on the acoustic analysis of primate vocalizations

The acoustic theory of speech production relates articulatory maneuvers to acoustic attributes of speech. Some procedures are discussed that make use of this theory to analyze the vocalizations of nonhuman primates. These procedures make use of sound spectrograms, oscillograms, and computer-implemented analogs of the primate vocal apparatus as well as anatomical measurements. The use of these techniques in recent studies of nonhuman primate vocalizations is reviewed. These studies show that nonhuman primates lack the anatomical apparatus that is necessary for the production of the full range of human speech. Some unresolved questions concerning the structure of nonhuman primate utterances are discussed with regard to human linguistic ability.     

A Primate Dictionary? Decoding the Function and Meaning of Another Species' Vocalizations

Decoding the function and meaning of a foreign culture's sounds and gestures is a notoriously difficult problem. It is even more challenging when we think about the sounds and gestures of nonhuman animals. This essay provides a review of what is currently known about the informational content and function of primate vocalizations, emphasizing the problems underlying the construction of a primate “dictionary.” In contrast to the Oxford English Dictionary, this dictionary provides entries to emotional expressions as well as potentially referential expressions. It therefore represents a guide to what animals do with their vocalizations, as well as how they are represented by signalers and perceivers. I begin by a discussion of the unit problem, of how an acoustic space is carved up into functionally significant components leading to a species‐specific repertoire or lexicon of sorts. This section shows how little we know about the units of organization within animal vocal repertoires, and how such lack of information constrains our ability to tackle the problem of syntactic structure. In Section III, I review research on the production and perception of vocal signals that appear to be functionally referential. This work shows that several nonhuman primates produce vocalizations that share some of the key properties of reference, but certainly not all; the components that are missing raises questions about their role as precursors to human words. In Section IV, I explore the social uses of vocalizations, assessing whether the signal contains sufficient information for listeners to judge a caller's credibility; ultimately, caller credibility determines how receivers select an appropriate response. Results show that individuals can use calls to assess whether someone is reliable or unreliable, and that such attributes are associated with individuals and particular contexts. I conclude by synthesizing the issues presented and then raise some directions for future conceptual and methodological progress.     

Social processes in communication and cognition in callitrichid monkeys: a review

Studies of vocal development in nonhuman primates have found little evidence for plasticity in vocal production, somewhat more for usage of calls, with the greatest plasticity arising in response to calls of others. Generally, similar results were obtained with callitrichid monkeys, the marmosets and tamarins, but with several interesting exceptions. Infant pygmy marmosets show babbling behavior with improvement in adult call structure related to the amount and diversity of babbling. Adult marmosets alter call structure in response to changes in social partners, and wild marmosets have vocal dialects and modify call structure according to how far they are from other group members, suggesting the potential to modify call structure in different social and environmental contexts, though direct learning of novel vocalizations has not been observed. Infant cotton-top tamarins do not produce adult-like calls in appropriate contexts, at least in the first few months of life, but through food sharing from adults infants learn about appropriate foods and the appropriate contexts for food vocalizations. Tamarins modify call structure and usage with changes in social status. Tamarins, unlike other monkeys tested, can learn to avoid noxious foods through observation of other group members, and can learn about novel food locations. Recent studies provide evidence of contextual imitation in marmosets. The plasticity in vocal communication and evidence of social learning in marmosets and tamarins relative to other monkeys may be related to the cooperative breeding system of marmosets and tamarins. With a high degree of behavioral coordination among group members, there is a priority on monitoring signals and behavior of others and adjusting one's own signals and behavior. This creates the context for vocal plasticity and social learning. Accepted after revision: 23 May 2001 Electronic Publication     

The neuroethology of primate vocal communication: substrates for the evolution of speech

In this article, we review behavioral and neurobiological studies of the perception and use of species-specific vocalizations by non-human primates. At the behavioral level, primate vocal perception shares many features with speech perception by humans. These features include a left-hemisphere bias towards conspecific vocalizations, the use of temporal features for identifying different calls, and the use of calls to refer to objects and events in the environment. The putative neural bases for some of these behaviors have been revealed by recent studies of the primate auditory and prefrontal cortices. These studies also suggest homologies with the human language circuitry. Thus, a synthesis of cognitive, ethological and neurobiological approaches to primate vocal behavior is likely to yield the richest understanding of the neural bases of speech perception, and might also shed light on the evolutionary precursors to language.     

