Pawel Fedurek

Primate Vocal Communication: A Useful Tool for Understanding Human Speech and Language Evolution?

Abstract Language is a uniquely human trait, and questions of how and why it evolved have been intriguing scientists for years. Nonhuman primates (primates) are our closest living relatives, and their behavior can be used to estimate the capacities of our extinct ancestors. As humans and many primate species rely on vocalizations as their primary mode of communication, the vocal behavior of primates has been an obvious target for studies investigating the evolutionary roots of human speech and language. By studying the similarities and differences between human and primate vocalizations, comparative research has the potential to clarify the evolutionary processes that shaped human speech and language. This review examines some of the seminal and recent studies that contribute to our knowledge regarding the link between primate calls and human language and speech. We focus on three main aspects of primate vocal behavior: functional reference, call combinations, and vocal learning. Studies in these areas indicate that despite important differences, primate vocal communication exhibits some key features characterizing human language. They also indicate, however, that some critical aspects of speech, such as vocal plasticity, are not shared with our primate cousins. We conclude that comparative research on primate vocal behavior is a very promising tool for deepening our understanding of the evolution of human speech and language, but much is still to be done as many aspects of monkey and ape vocalizations remain largely unexplored.

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.

The acoustic structure of chimpanzee pant-hooting facilitates chorusing

Duetting or chorusing behaviour occurs in a wide variety of animals and is posited to fulfil various important functions including territory defence and social bonding. The structure of calls produced in choruses might be shaped in a way that facilitates such joint vocal displays. In this study, we test the hypothesis that flexibility to modify the temporal structure of chimpanzee pant-hoots, vocalisations often given jointly with other individuals, facilitates chorusing. The results of this study, which was conducted on two communities of wild chimpanzees in Uganda, support this hypothesis. First, the duration of the build-up phase of the pant-hoot correlated with the latency with which the partner joined in the call, suggesting that males prolong the duration of the build-up to allow others to join in the call and to increase the likelihood of a chorus occurring. Second, the loud climax phases were significantly longer when produced in choruses than alone, which suggests that males prolong this part of the call when calling in choruses. Within chorus pant-hoots, there was a positive relationship between the number of climax elements given by two calling partners, suggesting that males adjust the temporal structure of their call to mirror their partners call. We conclude that the basic acoustic structure of chimpanzee pant-hoots and the flexibility with which males adjust the duration of the constituent phases promote chorusing, and that the temporal structure of this rather stereotyped vocalisation is sensitive to fine details of the vocal responses of the audience.

Chimpanzee lip-smacking facilitates cooperative behaviour

Signalling plays an important role in facilitating and maintaining affiliative or cooperative interactions in social animals. Social grooming in primates is an example of an interaction that requires coordination between partners but little is known about communicative behaviours facilitating this activity. In this study, we analysed the communication of wild chimpanzees of Budongo Forest, Uganda, as they entered and maintained a naturally occurring cooperative interaction: social grooming. We found that lip-smacking, a distinct multimodal oral gesture produced during grooming, coordinated this activity. Lip-smacking at the beginning of grooming bouts was significantly more often followed by longer and reciprocated bouts than silent grooming initiations. Lip-smacks were more likely to be produced when the risk of termination of the interaction by the recipient was high, for instance when grooming vulnerable body parts. Groomers were also more likely to produce lip-smacks during face-to-face grooming where the visual aspect of the signal could be perceived. Data are consistent with the hypothesis that chimpanzee lip-smacks function to coordinate and prolong social grooming, suggesting that this oral signal is an example of a communicative behaviour facilitating cooperative behaviour in chimpanzees.

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.

