Salience of Caller Identity in Rhesus Monkey (Macaca mulatta) Coos and Screams: Perceptual Experiments With Human (Homo sapiens) Listeners

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.

Evolutionary loss of complexity in human vocal anatomy as an adaptation for speech

Human speech production obeys the same acoustic principles as vocal production in other animals but has distinctive features: A stable vocal source is filtered by rapidly changing formant frequencies. To understand speech evolution, we examined a wide range of primates, combining observations of phonation with mathematical modeling. We found that source stability relies upon simplifications in laryngeal anatomy, specifically the loss of air sacs and vocal membranes. We conclude that the evolutionary loss of vocal membranes allows human speech to mostly avoid the spontaneous nonlinear phenomena and acoustic chaos common in other primate vocalizations. This loss allows our larynx to produce stable, harmonic-rich phonation, ideally highlighting formant changes that convey most phonetic information. Paradoxically, the increased complexity of human spoken language thus followed simplification of our laryngeal anatomy.

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.

Experimental evidence for delayed post-conflict management behaviour in wild dwarf mongooses

In many species, within-group conflict leads to immediate avoidance of potential aggressors or increases in affiliation, but no studies have investigated delayed post-conflict management behaviour. Here, we experimentally test that possibility using a wild but habituated population of dwarf mongooses (Helogale parvula). First, we used natural and playback-simulated foraging displacements to demonstrate that bystanders take notice of the vocalisations produced during such within-group conflict events but that they do not engage in any immediate post-conflict affiliative behaviour with the protagonists or other bystanders. We then used another playback experiment to assess delayed effects of within-group conflict on grooming interactions: we examined affiliative behaviour at the evening sleeping burrow, 30–60 min after the most recent simulated foraging displacement. Overall, fewer individuals groomed on evenings following an afternoon of simulated conflict, but those that did groomed more than on control evenings. Subordinate bystanders groomed with the simulated aggressor significantly less, and groomed more with one another, on conflict compared to control evenings. Our study provides experimental evidence that dwarf mongooses acoustically obtain information about within-group contests (including protagonist identity), retain that information, and use it to inform conflict-management decisions with a temporal delay.

Social animals that live in groups often have disagreements over access to mates and food. Even fleeting in-group disputes can be costly, disrupting relationships, wasting time and energy, or causing injury if aggression escalates. So, much like humans, many social animals, including primates, birds and dogs, have evolved conflict management strategies to prevent and resolve in-group disagreements. In the immediate aftermath of a conflict, this usually involves changes in the interactions between those involved in the disagreement, or between bystander groupmates and either the victim or aggressor.

Less is known about whether social animals can recall past disputes and if they can use conflict management strategies some time after a quarrel has occurred. That is, do aggressive interactions between groupmates influence later social decisions of bystanders in the group?

To investigate, Morris-Drake et al. studied groups of wild dwarf mongooses (Helogale parvula) that have become accustomed to living alongside humans in Limpopo Province, South Africa. Dwarf mongooses live in groups of up to 30 individuals, with one dominant breeding pair and lower-ranked helpers. When disagreements arise over food, an aggressor growls deeply and hip-slams the victim away from their foraging patch, stealing the victim’s prey in the process. Victims often produce high-pitched squeals in retreat.

Using recordings of these calls, Morris-Drake et al. devised a field experiment to investigate how mongooses responded to nearby conflicts between other group members. Recordings simulating a conflict over food were played to groups of foraging mongooses over the course of an afternoon, so that group members effectively heard what sounded like repeated squabbles between two out-of-sight individuals. Similar to natural conflicts, the mongooses did not engage in any obvious conflict management behaviour immediately after hearing these disputes. But when the group returned to their sleeping burrow that evening, subordinate group members shunned the perceived aggressors from grooming, a key social activity.

This work provides evidence that dwarf mongooses keep tabs on conflicts that occur between groupmates. It shows these animals can extract information about conflicts from vocal cues alone and that bystanders use this information when making later social decisions impacting group dynamics. It also adds to growing evidence from baboons, monkeys and chimpanzees that social animals can remember past events and take these into account when interacting with groupmates.

