Acoustic Variability in Nonhuman Primates: Individuality and Social Context
Résumé
A growing number of studies revealed that, to some extent, parallels can be found between nonhuman primates' vocal communication and human language (e.g. conversation-like vocal exchanges, referential communication and call combination). But at the same time, nonhuman primate calls are different from speech because of their very limited flexibility. This is intriguing for people interested in the evolution of communicative abilities and in the phylogenetic origin of language. It is now commonly admitted by the scientific community that social life played an essential role in this evolution. Interestingly, vocal plasticity in nonhuman primates is certainly limited but not inexistent and they are nice models to test the influence of social factors on acoustic variability. Several studies have shown that changing the group social composition triggered changes in individual vocal signatures. For example, replacing the harem male in a captive Campbell's monkeys group leads to a rearrangement of social networks and, in parallel, to fine acoustic modifications of the female contact calls conducting to vocal sharing between preferential partners. Here we propose two ways to test the potential influence of social factors on acoustic variability. Our questions were: 1) Does the social context of calling influence the level of intra/inter-individual variability? 2) Does the social system of the species play a role on the shaping of the vocal repertoire's variability?
We conducted call recordings in two primate species presenting interesting social differences. Red-capped mangabeys (Cercocebus torquatus) live in the wild in large multi-male and multi-female groups and their social organisation is, like most baboons and macaques, strongly based on frequent peaceful and agonistic interactions. Campbell's monkeys (Cercopithecus campbelli campbelli) live in the wild in small harem groups and their social organisation is, like most forest guenons, based on rare physical interactions and a discrete hierarchy. We investigated whether the level of intra/inter-individual variability depended on the call social function. We hypothesized that, as in several other animal species, the level of acoustic variability, notably the level of individual distinctiveness, would be higher in calls presenting a high social value (e.g. contact call types) than in calls presenting a low social value (e.g. alarm call types), the former being involved in affiliative dyadic interactions and the latter consisting in a communication at the group level. In humans, individual distinctiveness can also be accentuated or attenuated by speakers according to the macro context of communication depending on the size and the composition of the audience. We then hypothesized that the variability found in affiliative versus agonistic calls would depend on the species social system.
We studied in mangabeys and guenons respectively six and five call types for which we could associate a high, intermediary and low social value. Among highly social calls we compared in both species affiliative contact calls and agonistic threat calls. Intra- and inter-individual acoustic variability was assessed by measuring a set of temporal and frequential values in the same way for all call types. We measured respectively 1416 calls from 14 individuals in mangabeys and 1348 calls from 6 individuals in guenons. We found that the degree of the call social value predicted the level of variability. For instance, we found in both species a much higher potential for identity coding in affiliative calls used during dyadic exchanges than in less social calls like alarm calls (two to four times more variable). A strong difference between the two species was found in social calls associated to a negative value. While the level of variability of threat calls was almost as high as the one of affiliative calls in mangabeys, it was as low as the one of alarm calls in Campbell's monkeys, supporting the social system effect hypothesis.
This study highlights the determinant role played by social factors on the structuring of vocal repertoires and individual acoustic variability. It opens new perspectives of comparative research in animals for understanding how human language evolved and supports the general theory of a social-vocal co-evolution.