Function of loud calls in wild bonobos

Under the social origins hypothesis, human language is thought to have evolved within the framework of non-human primate social contexts and relationships. Our two closest relatives, chimpanzees and bonobos, however, have very different social relationships and this may be reflected in their use of loud calls. Much of loud calling in the male-bonded and aggressive chimpanzee functions for male alliance formation and intercommunity aggression. Bonobos, however, are female bonded and less aggressive and little is known on the use and function of their loud calls. Data on frequencies, context, and locations of vocalizations were collected for wild bonobos, Pan paniscus, at the Lomako Forest study site in the Democratic Republic of the Congo from 1983 to 2009. Both males and females participated in loud calls used for inter-party communication. Calling and response rates by both males and females were higher during party fusion than party fission and were common at evening nesting. The distribution of loud calls within the community range of loud calls was not random with males calling significantly more towards the periphery of the range and females calling significantly more in central areas. Calling and party fission were common at food patches. Responses were more frequent for female calls than for male calls. Calling, followed by fusion, was more frequent when a small party called from a large patch. We conclude that bonobo females and males loud calls can function in inter-party communication to call others to large food patches. Females call to attract potential allies and males call to attract potential mates. Our results support the social hypothesis of the origin of language because differences in the function and use of loud calls reflect the differing social systems of chimpanzees and bonobos. Bonobo loud calls are important for female communication and function in party coordination and, unlike chimpanzees, are less important in male cooperative aggression.

Scientific Reports

the origin of human speech is still a hotly debated topic in science. evidence of socially-guided acoustic flexibility and proto-conversational rules has been found in several monkey species, but is lacking in social and cooperative great apes. Here we investigated spontaneous vocal interactions within a peaceful context in captive bonobos to reveal that vocal interactions obey temporally and social rules. Dyadic vocal interactions were characterized by call overlap avoidance and short inter-call intervals. Bonobos preferentially responded to conspecifics with whom they maintained close bonds. We also found that vocal sharing rate (production rate of shared acoustic variants within each given dyad) was mostly explained by the age difference of callers, as other individual characteristics (sex, kinship) and social parameters (affinity in spatial proximity and in vocal interactions) were not. Our results show that great apes spontaneously display primitive conversation rules guided by social bonds. The demonstration that such coordinated vocal interactions are shared between monkeys, apes and humans fills a significant gap in our knowledge of vocal communication within the primate phylogeny and highlights the universal feature of social influence in vocal interactions. The evolutionary origins of language and speech remains a fundamental question in science. In particular, whether clues to the origins of speech are present in nonhuman primate communication remains a hotly debated topic 1-4. Despite the diversity of social cultures and languages in humans, universal features in conversations are found across all languages, such as the avoidance of overlapping and a minimum gap between turns 5-8. Orderly vocal exchanges (antiphony between two or more animals or duets within male-female pairs 9) have been found across the primate order: from lemurs 10 , to New World monkeys 11-15 , Old World monkeys 16 and lesser apes 17,18. Vocal turn-taking appears to be associated with social life and cooperation capacities 1,2,5,9,19-21. It is thought to maintain and reinforce social bonds between individuals (e.g. in non-human primates 10,22), enable the extraction of information in the absence of overlap (e.g. 23 but see 24) and reduce stress as in the case of social grooming 25. Vocal exchange is "a characteristic communication style in which a sender produces a vocalization to address a receiver, and the receiver emits a call in response within a brief interval" (cited from 26). Vocal exchange patterns are influenced by social factors in non-human primates. 'Interlocutors' are not randomly selected, and preference is given to elders 11,27-29 , social allies 12,14,22 or dominant individuals 30,31. The attention of the audience also influences vocal outputs leading to persistence (repetition of calls) and elaboration (changes in the acoustic structure of calls) in situations where no response has been received 32,33. Shared primitive forms of vocal turn-taking within non-human primate species might suggest an ancient evolutionary origin 1,34. Surprisingly, however, studies based on great apes are scarce and controversial. No evidence of spontaneous vocal coordinated exchanges has been found in wild chimpanzees 35 , who display complex social interactions and cooperative abilities 36. Indeed, Arcadi 35 found that chimpanzees do not "respond" to the majority of calls they heard (within 5 sec), and that instead, bonded males tend to chorus together, matching each-other's pant hoots 37,38. Nevertheless, a recent study in great apes found for the first time that captive gorillas display some rule-governed call exchanges 31. Relying on our current knowledge, vocal turn-taking is thus reported across phylogenetically distant species (monkeys and more generally in some social mammals such as African elephants 39 , bottlenose dolphins 40 , bats 41,42 , naked mole-rats 43) but with some apparent discontinuities among great apes. More investigations among great ape species, our closest, highly social, relatives, are thus necessary in order to ascertain if vocal-turn taking behavior is as a result of convergent evolution (analogies as adaptations to similar social requirements) or is shared ancestry (homologies which are inheritance behaviours) 34 .