The Indexical Voice: Communication of Personal States and Traits in Humans and Other Primates

Many studies of primate vocalization have been undertaken to improve our understanding of the evolution of language. Perhaps, for this reason, investigators have focused on calls that were thought to carry symbolic information about the environment. Here I suggest that even if these calls were in fact symbolic, there were independent reasons to question this approach in the first place. I begin by asking what kind of communication system would satisfy a species' biological needs. For example, where animals benefit from living in large groups, I ask how members would need to communicate to keep their groups from fragmenting. In this context, I discuss the role of social grooming and "close calls," including lip-smacking and grunting. Parallels exist in human societies, where information is exchanged about all kinds of things, often less about the nominal topic than the communicants themselves. This sort of indexical (or personal) information is vital to group living, which presupposes the ability to tolerate, relate to, and interact constructively with other individuals. Making indexical communication the focus of comparative research encourages consideration of somatic and behavioral cues that facilitate relationships and social benefits, including cooperation and collaboration. There is ample room here for a different and potentially more fruitful approach to communication in humans and other primates, one that focuses on personal appraisals, based on cues originating with individuals, rather than signals excited by environmental events.

Coevolution of vocal signal characteristics and hearing sensitivity in forest mammals

Although signal characteristics and sensory systems are predicted to co-evolve according to environmental constraints, this hypothesis has not been tested for acoustic signalling across a wide range of species, or any mammal sensory modality. Here we use phylogenetic comparative techniques to show that mammal vocal characteristics and hearing sensitivity have co-evolved to utilise higher frequencies in forest environments - opposite to the general prediction that lower frequencies should be favoured in acoustically cluttered habitats. We also reveal an evolutionary trade-off between high frequency hearing sensitivity and the production of calls with high frequency acoustic energy that suggests forest mammals further optimise vocal communication according to their high frequency hearing sensitivity. Our results provide clear evidence of adaptive signal and sensory system coevolution. They also emphasize how constraints imposed by the signalling environment can jointly shape vocal signal structure and auditory systems, potentially driving acoustic diversity and reproductive isolation.

Do human screams permit individual recognition?

The recognition of individuals through vocalizations is a highly adaptive ability in the social behavior of many species, including humans. However, the extent to which nonlinguistic vocalizations such as screams permit individual recognition in humans remains unclear. Using a same-different vocalizer discrimination task, we investigated participants' ability to correctly identify whether pairs of screams were produced by the same person or two different people, a critical prerequisite to individual recognition. Despite prior theory-based contentions that screams are not acoustically well-suited to conveying identity cues, listeners discriminated individuals at above-chance levels by their screams, including both acoustically modified and unmodified exemplars. We found that vocalizer gender explained some variation in participants' discrimination abilities and response times, but participant attributes (gender, experience, empathy) did not. Our findings are consistent with abundant evidence from nonhuman primates, suggesting that both human and nonhuman screams convey cues to caller identity, thus supporting the thesis of evolutionary continuity in at least some aspects of scream function across primate species.

The acoustic structure of male giant panda bleats varies according to intersexual context

Although the acoustic structure of mammal vocal signals often varies according to the social context of emission, relatively few mammal studies have examined acoustic variation during intersexual advertisement. In the current study male giant panda bleats were recorded during the breeding season in three behavioural contexts: vocalising alone, during vocal interactions with females outside of peak oestrus, and during vocal interactions with peak-oestrous females. Male bleats produced during vocal interactions with peak-oestrous females were longer in duration and had higher mean fundamental frequency than those produced when males were either involved in a vocal interaction with a female outside of peak oestrus or vocalising alone. In addition, males produced bleats with higher rates of fundamental frequency modulation when they were vocalising alone than when they were interacting with females. These results show that acoustic features of male giant panda bleats have the potential to signal the caller's motivational state, and suggest that males increase the rate of fundamental frequency modulation in bleats when they are alone to maximally broadcast their quality and promote close-range contact with receptive females during the breeding season.

Individual distinctiveness in call types of wild western female gorillas

Individually distinct vocalizations play an important role in animal communication, allowing call recipients to respond differentially based on caller identity. However, which of the many calls in a species' repertoire should have more acoustic variability and be more recognizable is less apparent. One proposed hypothesis is that calls used over long distances should be more distinct because visual cues are not available to identify the caller. An alternative hypothesis proposes that close calls should be more recognizable because of their importance in social interactions. To examine which hypothesis garners more support, the acoustic variation and individual distinctiveness of eight call types of six wild western gorilla (Gorilla gorilla) females were investigated. Acoustic recordings of gorilla calls were collected at the Mondika Research Center (Republic of Congo). Acoustic variability was high in all gorilla calls. Similar high inter-individual variation and potential for identity coding (PIC) was found for all call types. Discriminant function analyses confirmed that all call types were individually distinct (although for call types with lowest sample size - hum, grumble and scream - this result cannot be generalized), suggesting that neither the distance at which communication occurs nor the call social function alone can explain the evolution of identity signaling in western gorilla communication.