Auditory--visual cross-modal perception of communicative stimuli in tufted capuchin monkeys (Cebus apella)

Research on cross-modal performance in nonhuman primates is limited to a small number of sensory modalities and testing methods. To broaden the scope of this research, the authors tested capuchin monkeys (Cebus apella) for a seldom-studied cross-modal capacity in nonhuman primates, auditory-visual recognition. Monkeys were simultaneously played 2 video recordings of a face producing different vocalizations and a sound recording of 1 of the vocalizations. Stimulus sets varied from naturally occurring conspecific vocalizations to experimentally controlled human speech stimuli. The authors found that monkeys preferred to view face recordings that matched presented vocal stimuli. Their preference did not differ significantly across stimulus species or other stimulus features. However, the reliability of the latter set of results may have been limited by sample size. From these results, the authors concluded that capuchin monkeys exhibit auditory-visual cross-modal perception of conspecific vocalizations.     

Effect of acoustic cue modifications on evoked vocal response to calls in zebra finches (Taeniopygia guttata)

Besides their song, which is usually a functionally well-defined communication signal with an elaborate acoustic structure, songbirds also produce a variety of shorter vocalizations named calls. While a considerable amount of work has focused on information coding in songs, little is known about how calls' acoustic structure supports communication processes. Because male and female zebra finches use calls during most of their interactions and answer to conspecific calls without visual contact, we aimed at identifying which calls' acoustic cues are necessary to elicit a vocal response. Using synthetic zebra finch calls, we examined evoked vocal response of male and female zebra finches to modified versions of the distance calls. Our results show that the vocal response of zebra finches to female calls requires the full harmonic structure of the call, whereas the frequency downsweep of male calls is necessary to evoke a vocal response. It is likely that both female and male calls require matching a similar frequency bandwidth to trigger a response in conspecific individuals.     

Discriminative vocal conditioning in rhesus monkeys: Evidence for volitional control?

Both emotional and volitional systems are invoked in explaining the control of human vocalizations, but vocal behavior of nonhuman primates is often believed to be totally under emotional control. Monkeys' poor performance on conditioning tasks, especially discriminative conditioning of vocalization (DCV), has been cited as evidence against volitional control. In Experiment 1, rhesus monkeys failed a DCV task in which food was given for vocalizations emitted during an arbitrary visual stimulus, but in Experiment 2, monkeys showed clear discriminative performance when an otherwise comparable shock-avoidance DCV procedure was used. This evidence that monkeys possess some degree of volitional control has implications for the relation between animal vocalizations and the origin of human speech.     

Les aspects pragmatiques de la communication chez les primates

Pragmatics is about how individuals use their inventory of semiotic devices, the strategic choices they make, in particular acts of communication. An interesting question is the degree to which other animal species, especially our nearest primate relatives, employ pragmatic strategies in their vocal and gestural communication. Based on a review of the evidence, it is concluded in this essay that primate communication displays almost none of the pragmatic dimensions that characterize human linguistic communication. The most fundamental reason is that nonhuman animal communication does not really take place on the mental or intersubjective plane at all. It is directed at the behavior and emotional states of others, not at their attentional or mental states.     

The vocal repertoire of the domesticated zebra finch: a data-driven approach to decipher the information-bearing acoustic features of communication signals

Although a universal code for the acoustic features of animal vocal communication calls may not exist, the thorough analysis of the distinctive acoustical features of vocalization categories is important not only to decipher the acoustical code for a specific species but also to understand the evolution of communication signals and the mechanisms used to produce and understand them. Here, we recorded more than 8000 examples of almost all the vocalizations of the domesticated zebra finch, Taeniopygia guttata: vocalizations produced to establish contact, to form and maintain pair bonds, to sound an alarm, to communicate distress or to advertise hunger or aggressive intents. We characterized each vocalization type using complete representations that avoided any a priori assumptions on the acoustic code, as well as classical bioacoustics measures that could provide more intuitive interpretations. We then used these acoustical features to rigorously determine the potential information-bearing acoustical features for each vocalization type using both a novel regularized classifier and an unsupervised clustering algorithm. Vocalization categories are discriminated by the shape of their frequency spectrum and by their pitch saliency (noisy to tonal vocalizations) but not particularly by their fundamental frequency. Notably, the spectral shape of zebra finch vocalizations contains peaks or formants that vary systematically across categories and that would be generated by active control of both the vocal organ (source) and the upper vocal tract (filter).     

