Relation between the level of acoustic variability and the call social function in red-capped mangabeys
Abstract
Despite a strong influence of genetic factors on the structuring of nonhuman primate vocal
repertoires, the last decade evidenced to some extent abilities in vocal plasticity. Snowdon et
al. (1997) suggested a relation between the level of acoustic variability and the call function.
The highest variability is expected in calls with an affiliative social value. But this hypothesis
has rarely been tested. Here, we conducted a quantitative analysis of acoustic structures’
variability and associated contexts of emission by studying the, still unknown, vocal
repertoire of red-capped mangabeys (Cercocebus torquatus torquatus). Seven adult males and
seven adult females were studied in captivity. We found 13 structurally-different sound units
(4 peculiar to females, 3 to males and 6 shared). These units were combined to form 12 call
types (including 6 sex-specific) that were also contextually differentiable. Interestingly,
females, who are more socially integrated than males, produced a higher number of call types
and unclassifiable graded acoustic structures. The highest level of inter- and intra-individual
acoustic variability was found in the intra-group socially affiliative grunts and the lowest in
non-affiliative calls like Wahoo alarm calls and inter-group Whoop-gobble loud calls. An
intermediate pattern of variability was found in food calls for instance. Thus, our data support
Snowdon et al.’s hypothesis.