Natural variability in species-specific vocalizations constrains behavior and neural activity

A listener's capacity to discriminate between sounds is related to the amount of acoustic variability that exists between these sounds. However, a full understanding of how this natural variability impacts neural activity and behavior is lacking. Here, we tested monkeys' ability to discriminate between different utterances of vocalizations from the same acoustic class (i.e., coos and grunts), while neural activity was simultaneously recorded in the anterolateral belt region (AL) of the auditory cortex, a brain region that is a part of a pathway that mediates auditory perception. Monkeys could discriminate between coos better than they could discriminate between grunts. We also found AL activity was more informative about different coos than different grunts. This difference could be attributed, in part, to our finding that coos had more acoustic variability than grunts. Thus, intrinsic acoustic variability constrained the discriminability of AL spike trains and the ability of rhesus monkeys to discriminate between vocalizations.

Cues to androgens and quality in male gibbon songs

Animal vocal signals may provide information about senders and mediate important social interactions like sexual competition, territory maintenance and mate selection. Hence, it is important to understand whether vocal signals provide accurate information about animal attributes or status. Gibbons are non-human primates that produce loud, distinctive and melodious vocalizations resembling more those of birds than of other non-human primates. Wild gibbons are characterized by flexibility in social organization (i.e., pairs and multimale units) as well as in mating system (i.e., monogamy and polyandry). Such features make them a suitable model to investigate whether the physiology (hormonal status) and socio-demographic features find their correspondence in the structure of their songs. By combining male solo song recordings, endocrine outputs using non-invasive fecal androgen measures and behavioral observations, we studied 14 groups (10 pair-living, 4 multimale) of wild white-handed gibbons (Hylobates lar) residing at Khao Yai National Park, Thailand. We collected a total of 322 fecal samples and recorded 48 songs from 18 adult animals. Our results confirmed inter-individuality in male gibbon songs, and showed a clear correlation between androgen levels and song structures. Gibbons with higher androgen levels produced calls having higher pitch, and similarly adult individuals produced longer calls than senior males. Thus, it is plausible that gibbon vocalizations provide receivers with information about singers' attributes.

Coding of vocalizations by single neurons in ventrolateral prefrontal cortex

Neuronal activity in single prefrontal neurons has been correlated with behavioral responses, rules, task variables and stimulus features. In the non-human primate, neurons recorded in ventrolateral prefrontal cortex (VLPFC) have been found to respond to species-specific vocalizations. Previous studies have found multisensory neurons which respond to simultaneously presented faces and vocalizations in this region. Behavioral data suggests that face and vocal information are inextricably linked in animals and humans and therefore may also be tightly linked in the coding of communication calls in prefrontal neurons. In this study we therefore examined the role of VLPFC in encoding vocalization call type information. Specifically, we examined previously recorded single unit responses from the VLPFC in awake, behaving rhesus macaques in response to 3 types of species-specific vocalizations made by 3 individual callers. Analysis of responses by vocalization call type and caller identity showed that ∼19% of cells had a main effect of call type with fewer cells encoding caller. Classification performance of VLPFC neurons was ∼42% averaged across the population. When assessed at discrete time bins, classification performance reached 70 percent for coos in the first 300 ms and remained above chance for the duration of the response period, though performance was lower for other call types. In light of the sub-optimal classification performance of the majority of VLPFC neurons when only vocal information is present, and the recent evidence that most VLPFC neurons are multisensory, the potential enhancement of classification with the addition of accompanying face information is discussed and additional studies recommended. Behavioral and neuronal evidence has shown a considerable benefit in recognition and memory performance when faces and voices are presented simultaneously. In the natural environment both facial and vocalization information is present simultaneously and neural systems no doubt evolved to integrate multisensory stimuli during recognition. This article is part of a Special Issue entitled "Communication Sounds and the Brain: New Directions and Perspectives".