Agonistic screams in wild chimpanzees (Pan troglodytes schweinfurthii) vary as a function of social role

Some nonhuman primates have demonstrated the capacity to communicate about external objects or events, suggesting primate vocalizations can function as referential signals. However, there is little convincing evidence for functionally referential communication in any great ape species. Here, the authors demonstrate that wild chimpanzees (Pan troglodytes schweinfurthii) of Budongo forest, Uganda, give acoustically distinct screams during agonistic interactions depending on the role they play in a conflict. The authors analyzed the acoustic structure of screams of 14 individuals, in the role of both aggressor and victim. The authors found consistent differences in the acoustic structure of the screams, across individuals, depending on the social role the individual played during the conflict. The authors propose that these 2 distinct scream variants, produced by victims and aggressors during agonistic interactions, may be promising candidates for functioning as referential signals.     

Context-dependent alarm responses in wild vervet monkeys

Animal Cognition - 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...     

Context-related call combinations in female Diana monkeys

Non-human primates possess species-specific repertoires of acoustically distinct call types that can be found in adults in predictable ways. Evidence for vocal flexibility is generally rare and typically restricted to acoustic variants within the main call types or sequential production of multiple calls. So far, evidence for context-specific call sequences has been mainly in relation to external disturbances, particularly predation. In this study, we investigated extensively the vocal behaviour of free-ranging and individually identified Diana monkeys in non-predatory contexts. We found that adult females produced four vocal structures alone (‘H’, ‘L’, ‘R’ and ‘A’ calls, the latter consisting of two subtypes) or combined in non-random ways (‘HA’, ‘LA’ and ‘RA’ call combinations) in relation to ongoing behaviour or external events. Specifically, the concatenation of an introductory call with the most frequently emitted and contextually neutral ‘A’ call seems to function as a contextual refiner of this potential individual identifier. Our results demonstrate that some non-human primates are able to increase the effective size of their small vocal repertoire not only by varying the acoustic structure of basic call types but also by combining them into more complex structures. We have demonstrated this phenomenon for a category of vocalisations with a purely social function and discuss the implications of these findings for evolutionary theories of primate vocal communication.     

The function of primate multimodal communication

Language is commonly narrowed down to speech, but human face-to-face communication is in fact an intrinsically multimodal phenomenon. Despite growing evidence that the communication of non-human primates, our main model for the evolution of language, is also inherently multimodal, most studies on primate communication have focused on either gestures or vocalizations in isolation. Accordingly, the biological function of multimodal signalling remains poorly understood. In this paper, we aim to merge the perspectives of comparative psychology and behavioural ecology on multimodal communication, and review existing studies in great apes for evidence of multimodal signal function based on content-based, efficacy-based and inter-signal interaction hypotheses. We suggest that cross-species comparisons of great ape interactions in both captive and wild settings will allow us to test the conditions in which these hypotheses apply. We expect such studies to provide novel insights into the function of speech-accompanying signals and cues, such as gestures, facial expressions, and eye gaze.     

`Babbling' and social context in infant monkeys: parallels to human infants

Although only humans use spoken language, the vocal communication of many animals shares some features with language. Within the context of their family, normal children and young non-human primates develop proficiency in the nuances of their species-specific vocal communication system. Engaging in speech-like phonetic activity, or babbling, occurs in all normal children regardless of their native language. Similar periods of vocal development have not been described previously for non-human primates. However, in the pygmy marmoset, a South American monkey, we found that the primary vocal behavior of infants parallels many characteristics of human infant babbling. These analogous features include universality, repetition, use of a subset of the adult vocal repertoire, recognizably adult-like vocal structure and lack of a clear vocal referent. Also, like human infants, young marmosets develop in a closely knit family unit that includes both parents and often older siblings. In this context, the babbling-like behavior of the marmoset infant stimulates interaction with caregivers, thereby serving a key role in the infant's own development. These developmental and social processes indicate that the study of vocal development in non-human primate species can provide insights into the function of babbling in humans.     

Are temporal features crucial acoustic cues in dog vocal recognition?

To investigate the perceptual mechanisms underlying conspecific vocal recognition in canine species, eighteen dogs were presented with playbacks of normal and reversed versions of typical dog vocalizations. Auditory perception was analysed using the head-turn paradigm, a non-invasive technique extensively employed to study hemispheric specializations for processing conspecific vocalizations in primates. The results revealed that dogs usually turn their heads with the right ear leading (left hemisphere activation) in response to the forward version of their typical calls, and with either no bias and the left ear leading (right hemisphere activation) in response to the reversed call versions. Overall, our findings suggest that temporal features are determinant auditory cues for call sound recognition in dogs, and support earlier findings of the role of the left hemisphere in the analyses of intraspecific communication.     

