handicaps for the sender, and since language cannot entail high cost, the reasoning has suggested an insurmountable obstacle to the evolution of language. We think this presumed honesty barrier is an illusion that can be revealed by recognition of the fact that language is not inherently honest and in light of the distinction between illocutionary force and semantics. Our paper also considers barriers to the evolution of language (not having to do with honesty) that we think may have actually played important roles in preventing species other than humans from evolving language.

Vocal convergence in a multi-level primate society: insights into the evolution of vocal learning

The extent to which nonhuman primate vocalizations are amenable to modification through experience is relevant for understanding the substrate from which human speech evolved. We examined the vocal behaviour of Guinea baboons, Papio papio, ranging in the Niokolo Koba National Park in Senegal. Guinea baboons live in a multi-level society, with units nested within parties nested within gangs. We investigated whether the acoustic structure of grunts of 27 male baboons of two gangs varied with party/gang membership and genetic relatedness. Males in this species are philopatric, resulting in increased male relatedness within gangs and parties. Grunts of males that were members of the same social levels were more similar than those of males in different social levels (N = 351 dyads for comparison within and between gangs, and N = 169 dyads within and between parties), but the effect sizes were small. Yet, acoustic similarity did not correlate with genetic relatedness, suggesting that higher amounts of social interactions rather than genetic relatedness promote the observed vocal convergence. We consider this convergence a result of sensory-motor integration and suggest this to be an implicit form of vocal learning shared with humans, in contrast to the goal-directed and intentional explicit form of vocal learning unique to human speech acquisition.

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.

Rapid modulations of the vocal structure in marmoset monkeys

Humans and some animal species show flexibility in vocal production either voluntarily or in response to environmental cues. Studies have shown rapid spectrotemporal changes in speech or vocalizations during altered auditory feedback in humans, songbirds and bats. Non-human primates, however, have long been considered lacking the ability to modify spectrotemporal structures of their vocalizations. Here we tested the ability of the common marmoset (Callithrix jacchus), a highly vocal New World primate species to alter spectral and temporal structures of their species-specific vocalizations in the presence of perturbation signals. By presenting perturbation noises while marmosets were vocalizing phee calls, we showed that they were able to change in real-time the duration or spectral trajectory of an ongoing phee phrase by either terminating it before its completion, making rapid shifts in fundamental frequency or in some cases prolonging the duration beyond the natural range of phee calls. In some animals, we observed fragmented phee calls which were not produced by marmosets in their natural environment. Interestingly, some perturbation-induced changes persisted even in the absence of the perturbation noises. These observations provide further evidence that marmoset monkeys are capable of rapidly modulating their vocal structure and suggested potential voluntary vocal control by this non-human primate species.

Are dialects socially learned in marmoset monkeys? Evidence from translocation experiments

The acoustic properties of vocalizations in common marmosets differ between populations. These differences may be the result of social vocal learning, but they can also result from environmental or genetic differences between populations. We performed translocation experiments to separately quantify the influence of a change in the physical environment (experiment 1), and a change in the social environment (experiment 2) on the acoustic properties of calls from individual captive common marmosets. If population differences were due to genetic differences, we expected no change in the vocalizations of the translocated marmosets. If differences were due to environmental factors, we expected vocalizations to permanently change contingent with environmental changes. If social learning was involved, we expected that the vocalizations of animals translocated to a new population with a different dialect would become more similar to the new population. In experiment 1, we translocated marmosets to a different physical environment without changing the social composition of the groups or their neighbours. Immediately after the translocation to the new facility, one out of three call types showed a significant change in call structure, but 5-6 weeks later, the calls were no longer different from before the translocation. Thus, the novel physical environment did not induce long lasting changes in the vocalizations of the marmosets. In experiment 2, we translocated marmosets to a new population with a different dialect. Importantly, our previous work had shown that these two populations differed significantly in vocalization structure. The translocated marmosets were still housed in their original social group, but after translocation they were surrounded by the vocalizations from neighbouring groups of the new population. The vocal distance between the translocated individuals and the new population decreased for two out of three call types over 16 weeks. Thus, even without direct social contact or interaction, the vocalizations of the translocated animals converged towards the new population, indicating that common marmosets can modify their calls due to acoustic input from conspecifics alone, via crowd vocal learning. To our knowledge, this is the first study able to distinguish between different explanations for vocal dialects as well as to show crowd vocal learning in a primate species.

Long-lasting vocal plasticity in adult marmoset monkeys

Humans exhibit a high level of vocal plasticity in speech production, which allows us to acquire both native and foreign languages and dialects, and adapt to local accents in social communication. In comparison, non-human primates exhibit limited vocal plasticity, especially in adulthood, which would limit their ability to adapt to different social and environmental contexts in vocal communication. Here, we quantitatively examined the ability of adult common marmosets (Callithrix jacchus), a highly vocal New World primate species, to modulate their vocal production in social contexts. While recent studies have demonstrated vocal learning in developing marmosets, we know much less about the extent of vocal learning and plasticity in adult marmosets. We found, in the present study, that marmosets were able to adaptively modify the spectrotemporal structure of their vocalizations when they encountered interfering sounds. Our experiments showed that marmosets shifted the spectrum of their vocalizations away from the spectrum of the interfering sounds in order to avoid the overlap. More interestingly, we found that marmosets made predictive and long-lasting spectral shifts in their vocalizations after they had experienced a particular type of interfering sound. These observations provided evidence for directional control of the vocalization spectrum and long-term vocal plasticity by adult marmosets. The findings reported here have important implications for the ability of this New World primate species in voluntarily and adaptively controlling their vocal production in social communication.

Bottlenose Dolphins Retain Individual Vocal Labels in Multi-level Alliances

Cooperation between allied individuals and groups is ubiquitous in human societies, and vocal communication is known to play a key role in facilitating such complex human behaviors [1, 2]. In fact, complex communication may be a feature of the kind of social cognition required for the formation of social alliances, facilitating both partner choice and the execution of coordinated behaviors [3]. As such, a compelling avenue for investigation is what role flexible communication systems play in the formation and maintenance of cooperative partnerships in other alliance-forming animals. Male bottlenose dolphins in some populations form complex multi-level alliances, where individuals cooperate in the pursuit and defense of an important resource: access to females [4]. These strong relationships can last for decades and are critical to each male's reproductive success [4]. Convergent vocal accommodation is used to signal social proximity to a partner or social group in many taxa [5, 6], and it has long been thought that allied male dolphins also converge onto a shared signal to broadcast alliance identity [5-8]. Here, we combine a decade of data on social interactions with dyadic relatedness estimates to show that male dolphins that form multi-level alliances in an open social network retain individual vocal labels that are distinct from those of their allies. Our results differ from earlier reports of signature whistle convergence among males that form stable alliance pairs. Instead, they suggest that individual vocal labels play a central role in the maintenance of differentiated relationships within complex nested alliances.