Plasticity after perceptual narrowing for voice perception: reinstating the ability to discriminate monkeys by their voices at 12 months of age

Differentiating individuals by their voice is an important social skill for infants to acquire. In a previous study, we demonstrated that the ability to discriminate individuals by voice follows a pattern of perceptual narrowing (Friendly et al., 2013). Specifically, we found that the ability to discriminate between two foreign-species (rhesus monkey) voices decreased significantly between 6 and 12 months of age. Also during this period, there was a trend for the ability to discriminate human voices to increase. Here we investigate the extent to which plasticity remains at 12 months, after perceptual narrowing has occurred. We found that 12-month-olds who received 2 weeks of monkey-voice training were significantly better at discriminating between rhesus monkey voices than untrained 12-month-olds. Furthermore, discrimination was reinstated to a level slightly better than that of untrained 6-month-olds, suggesting that voice-processing abilities remain considerably plastic at the end of the first year.

Vocal cues indicate level of arousal in infant African elephant roars

Arousal-based physiological changes influence acoustic features of vocalizations in mammals. In particular, nonlinear phenomena are thought to convey information about the caller's arousal state. This hypothesis was tested in the infant African elephant (Loxodonta africana) roar, a call type produced in situations of arousal and distress. Ninety-two percent of roars exhibited nonlinear phenomena, with chaos being the most common type. Acoustic irregularities were strongly associated with elevated fundamental frequency values. Roars produced in situations of highest urgency, based on the occurrence of behavioral indicators of arousal, were characterized by the lowest harmonics-to-noise ratio; this indicates low tonality. In addition, roars produced in these situations lasted longer than those produced in contexts of lower presumed urgency. Testing the infant roars for individual distinctiveness revealed only a moderate classification result. Combined, these findings indicate that infant African elephant roars primarily function to signal the caller's arousal state. The effective communication of this type of information may allow mothers to respond differentially based on their infant's degree of need and may be crucial for the survival of infant African elephants in their natural environment.

Rhesus monkeys see who they hear: spontaneous cross-modal memory for familiar conspecifics

Rhesus monkeys gather much of their knowledge of the social world through visual input and may preferentially represent this knowledge in the visual modality. Recognition of familiar faces is clearly advantageous, and the flexibility and utility of primate social memory would be greatly enhanced if visual memories could be accessed cross-modally either by visual or auditory stimulation. Such cross-modal access to visual memory would facilitate flexible retrieval of the knowledge necessary for adaptive social behavior. We tested whether rhesus monkeys have cross-modal access to visual memory for familiar conspecifics using a delayed matching-to-sample procedure. Monkeys learned visual matching of video clips of familiar individuals to photographs of those individuals, and generalized performance to novel videos. In crossmodal probe trials, coo-calls were played during the memory interval. The calls were either from the monkey just seen in the sample video clip or from a different familiar monkey. Even though the monkeys were trained exclusively in visual matching, the calls influenced choice by causing an increase in the proportion of errors to the picture of the monkey whose voice was heard on incongruent trials. This result demonstrates spontaneous cross-modal recognition. It also shows that viewing videos of familiar monkeys activates naturally formed memories of real monkeys, validating the use of video stimuli in studies of social cognition in monkeys.

Estrogen and Progestogen Correlates of the Structure of Female Copulation Calls in Semi-Free-Ranging Barbary Macaques (Macaca sylvanus)

Females of many Old World primates produce conspicuous vocalizations in combination with copulations. Indirect evidence exists that in Barbary macaques (Macaca sylvanus), the structure of these copulation calls is related to changes in reproductive hormone levels. However, the structure of these calls does not vary significantly around the timing of ovulation when estrogen and progestogen levels show marked changes. We here aimed to clarify this paradox by investigating how the steroid hormones estrogen and progesterone are related to changes in the acoustic structure of copulation calls. We collected data on semi-free-ranging Barbary macaques in Gibraltar and at La Forêt des Singes in Rocamadour, France. We determined estrogen and progestogen concentrations from fecal samples and combined them with a fine-grained structural analysis of female copulation calls (N = 775 calls of 11 females). Our analysis indicates a time lag of 3 d between changes in fecal hormone levels, adjusted for the excretion lag time, and in the acoustic structure of copulation calls. Specifically, we found that estrogen increased the duration and frequency of the calls, whereas progestogen had an antagonistic effect. Importantly, however, variation in acoustic variables did not track short-term changes such as the peak in estrogen occurring around the timing of ovulation. Taken together, our results help to explain why female Barbary macaque copulation calls are related to changes in hormone levels but fail to indicate the fertile phase.