Behavioral Ecology and Sociobiology, 2010

Chimpanzees produce acoustically distinct calls when encountering food. Previous research on a number of species has indicated that food-associated calls are relatively widespread in animal communication, and the production of these calls can be influenced by both ecological and social factors. Here, we investigate the factors influencing the production of food-associated calls in wild chimpanzees and examine whether male chimpanzees produce food-associated calls selectively in the presence of important social partners. Male chimpanzees form stable long-term social relationships with each other, and these social bonds are vital in enabling a range of cooperative activities, such as group hunting and territory defence. Our data show that males were significantly more likely to produce food-associated calls if an important social partner was nearby, regardless of the size of the audience or the presence of oestrus females. Call production was also mediated by the size of the food patch and by whether or not the food could be monopolised. The presence of important social partners explained most of the variation in male calling behaviour, indicating that food-associated calls are socially directed and serve a bonding function.

PLoS ONE, 2014

Contest hoots' are acoustically complex vocalisations produced by adult and subadult male bonobos (Pan paniscus). These calls are often directed at specific individuals and regularly combined with gestures and other body signals. The aim of our study was to describe the multi-modal use of this call type and to clarify its communicative and social function. To this end, we observed two large groups of bonobos, which generated a sample of 585 communicative interactions initiated by 10 different males. We found that contest hooting, with or without other associated signals, was produced to challenge and provoke a social reaction in the targeted individual, usually agonistic chase. Interestingly, 'contest hoots' were sometimes also used during friendly play. In both contexts, males were highly selective in whom they targeted by preferentially choosing individuals of equal or higher social rank, suggesting that the calls functioned to assert social status. Multi-modal sequences were not more successful in eliciting reactions than contest hoots given alone, but we found a significant difference in the choice of associated gestures between playful and agonistic contexts. During friendly play, contest hoots were significantly more often combined with soft than rough gestures compared to agonistic challenges, while the calls' acoustic structure remained the same. We conclude that contest hoots indicate the signaller's intention to interact socially with important group members, while the gestures provide additional cues concerning the nature of the desired interaction.

Ethology and Sociobiology

Journal of Comparative Psychology, 2005

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. Primates vocalize to one another most often during evolutionarily relevant events, such as predator avoidance, defense against aggressors, and food discovery. The relationship between the function of vocalizations and their acoustic structure has been the focus of much research, with relationships reported between the acoustic structure of calls and the caller's motivational state , physical attributes of the caller , and the occurrence of discrete external events (reviewed in . From a cognitive perspective, the most interesting studies are those showing that individuals produce calls in response to discrete external events, such as the appearance of a predator or the occurrence of a specific social context (e.g.,

Ethology and Sociobiology

Biology Letters, 2005

Understanding the rules that link communication and social behaviour is an essential prerequisite for discerning how a communication system as complex as human language might have evolved. The comparative method offers a powerful tool for investigating the nature of these rules, since it provides a means to examine relationships between changes in communication abilities and changes in key aspects of social behaviour over evolutionary time. Here we present empirical evidence from phylogenetically controlled analyses indicating that evolutionary increases in the size of the vocal repertoire among non-human primate species were associated with increases in both group size and time spent grooming (our measure of extent of social bonding).

Animal Behaviour, 2010

Biology Letters, 2011

During mating events, females of many primate species produce loud and distinct vocalizations known as ‘copulation calls’. The adaptive significance of these signals is considered to be in promoting the caller's direct reproductive success. Here, we investigated copulation calling in bonobos ( Pan paniscus ), a species in which females produce these vocalizations during sexual interactions with partners of both sexes. Females were more likely to call when mating with males than with females. We also observed a positive relationship between the likelihood of calling and partner rank, regardless of partner sex. Sexual activity generally increased with swelling size (an indicator of reproductive state) and, during their peak swelling, females called more with male than with female partners. Female bonobos are unusual among the non-human primates in terms of their heightened socio-sexuality. Our results suggest that in this species, copulation calls have undergone an evolutionary tr...