Pant Hoots Research Articles

The form and function of chimpanzee buttress drumming

Many animal species use vocal and nonvocal acoustic signals to communicate over large distances. Wild chimpanzees, Pan troglodytes , drum on the buttress roots of trees, generating low-frequency sounds that can reach distances of over 1 km. Buttress drumming is produced in bouts of beats and is often accompanied by pant hoots, the species-typical long-distance vocalization. We investigated whether individual differences exist in the acoustic structure of drumming bouts produced by male chimpanzees of the Waibira community in the Budongo Forest in Uganda, and whether individual, contextual and social factors affected their use of drumming. We found individual differences in drumming bouts produced by seven male chimpanzees during travel events as well as in their timing within the pant hoot, and discriminated specific patterns of beats for some chimpanzees. In contrast, we found no evidence for individual differences in the acoustic structure of drumming bouts produced by four males during displays. Together these findings suggest that chimpanzees may be able to choose to encode identity within individual drumming ‘signatures’. Chimpanzees drummed less frequently as their party size increased. We found no evidence that the age of the signaller or the presence of preferred social partners, higher-ranking males or females in oestrus affected the use of drumming. These findings suggest there may be flexibility in buttress drumming across social and behavioural contexts and provide support for the hypothesis that, by encoding individual identity, long-distance drumming may be used to facilitate chimpanzee fission–fusion social dynamics. • Chimpanzees produce low-frequency sounds by drumming on buttress roots of trees. • Drumming is used across contexts with apparently long- and short-distance functions. • Chimpanzee drums encode individual signatures in travel events but not in displays. • Patterns of production suggest drumming coordinates fission–fusion dynamics.

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.

Audience sensitivity in chimpanzee display pant hoots

Audience effects are key in studies of animal social cognition and are typically investigated during directed social interactions. Male chimpanzees, Pan troglodytes, regularly perform aggressive displays in the presence of others, often targeting a specific group member, and combine this agonistic behaviour with acoustic signals. Here, we were interested in whether the production and structure of pant hoots, a long-distance signal, were influenced by audience composition (i.e. presence and absence of specific individuals). We investigated pant hoots produced during displays by adult and subadult males of Budongo Forest, Uganda. We found that males overall called more often when their preferred social partners and females were absent from the party, as well as when more dominant males were present. We then separately analysed the four phases of pant hoots, introduction, build-up, climax and let-down, and found that audience composition and social context could often explain the presence or absence of each phase. In addition, displays were often accompanied by drumming, especially by older males and when male audiences were small. Our study adds to the growing body of literature on audience effects and other social factors and shows their impact on the structure of a sophisticated vocal sequence, which enhances the communicative capacity in a species with limited vocal control.

Male chimpanzees communicate to mediate competition and cooperation during feeding

An ongoing debate in animal behaviour research is whether food calls function to cooperatively inform others or provide the caller with competitive advantages. When feeding, chimpanzees, Pan troglodytes, produce two types of call: context-specific, close-range ‘rough grunts’ and context-general, long-range ‘pant hoots’. We investigated this dual signalling behaviour by wild male chimpanzees that were either actively joining others or passively being joined in food trees, considering the effects of the audience composition and the type of food encountered. For arriving individuals, we found that pant hoot production was best explained by the absence of socially important individuals (i.e. social bond partners and/or high-ranking males), suggesting that callers were cooperatively informing them about food availability, probably to strengthen social relationships. In contrast, rough grunts were mostly produced by low-ranking individuals, suggesting they were part of competitive interactions to avoid aggression. For individuals already in a tree, we found that both rough grunt and pant hoot production were most common in low-ranking individuals reacting to the arrival of high-ranking males and there was no significant effect of the presence, or absence, of social bond partners. We discuss these patterns and conclude that, when chimpanzees enter a food tree, their vocal behaviour functions to mediate both cooperative and competitive interactions.