Elasticity and stress relaxation of rhesus monkey (Macaca mulatta) vocal folds

Fundamental frequency is an important perceptual parameter for acoustic communication in mammals. It is determined by vocal fold oscillation, which depends on the morphology and viscoelastic properties of the oscillating tissue. In this study, I tested if stress-strain and stress-relaxation behavior of rhesus monkey (Macaca mulatta) vocal folds allows the prediction of a species' natural fundamental frequency range across its entire vocal repertoire as well as of frequency contours within a single call type. In tensile tests, the load-strain and stress-relaxation behavior of rhesus monkey vocal folds and ventricular folds has been examined. Using the string model, predictions about the species' fundamental frequency range, individual variability, as well as the frequency contour of 'coo' calls were made. The low- and mid-frequency range (up to 2 kHz) of rhesus monkeys can be predicted relatively well with the string model. The discrepancy between predicted maximum fundamental frequency and what has been recorded in rhesus monkeys is currently ascribed to the difficulty in predicting the behavior of the lamina propria at very high strain. Histological sections of the vocal fold and different staining techniques identified collagen, elastin, hyaluronan and, surprisingly, fat cells as components of the lamina propria. The distribution of all four components is not uniform, suggesting that different aspects of the lamina propria are drawn into oscillation depending on vocal fold tension. A differentiated recruitment of tissue into oscillation could extend the frequency range specifically at the upper end of the frequency scale.

Seeing with ears: Sightless humans’ perception of dog bark provides a test for structural rules in vocal communication

Prerecorded family dog ( Canis familiaris) barks were played back to groups of congenitally sightless, sightless with prior visual experience, and sighted people (none of whom had ever owned a dog). We found that blind people without any previous canine visual experiences can categorize accurately various dog barks recorded in different contexts, and their results are very close to those of sighted people in characterizing the emotional content of barks. These findings suggest that humans can recognize some of the most important motivational states reflecting, for example, fear or aggression in a dog's bark without any visual experience. It is very likely that this result can be generalized to other mammalian species—that is, no visual experience of another individual is needed for recognizing some of the most important motivational states of the caller.

Five-month-old infants' identification of the sources of vocalizations

Humans speak, monkeys grunt, and ducks quack. How do we come to know which vocalizations animals produce? Here we explore this question by asking whether young infants expect humans, but not other animals, to produce speech, and further, whether infants have similarly restricted expectations about the sources of vocalizations produced by other species. Five-month-old infants matched speech, but not human nonspeech vocalizations, specifically to humans, looking longer at static human faces when human speech was played than when either rhesus monkey or duck calls were played. They also matched monkey calls to monkey faces, looking longer at static rhesus monkey faces when rhesus monkey calls were played than when either human speech or duck calls were played. However, infants failed to match duck vocalizations to duck faces, even though infants likely have more experience with ducks than monkeys. Results show that by 5 months of age, human infants generate expectations about the sources of some vocalizations, mapping human faces to speech and rhesus faces to rhesus calls. Infants' matching capacity does not appear to be based on a simple associative mechanism or restricted to their specific experiences. We discuss these findings in terms of how infants may achieve such competence, as well as its specificity and relevance to acquiring language.

Vocal cues to identity and relatedness in giant pandas (Ailuropoda melanoleuca)

A range of acoustic characteristics typically carry information on individual identity in mammalian calls. In addition, physical similarities in vocal production anatomy among closely related individuals may result in similarities in the acoustic structure of vocalizations. Here, acoustic analyses based on source-filter theory were used to determine whether giant panda bleats are individually distinctive, to investigate the relative importance of different source-(larynx) and filter-(vocal tract) related acoustic features for coding individuality, and to test whether closely related individuals have similarities in call structure. The results revealed that giant panda bleats are highly individualized and indicate that source-related features, in particular, mean fundamental frequency, amplitude variation per second, and the mean extent of each amplitude modulation, contribute the most to vocal identity. In addition, although individual pairwise relatedness was not correlated with overall acoustic similarity, it was highly correlated with amplitude modulation rate and fundamental frequency range, suggesting that these acoustic features are heritable components of giant panda bleats that could be used as a measure of genetic relatedness. The ecological relevance of acoustically signaling information on caller identity and the potential practical implications for acoustic monitoring of population levels in this endangered species are discussed.

