Factors influencing acoustic variability in the contact calls of wild Geoffroy’s spider monkeys

Non-human primates have long been considered to show limited flexibility in their vocal production, with changes mainly due to maturational mechanisms. However, several studies suggest that emotional, contextual and social factors are likely to affect acoustic variability in non-human primates, although it is often difficult to untangle them. In this study, we investigated whether these factors influence the acoustic variability of high whinnies, the most frequent contact call of Geoffroy’s spider monkeys (Ateles geoffroyi). We used an acoustic similarity index for each pair of high whinnies and behavioural indicators of anxiety-like emotion, context and social relationships to examine intra- and inter-individual acoustic variability. We found that anxiety-like emotion explained some acoustic variability, whereas the time two monkeys spent together did not. The main factor influencing variability was the context of emission, with high whinnies being more similar during feeding at the inter-individual level. Overall, our findings indicate a vocal convergence at the group level during feeding, possibly to enhance tolerance in a risky context.