Chimpanzees combine pant hoots with food calls into larger structures

A growing body of observational and experimental data in nonhuman primates has highlighted the presence of rudimentary call combinations within the vocal communication system of monkeys. Such evidence suggests the ability to combine meaning-bearing units into larger structures, a key feature of language also known as syntax, could have its origins rooted within the primate lineage. However, the evolutionary progression of this trait remains ambiguous as evidence for similar combinations in great apes, our closest-living relatives, is sparse and incomplete. In this study, we aimed to bridge this gap by analysing the combinatorial properties of the pant hoot–food call combination in our closest-living relative, the chimpanzee, Pan troglodytes . To systematically investigate the syntactic-like potential of this structure, we adopted three levels of analysis. First, we applied collocation analyses, methods traditionally used in language sciences, to confirm the combination of pant hoots with food calls was not a random co-occurrence, but instead a consistently produced structure. Second, using acoustic analyses, we confirmed pant hoots and food calls comprising the combination were acoustically indistinguishable from the same calls produced in isolation, indicating the pant hoot–food call combination is composed of individually occurring meaning-bearing units, a key criterion of linguistic syntax. Finally, we investigated the context-specific nature of this structure, demonstrating that the call combination was more likely to be produced when feeding on larger patches and when a high-ranking individual joined the feeding party. Together our results converge to provide support for the systematic combination of calls in chimpanzees. We highlight that playback experiments are vital to robustly disentangle both the function this combination might serve and the similarities with combinations of meaning-bearing units (i.e. syntax) in language. • Chimpanzees combine pant hoots with food calls into larger nonrandom structures. • Calls in combinations are acoustically similar to individual ones. • They occur when feeding on large patches and when high-ranked individuals join. • Chimpanzees may combine calls to convey information on both ID and context.

Re-emergence of the leaf clip gesture during an alpha takeover affects variation in male chimpanzee loud calls.

Loud calls are used by many species as long-distance signals for group defense, mate attraction, and inter- and intragroup spacing. Chimpanzee loud calls, or pant hoots, are used in a variety of contexts including group coordination and during male contests. Here, we observed an alpha male takeover in wild chimpanzees (Pan troglodytes verus) during which the leaf clipping gesture re-emerged after disappearing for almost two years in this community. Leaf clipping only occurred in males and was observed almost exclusively prior to pant hoot vocalizations, as has been observed in other chimpanzee communities of the Taï forest in Côte d’Ivoire. Consequently, we hypothesized that leaf clipping may be important for male-male competition by affecting variation in the acoustic properties of male chimpanzee loud calls. We therefore investigated whether pant hoots preceded by leaf clipping differed acoustically from those without, while also testing the influence of social context on pant hoot variation, namely male dominance rank and hierarchy instability, i.e., before, during and after the alpha takeover. We found that pant hoots preceded by leaf clipping were longer, contained more call elements and drum beats, and lower fundamental and peak frequencies. Moreover, during the alpha takeover pant hoots were shorter, contained fewer drum beats and higher fundamental frequencies. Additionally, pant hoot and aggression rates were also highest during the alpha takeover with leaf clipping more likely to occur on days when pant hooting rates were high. Overall social rank had limited effects on pant hoot variation. We suggest that elevated arousal and aggression during the alpha takeover triggered the re-emergence of leaf clipping and the associated acoustic changes in pant hoots. Further research should focus on the potential mechanisms by which leaf clipping is connected to variation in pant hoots and cross-population comparisons of the behaviour.

Trade-offs in the production of animal vocal sequences: insights from the structure of wild chimpanzee pant hoots

BackgroundVocal sequences - utterances consisting of calls produced in close succession - are common phenomena in animal communication. While many studies have explored the adaptive benefits of producing such sequences, very little is known about how the costs and constraints involved in their production affect their form. Here, we investigated this issue in the chimpanzee (Pan troglodytes schweinfurthii) pant hoot, a long and structurally complex vocal sequence comprising four acoustically distinct phases – introduction, build-up, climax and let-down.ResultsWe found that in each of these phases, and for the sequence as a whole, there was a negative relationship between the number of calls produced and their average duration. There was also a negative relationship between the total duration of some adjacent phases. Significant relationships between the fundamental frequency of calls and their number or duration were found for some phases of the sequence, but the direction of these relationships differed between particular phases.ConclusionsThese results indicate that there are trade-offs in terms of signal duration at two levels in pant-hoot production: between call number and duration, and between the relative durations of successive phases. These trade-offs are likely to reflect biomechanical constraints on vocal sequence production. Phase-specific trade-offs also appear to occur between fundamental frequency and call number or duration, potentially reflecting that different phases of the sequence are associated with distinct types of information, linked in different ways to call pitch. Overall, this study highlights the important role of costs and constraints in shaping the temporal and acoustic structure of animal vocal sequences.

Sequential information in a great ape utterance.

