Pragmatic flexibility in primate vocal production
Introduction
Despite decades of research on non-human primate vocal communication, we still know little about the factors that cause individuals to give a vocalization. This is surprising, because we know a great deal about other common social behaviors, like grooming or the formation of alliances. In many mammals including primates, social bonds are adaptive: individuals who maintain close, enduring bonds with others live longer and experience greater reproductive success [1]. Measures of bond strength include grooming, proximity, and alliance formation. Notably, however, few studies of social relationships have included vocal communication in their assessments. The implicit assumption seems to be that non-vocal behaviors are sufficient to characterize the strength and quality of relationships, and that giving a vocalization is redundant or perhaps even irrelevant.
The omission of data on vocal production is puzzling because vocalizations are a pervasive feature of primate social life. Most primates vocalize far more often than they groom or form alliances. If vocalizations contribute little to the factor most closely associated with fitness — close social bonds — why vocalize at all?
Our limited understanding of call production is also striking when compared to call perception, which is known to be both nuanced and complex. Animals’ responses to calls depend on many social and contextual contingencies, including the identity of the signaller, the type of call given, the nature of recent interactions with the signaller or the signaller's close associates, and the recipient's apparent knowledge of the correlation between past and future interactions [2•, 3•, 4•]. Given that the same animals are both listeners and signallers, it seems reasonable to hypothesize that the factors affecting call production are just as complex and sophisticated as those affecting perception. Why, then, have production and perception been treated so differently?
Call production is typically assumed to be highly constrained. Primates have relatively small vocal repertoires that show little acoustic modification during development [5]. Within these constraints, however, there is considerable flexibility in call usage — the decision to call or remain silent, or to use different calls in different circumstances. Calling can be brought under operant control, and callers can modify the type of call given in different contexts [6, 7]. In the wild, call usage, like responses, depends on a multitude of cues, including the identity of potential listeners and the caller's relationship with them [3•, 4•].
Here we describe recent studies that suggest considerable flexibility in the usage of vocalizations by primates. Evidence suggests that many vocalizations function to signal the caller's intentions and to reduce uncertainty about the likely outcome of a social interaction. We briefly discuss the neural mechanisms that might underlie vocal usage and consider the implications for theories of language evolution.
Section snippets
Social and motivational correlates of vocal production
Most primate vocalizations are given during close range, face-to-face interactions. Unlike other affiliative behaviors like grooming that reflect existing social bonds, close-range calls seem to facilitate social interactions and reduce uncertainty between partners who might not otherwise interact often. For example, female chacma baboons (Papio cynocephalus ursinus) give low amplitude grunts as they approach other females [8]. Listeners can recognize the identity of callers based on acoustic
Mechanisms
Authors comparing vocal communication in human and nonhuman primates have often concluded that the former is learned and voluntary whereas the latter is involuntary, unlearned, and reflexive (e.g. [32, 33]). This dichotomy is not entirely accurate [34, 35]. In the acoustics of call production, nonhuman primates are indeed constrained: monkeys and apes seem incapable of learning entirely new calls or engaging in vocal imitation [5]. In other respects, however, call production is more flexible.
Relevance to theories of language evolution
If we assume that nonhuman primate call production is constrained by neural mechanisms rather than vocal anatomy [50•], and we further assume that vocal communication in contemporary monkeys and apes offers a reasonable approximation to the communication of our pre-linguistic hominoid ancestors, two conclusions follow.
First, long before language evolved, vocal communication was at its most complex during social interactions, where it served a somewhat different function from other social
Conclusions
Non-human primates have a relatively small repertoire of calls but use these vocalizations in a highly flexible manner. Like their responses to calls, their use of vocalizations depends upon contextual factors including the listener's identity, the listener's recent interactions with the caller and her close associates, and the caller's judgment of the listener's likely response. These assessments do not require that callers recognize mental states like ignorance in others; call usage could
Conflict of interest statement
Nothing declared.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
Acknowledgements
We thank Thore Bergman, Tecumseh Fitch, Asif Ghazanfar, and Cory Miller for comments on an earlier draft.
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