New Developments in Understanding Emotional Facial Signals in Chimpanzees

ABSTRACT— There has been little research over the past few decades focusing on similarities and differences in the form and function of emotional signals in nonhuman primates, or whether these communication systems are homologous with those of humans. This is, in part, due to the fact that detailed and objective measurement tools to answer such questions have not been systematically developed for nonhuman primate research. Despite this, emotion research in humans has benefited for over 30 years from an objective, anatomically based facial-measurement tool: the Facial Action Coding System (FACS). In collaboration with other researchers, we have now developed a similar system for chimpanzees (ChimpFACS) and, in the process, have made exciting new discoveries regarding chimpanzees' perception and categorization of emotional facial expressions and similarities in the facial anatomy of chimpanzees and humans, and we have identified homologous facial movements in the two species. Investigating similarities and differences in primate emotional communication systems is essential if we are to understand unique evolutionary specializations among different species.     

What bats have to say about speech and language

Understanding the biological foundations of language is vital to gaining insight into how the capacity for language may have evolved in humans. Animal models can be exploited to learn about the biological underpinnings of shared human traits, and although no other animals display speech or language, a range of behaviors found throughout the animal kingdom are relevant to speech and spoken language. To date, such investigations have been dominated by studies of our closest primate relatives searching for shared traits, or more distantly related species that are sophisticated vocal communicators, like songbirds. Herein I make the case for turning our attention to the Chiropterans, to shed new light on the biological encoding and evolution of human language-relevant traits. Bats employ complex vocalizations to facilitate navigation as well as social interactions, and are exquisitely tuned to acoustic information. Furthermore, bats display behaviors such as vocal learning and vocal turn-taking that are directly pertinent for human spoken language. Emerging technologies are now allowing the study of bat vocal communication, from the behavioral to the neurobiological and molecular level. Although it is clear that no single animal model can reflect the complexity of human language, by comparing such findings across diverse species we can identify the shared biological mechanisms likely to have influenced the evolution of human language.     

Is acoustic evaluation in a non-primate mammal,the tree shrew,affected by context?

Sound categorisation plays a crucial role for processing ecological and social stimuli in a species’ natural environment. To explore the discrimination and evaluation of sound stimuli in human babies and nonhuman primates, a reciprocal habituation-dishabituation paradigm has been successfully introduced into auditory research. We applied the reciprocal paradigm for the first time to a non-primate mammal, the tree shrew (Tupaia belangeri), to examine to what extent non-primate mammals share the ability to evaluate communication calls with primates. Playback stimuli were three types of communication calls, differing distinctively in context and acoustic structure, as well as two artificial control sounds, differing solely in frequency. We assessed the attention towards the playback stimuli by the latency to respond to the test stimulus. Subjects evaluated pairs of communication call types as well as the artificial playback stimuli. Attention towards the test stimuli differed significantly in strength for one pair of communication calls, with subjects dishabituating faster to one category than the other. The comparison of a second pair of communication calls did not show significant differences. Interestingly, subjects also evaluated the artificial control sounds. Findings are only partly in line with results on human and non-human primates. They provided first evidence that in non-primate mammals acoustic evaluation is not solely affected by the sound-associated context but is also linked to unusualness and acoustic cues, such as peak frequency.     

Convergence of calls as animals form social bonds, active compensation for noisy communication channels, and the evolution of vocal learning in mammals

The classic evidence for vocal production learning involves imitation of novel, often anthropogenic sounds. Among mammals, this has been reported for dolphins, elephants, harbor seals, and humans. A broader taxonomic distribution has been reported for vocal convergence, where the acoustic properties of calls from different individuals converge when they are housed together in captivity or form social bonds in the wild. Vocal convergence has been demonstrated for animals as diverse as songbirds, parakeets, hummingbirds, bats, elephants, cetaceans, and primates. For most species, call convergence is thought to reflect a group-distinctive identifier, with shared calls reflecting and strengthening social bonds. A ubiquitous function for vocal production learning that is starting to receive attention involves modifying signals to improve communication in a noisy channel. Pooling data on vocal imitation, vocal convergence, and compensation for noise suggests a wider taxonomic distribution of vocal production learning among mammals than has been generally appreciated. The wide taxonomic distribution of this evidence for vocal production learning suggests that perhaps more of the neural underpinnings for vocal production learning are in place in mammals than is usually recognized.