The Social Function of Food-Associated Calls in Male Chimpanzees

There is an ongoing debate about the adaptive function of chimpanzees food‐associated calls. Here, we tested a new hypothesis that food‐associated calls in male chimpanzees function to signal that the calling individual is likely to initiate or prolong feeding. We propose that the signal functions to coordinate activities between individuals and that its ultimate function is to retain the nearby individuals in proximity. To test this hypothesis, we collected data on social and ecological correlates of food‐associates calls in male chimpanzees. The results of this study, which was conducted on the Kanyawara community in the Kibale National Park, Uganda, show that males tended to feed for significantly longer after giving food‐associated calls upon initiating feeding than after remaining silent. The type of audience had a significant effect on food calling, with males producing food‐associated calls more often when males rather than females and preferred rather than neutral male social partners were in close proximity. However, the total party or male party size did not correlate with food calling behaviors, suggesting that the signal “targets” those in close proximity. Finally, a male feeding partner was more likely to remain with the focal until the end of a feeding bout after the focal gave a food‐associated call at the beginning of the feeding bout than when he was silent. These results support our hypothesis and suggest that one of the functions of food calling in chimpanzees might be signaling that the caller is likely to initiate and prolong a feeding bout. This information might be used by receivers to make the decision whether or not to stay with the calling individual on a feeding patch or leave him (fission). The study suggests therefore that ultimately the function of food calling might be to coordinate feeding behaviors between males. Am. J. Primatol. 75:726‐739, 2013.

The relationship between testosterone and long-distance calling in wild male chimpanzees

Long-distance calling is a common behaviour in animals, which has various important social functions. At a physiological level, calling is often mediated by gonadal hormones such as testosterone (T), particularly when its function is linked to intra-sexual competition for mates or territory. T also plays an important role in the development of vocal characteristics associated with dominance in humans. However, the few available studies of T and vocal behaviour in non-human primates suggest that in primates, T has less influence on call production than in other animals. We tested this hypothesis by studying the relationship between T concentrations and pant-hooting in wild male chimpanzees (Pan troglodytes schweinfurthii) of the Kanyawara community in the Kibale National Park, Uganda. We found three kinds of correlation. Hourly T averages were positively associated with hourly rates of pant-hooting. Monthly T levels were likewise correlated with monthly rates of pant-hooting after controlling for other influences such as fission-fusion rates. Finally, males with high T levels had higher peak frequency at the start of the call climax. These results suggest that T affects the production of pant-hoots in chimpanzees. This implies that the pant-hoot call plays a role in male-male competition. We propose that even in cognitively sophisticated species, endocrine mechanisms can contribute to regulating vocal production.Significance statementMany animals produce long-distance calls. The production of these calls is often modulated by gonadal hormones such as testosterone, especially if the calls are involved in competition between males for mates or territory. However, comparatively little is known about the influence of testosterone over the vocal behaviour of non-human primates, especially among great apes. In this study, we examined the relationship between testosterone and pant-hooting in wild male chimpanzees. We found that testosterone levels were associated with pant-hoot rates and one acoustic feature of the call. More specifically, males pant-hooted more often and produced pant-hoots with higher peak frequencies during periods of elevated testosterone levels. These results imply that gonadal hormones are involved in regulating vocal behaviour in chimpanzees and support the view that pant-hoots play a role in male-male competition.

Chimpanzees communicate to two different audiences during aggressive interactions

Conflict and aggressive interactions are common phenomena in group-living animals and vocal behaviour often plays an important role in determining their outcomes. In some species, vocal signals seem to provide bystanders with information about the nature of an ongoing aggressive interaction, which can be beneficial for the victims. For example, in chimpanzees and some other primates, victims adjust their screams depending on the composition of the by-standing audience, probably to solicit their support. Considerably less is known, however, about the role of other call types produced by victims of aggression. In this study, we focused on the fact that, immediately after screams, chimpanzee, Pan troglodytes schweinfurthii, victims often produce ‘waa’ barks, but little is known about their function. Our results showed that for screams, but not ‘waa’ barks, production was dependent on the audience composition with victims being more likely to scream when adult or late-adolescent males were in close proximity. We also found that after ‘waa’ barking, but not screaming, victims were more likely to retaliate against and less likely to reconcile with their aggressors, and that ‘waa’ barking was more common after victims had received support from other party members. These results suggest that, in chimpanzees, victims of aggression vocalize with a dual social strategy of attempting to recruit support from bystanders and to repel their attackers by signalling readiness to retaliate. We conclude that victim scream and ‘waa’ bark calls, although often produced during the same agonistic event, are directed at different audiences and fulfil different social functions, and that these calls can mediate both aggressive interactions and aggressor–victim relationships following aggression.

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.

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.