Birdsong is a prime example of acoustically sophisticated vocal behaviour, but its complexity has evolved mainly through sexual selection to attract mates and repel sexual rivals. In contrast, non-human primate calls often mediate complex social interactions, but are generally regarded as acoustically simple. Here, we examine arguably the most complex call in great ape vocal communication, the chimpanzee (Pan troglodytes schweinfurthii) ‘pant hoot’. This signal consists of four acoustically distinct phases: introduction, build-up, climax and let-down. We applied state-of-the-art Support Vector Machines (SVM) methodology to pant hoots produced by wild male chimpanzees of Budongo Forest, Uganda. We found that caller identity was apparent in all four phases, but most strongly in the low-amplitude introduction and high-amplitude climax phases. Age was mainly correlated with the low-amplitude introduction and build-up phases, dominance rank (i.e. social status) with the high-amplitude climax phase, and context (reflecting activity of the caller) with the low-amplitude let-down phase. We conclude that the complex acoustic structure of chimpanzee pant hoots is linked to a range of socially relevant information in the different phases of the call, reflecting the complex nature of chimpanzee social lives.

Audience effects in chimpanzee food calls and their potential for recruiting others

One proposed benefit for the seemingly costly behaviour of food calling is the recruitment of social allies and mates by the signaller. In chimpanzees, food calls are only produced for approximately half of all feeding events. Therefore, we investigated the influence of social and ecological context on the probability of making a food call upon arriving to a food patch in a group of wild chimpanzees. First, we tested whether feeding events where food calls had been uttered did in fact attract more individuals to join the caller. Secondly, we examined the influence of two sources of audience effects: those who were physically present with the caller and those who were presumed nearby but out of sight, and thirdly the effect of various ecological factors. We found that when feeding on fruit species, events where food calls had been produced had a higher probability of group mates arriving, even whilst controlling for the effect of pant hoots. Furthermore, the probability of uttering a food call was motivated by social more than ecological context. Specifically, high-ranking males were more likely to make food calls when estrous females were nearby, while low-ranking males and females generally called more when more females were nearby, irrespective of their reproductive state. These effects were independent of the increase in food call probability when male callers were accompanied by more males. Our findings support the recruitment function of food calls and suggest that high-ranking males call to attract estrous females to food patches to obtain mating opportunities.

Social, contextual, and individual factors affecting the occurrence and acoustic structure of drumming bouts in wild chimpanzees (Pan troglodytes).

The production of structured and repetitive sounds by striking objects is a behavior found not only in humans, but also in a variety of animal species, including chimpanzees (Pan troglodytes). In this study we examined individual and social factors that may influence the frequency with which individuals engage in drumming behavior when producing long distance pant hoot vocalizations, and analyzed the temporal structure of those drumming bouts. Male chimpanzees from Budongo Forest, Uganda, drummed significantly more frequently during travel than feeding or resting and older individuals were significantly more likely to produce drumming bouts than younger ones. In contrast, we found no evidence that the presence of estrus females, high ranking males and preferred social partners in the caller's vicinty had an effect on the frequency with which an individual accompanied their pant hoot vocalization with drumming. Through acoustic analyses, we demonstrated that drumming sequences produced with pant hoots may have contained information on individual identity and that qualitatively, there was individual variation in the complexity of the temporal patterns produced. We conclude that drumming patterns may act as individually distinctive long-distance signals that, together with pant hoot vocalizations, function to coordinate the movement and spacing of dispersed individuals within a community, rather than as signals to group members in the immediate audience.

Social and ecological correlates of long-distance pant hoot calls in male chimpanzees

Long-distance calls have a variety of functions in different animal species. However, where multiple functions are proposed for a single long-distance call type, little is known about their relative importance. Chimpanzees are one species where several functions have been proposed for their long-distance call, the pant hoot. In this study, we investigated the effect of social factors, including the rank of the caller, party size, fission–fusion rates, and the presence of estrus females as well as ecological factors including the type of food consumed and travel time, on male chimpanzee (Pan troglodytes schweinfurthii) pant hooting, in order to identify the key correlates of this behavior. The wild male chimpanzees of the Kanyawara community, Uganda, produced more pant hoots on days when there were frequent changes in the male, but not female, composition of the focal’s party. This factor accounted for the largest amount of variation in pant hoot production, and we found that males were more likely to repeat a call prior to rather than after fusion with other males, suggesting that the calls facilitate fusion. Pant hoots therefore seem to play a pivotal role in regulating grouping dynamics in chimpanzees. Our study also shows that pant hooting was positively correlated with the rank of the caller, the presence of parous females in estrus, and the consumption of high-quality food, suggesting that pant hoots signal social status or social bonds when between-male competition is high. This study supports the view that pant hoots fulfill a complex social function.