Asymmetries in the individual distinctiveness and maternal recognition of infant contact calls and distress screams in baboons

A key component of nonhuman primate vocal communication is the production and recognition of clear cues to social identity that function in the management of these species' individualistic social relationships. However, it remains unclear how ubiquitous such identity cues are across call types and age-sex classes and what the underlying vocal production mechanisms responsible might be. This study focused on two structurally distinct call types produced by infant baboons in contexts that place a similar functional premium on communicating clear cues to caller identity: (1) contact calls produced when physically separated from, and attempting to relocate, mothers and (2) distress screams produced when aggressively attacked by other group members. Acoustic analyses and field experiments were conducted to examine individual differentiation in single vocalizations of each type and to test mothers' ability to recognize infant calls. Both call types showed statistically significant individual differentiation, but the magnitude of the differentiation was substantially higher in contact calls. Mothers readily discriminated own-offspring contact calls from those of familiar but unrelated infants, but did not do so when it came to distress screams. Several possible explanations for these asymmetries in call differentiation and recognition are considered.

Recognition of rhesus macaque (Macaca mulatta) noisy screams: evidence from conspecifics and human listeners

Vocalizations are among the diverse cues that animals use to recognize individual conspecifics. For some calls, such as noisy screams, there is debate over whether such recognition occurs. To test recognition of rhesus macaque noisy screams, recorded calls were played back to unrelated and related conspecific group members as either single calls or short bouts. Higher-ranking, but not lower-ranking, monkeys looked longer toward the playback speaker in trials containing screams from kin than in those composed of screams from nonkin. In a second study, human listeners performed a "same/different" discrimination task between presentations of rhesus screams from either the same or two different monkeys. Listeners discriminated between "same" and "different" callers above an established empirical threshold, whether screams were presented singly or in short bouts. Together, these results suggest that rhesus monkeys can distinguish noisy screams between kin and nonkin, and humans are able to discriminate different individuals' noisy screams, even when the duration of the bout is short. Whether noisy screams are ideally designed signals for individual recognition is discussed with respect to possible evolutionary origins of the calls.

Voice processing in human and non-human primates

Humans share with non-human primates a number of voice perception abilities of crucial importance in social interactions, such as the ability to identify a conspecific individual from its vocalizations. Speech perception is likely to have evolved in our ancestors on the basis of pre-existing neural mechanisms involved in extracting behaviourally relevant information from conspecific vocalizations (CVs). Studying the neural bases of voice perception in primates thus not only has the potential to shed light on cerebral mechanisms that may be--unlike those involved in speech perception--directly homologous between species, but also has direct implications for our understanding of how speech appeared in humans. In this comparative review, we focus on behavioural and neurobiological evidence relative to two issues central to voice perception in human and non-human primates: (i) are CVs 'special', i.e. are they analysed using dedicated cerebral mechanisms not used for other sound categories, and (ii) to what extent and using what neural mechanisms do primates identify conspecific individuals from their vocalizations?

Can humans discriminate between dogs on the base of the acoustic parameters of barks?

In this study we tested the often suggested claim that people are able to recognize their dogs by their barks. Earlier studies in other species indicated that reliable discrimination between individuals cannot be made by listening to chaotically noisy vocalizations. As barking is typically such a chaotic noisy vocalization, we have hypothesized that reliable discrimination between individuals is not possible by listening to barks. In this study, playback experiments were conducted to explore (1) how accurately humans discriminate between dogs by hearing only their barks, (2) the impact of the eliciting context of calls on these discrimination performances, and (3) how much such discrimination depends on acoustic parameters (tonality and frequency of barks, and the intervals between the individual barks). Our findings were consistent with the previous studies: human performances did not pass the empirical threshold of reliable discrimination in most cases. But a significant effect of tonality was found: discrimination between individuals was more successful when listeners were listening to low harmonic-to-noise ratio (HNR) barks. The contexts in which barks were recorded affected significantly the listeners' performances: if the dog barked at a stranger, listeners were able to discriminate the vocalizations better than if they were listening to sounds recorded when the dog was separated from its owner. It is rendered probable that the bark might be a more efficient communication system between humans and dogs for communicating the motivational state of an animal than for discrimination among strange individuals.