Chimpanzee food calls are directed at specific individuals

If primates were capable of vocalizing to inform a receiver about an external entity, it would represent an important element of continuity with human language. We tested experimentally whether chimpanzee rough grunts, which function to refer to food, are produced selectively, indicating voluntary control, and whether they are directed at specific individuals. These are prerequisites for a system capable of actively informing others about external events. We conducted a field playback experiment in which we presented silently feeding male chimpanzees, Pan troglodytes , with arrival pant hoots of a familiar group member. We found that subjects were significantly more likely to respond with food calls to the simulated arrival of an individual with whom the caller had a high rather than low level of friendship and where there was a large rather than small positive dominance rank difference between the individuals (i.e. caller was lower ranking). We concluded that chimpanzee food calls are not simply reflexive responses to food, but can be selectively directed at socially important individuals. Our findings are thus inconsistent with traditional views of primate vocalizations as inflexibly and indiscriminately produced. Instead, our results indicate that great apes can produce semantically meaningful calls in a highly selective, recipient-directed manner. Further research is needed to test whether chimpanzees use this flexible system to inform ignorant individuals about food, but the prerequisites to support this type of communication seem to be present. • Field playback experiment shows chimpanzees direct food calls to specific recipients. • Male chimpanzees selectively produce calls for the arrival of high-ranking friends. • Functionally referential chimpanzee food calls are not reflexive responses to food. • Chimpanzees produce referential calls in a voluntary recipient-directed manner. • Challenges view that primate vocal production is inflexible and indiscriminate.

Human Identity and the Evolution of Societies

Human societies are examined as distinct and coherent groups. This trait is most parsimoniously considered a deeply rooted part of our ancestry rather than a recent cultural invention. Our species is the only vertebrate with society memberships of significantly more than 200. We accomplish this by using society-specific labels to identify members, in what I call an anonymous society. I propose that the human brain has evolved to permit not only the close relationships described by the social brain hypothesis, but also, at little mental cost, the anonymous societies within which such alliances are built. The human compulsion to discover or invent labels to "mark" group memberships may originally have been expressed in hominins as vocally learned greetings only slightly different in function from chimpanzee pant hoots (now known to be society-specific). The weight of evidence suggests that at some point, conceivably early in the hominin line, the distinct groups composed of several bands that were typical of our ancestors came to be distinguished by their members on the basis of multiple labels that were socially acquired in this way, the earliest of which would leave no trace in the archaeological record. Often overlooked as research subjects, these sizable fission-fusion communities, in recent egalitarian hunter-gatherers sometimes 2,000 strong, should consistently be accorded the status of societies, in the same sense that this word is used to describe tribes, chiefdoms, and other cultures arising later in our history. The capacity of hunter-gatherer societies to grow sufficiently populous that not all members necessarily recognize one another would make the transition to larger agricultural societies straightforward. Humans differ from chimpanzees in that societal labels are essential to the maintenance of societies and the processes giving birth to new ones. I propose that anonymous societies of all kinds can expand only so far as their labels can remain sufficiently stable.

Pant hoot chorusing and social bonds in male chimpanzees

Vocal interactions, such as call exchanges or chorusing, are common behaviours in many animal species and their function has often been attributed to social bonding. However, few studies have investigated the effectiveness of vocalizations as bonding signals in comparison to other affiliative behaviours. We tested the hypothesis that male chimpanzee, Pan troglodytes schweinfurthii, pant hoot chorusing, a common behaviour in these primates, is a reliable but also flexible signal of affiliative relationships. The results of our study, conducted on the Kanyawara community of chimpanzees in Uganda, show that males were more likely to join in with the pant hoot of preferred long-term social partners to form a chorus. This supports the hypothesis that this behaviour is a good indicator of strong or long-term social bonds between male chimpanzees. However, our results also show that pant hoot chorusing reliably reflects short-term affiliations between males. For instance, male dyads were more likely to be involved in affiliative behaviours, such as reciprocated grooming, joint nonvocal displays and coalitions, on days when they chorused together, compared to days when they did not. This pattern applied to both preferred and neutral social partners. Moreover, on a short-term basis chorusing was a better indicator of other affiliative behaviours than grooming. We suggest that in male chimpanzees pant hoot choruses are efficient signals of short-term affiliative relationships. We conclude that potentially low-cost bonding behaviours such as coordinated vocal displays might be especially adaptive in highly fluid fission–fusion societies where grouping patterns are often unpredictable.

社会の学としての霊長類学：『他者』としての他個体と『社会的な複雑さ』チンパンジーの長距離音声を介した行為接続のやり方と視界外に拡がる場の様態

Chimpanzees form a multi-male/multi-female unit-group and the members of the group usually split into temporary parties. They sometimes exchange long-distance calls, pant-hoots (PH) between parties and utter PH in chorus within a party. Although most studies on PH have focused on the sociobiological functions for vocalizers or the referential functions, such studies based on individualistic or anthropocentric viewpoint do not tell us how chimpanzees interact with others by using PH. This study aims to analyze how the chimpanzees of Mahale connect their actions through PH and how they organize their social fields beyond visual contact with involvement of others in sight, by applying ethnomethodology and focusing on hearers’ actions. Case analyses based on the result of their usual interval for exchanging PH (10 sec.) revealed that both vocalizers and hearers practiced the same “call-answer” form. Utilizing this form, not only vocalizers could construct an auditory social field between parties but also non-answering hearers could observe the field. Even if PH hearers headed to the place from which they heard the PH, they sometimes restrained themselves from answering to hear whether or not the answer was uttered by other parties, and anew uttered PH to elicit the voluntary answer from the party with whom they tried to interact. Meanwhile, PH hearers sometimes answered immediately regardless of the contexts in which the PH was uttered. Employing these two hands, they would organize their auditory social fields. Once two parties constructed their auditory social field by exchanging PH, one could wait for next call from another or they could get to meet visually with repeating PH exchanges. On the other hand, PH hearers sometimes terminated or deferred those interactions, which hearers’ attitudes were supposed to generate the society in which the members could stay apart from each other even beyond PH distance. In conclusion, these hearers’ various actions show the sociality related to their dynamic fission-fusion society and engaging in non-referential interactions. When we say that chimpanzees’ society is complex, we may be often thinking of the process of interactions resulted from such their unique sociality.

Vocal, gestural and locomotor responses of wild chimpanzees to familiar and unfamiliar intruders: a playback study

Wild chimpanzees can be dangerously violent against individuals that do not belong to their community, indicating a strong selection pressure on decision-making abilities in this context. The presence of a neighbouring male indicates a serious threat, although encountering an unfamiliar male is potentially even more dangerous because it indicates the arrival of a new group with whom the subjects have no previous history of interaction. We conducted playback experiments with members of three chimpanzee, Pan troglodytes verus, communities in the TaiNational Park, Cote d'Ivoire, in which we simulated the unexpected presence of another chimpanzee. We tested subjects' responses to pant hoots of familiar group members, neighbouring individuals and unfamiliar strangers. We found that neighbours and strangers elicited a wider range of gestural signals than familiar group members. Vocal responses were common in all conditions. Familiar group members mainly triggered pant hoot replies, whereas neighbour and stranger trials caused screams. Across conditions, males responded more strongly than females. Male party size had an effect on subjects' vocal responses to strangers but not to neighbours, although neighbours were approached more closely. Our results show that chimpanzees are able to identify different classes of individuals by their pant hoot vocalizations, as judged by their differential responses to the calls of familiar group members, neighbours or strangers. The overall response patterns suggest that chimpanzees are aware of the different social consequences associated with encountering a neighbouring group or a group of strangers. 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Nonlinear acoustics in the pant hoots of common chimpanzees (Pan troglodytes): Vocalizing at the edge

Common chimpanzee (Pan troglodytes) "pant hoots" are multi-call events that build from quiet, consistently harmonic introductory sounds to loud, screamlike "climax" calls with acoustic irregularities known as "nonlinear phenomena" (NLP). Two possible functions of NLP in climax calls are to increase direct auditory impact on listeners and to signal physical condition. These possibilities were addressed by comparing climax calls from 12 wild chimpanzee males with "screams" and pant hoot "introduction" calls from the same individuals. Climax calls that included NLP were found to have higher fundamental frequencies (F0s) than introduction or climax calls that were purely harmonic. NLP onsets within climax calls were also specifically associated with local F0 maxima, suggesting vocalizers are vibrating their vocal folds at the upper limits of stability. Furthermore, climax calls showed far fewer NLP than did screams recorded from the same individuals, while showing equivalent or higher F0 values. Overall, the results are consistent with the hypothesis that the relative prevalence of NLP is a signal of physical condition, with callers "vocalizing at the edge" of regular, stable production while producing few NLP. The results are discussed in light of the initial hypotheses.

Contextual variation in chimpanzee pant hoots and its implications for referential communication

There have been several previous studies of the loud, long-distance call of chimpanzees, termed a ‘pant hoot’. Some have explored the possibility that there are acoustically distinct subtypes of pant hoots that communicate to distant listeners different information about the caller's behaviour, or ecological and social circumstances. However, results to date have been either inconclusive or conflicting. To help resolve these issues, we undertook research on pant hoots produced by wild chimpanzees, Pan troglodytes schwienfurthii, living in the Budongo Forest, Uganda. In this paper, we report the results of acoustic analysis of 201 pant hoot series produced by seven adult males. Principal components analysis (PCA) was used to identify key dimensions of structural variation in the calls, which were then related to a large set of behavioural, social and ecological variables associated with call production. Although there was little evidence for distinctive pant hoot subtypes according to many of the social and ecological dimensions traditionally recognized, a number of significant patterns were identified. Specifically, pant hoots were produced with higher probability at abundant food sources, and they were more likely to contain a ‘let-down’ phase when produced in specific behavioural contexts, such as travelling and upon arrival at a food source. In addition, pant hoots produced while travelling along the ground in small parties prior to joining-up with other community members were consistently different from all other pant hoots, varying reliably in the fundamental frequency of their build-up elements, the tonal quality of climax elements and in the presence of a let-down phase. We discuss the possible mechanistic bases for these differences in the pattern of calling and detailed spectral structure of the calls and consider the implications for referential communication.

Nonlinear acoustics in pant hoots of common chimpanzees (Pan troglodytes): Frequency jumps, subharmonics, biphonation, and deterministic chaos

The pant hoot calls produced by common chimpanzees (Pan troglodytes) are multi-call vocalizations that have figured prominently in investigations of acoustic communication in this species. Although pant hoots are predominantly harmonically structured, they can exhibit an acoustic complexity that has recently been linked to nonlinearity in the vocal-fold dynamics underlying typical mammalian sound production. We examined the occurrence of these sorts of nonlinear phenomena in pant hoot vocalizations, contrasting quieter and lower-pitched "introduction" components with loud and high-pitched "climax" calls in the same bouts. Spectrographic evidence revealed four kinds of nonlinear phenomena, including discrete frequency jumps, subharmonics, biphonation, and deterministic chaos. While these events were virtually never observed during the introduction, they occurred in more than half of the climax calls. Biphonation was by far the most common phenomenon, followed by subharmonics, chaos, and frequency jumps. Individual callers varied in the degree to which their climax calls exhibited nonlinear phenomena, but were consistent in showing more biphonation than other forms. These outcomes show that nonlinear phenomena are routinely present in chimpanzee pant hoots, and help lay the foundation for investigating the function of such events.

Wild Chimpanzees Produce Group‐Specific Calls: a Case for Vocal Learning?

AbstractVocal learning, where animals can modify the structure of their vocalizations as a result of experience, has been found in a range of birds and mammals. Although vocal learning is a fundamental aspect of developing spoken language, there is as yet little evidence that vocal learning occurs in primates. Here we examine whether vocal learning may occur in chimpanzees. We analysed whether wild male chimpanzees,Pan troglodytes verus,of four communities living in a similar habitat in the Taï Forest, Côte d'Ivoire, developed community specific pant hoots. If so, we expected males of three contiguous communities to have distinct pant hoots, while pant hoots of males from a fourth, distant community, located 70 km away, should only differ from those of the contiguous communities by chance. Our analysis confirmed these expectations. In addition, the acoustic distances between the pant hoots of pairs of individuals did not correlate with the genetic relatedness of those pairs, where genetic relatedness was determined using nuclear DNA analysis. Thus, neither habitat nor genetic differences accounted for the observation that there were acoustic differences in the pant hoot structure of males living in neighbouring communities, but not in those of males from a distant community. This suggests that chimpanzees may actively modify pant hoots to be different from their neighbours, providing support for the vocal learning hypothesis.