Social coordination by vocal interactions in spider monkeys
Résumé
Nonhuman primate vocal repertoires are largely genetically determined. However, a growing number of studies evidences flexibility under social influences via subtle modifications in the acoustic structure of certain call types and clear context-dependent variations in call usage. Vocal interaction patterns are particularly interesting to study in this regard. Call exchanges are crucial to regulate socio-spatial cohesion in forest-dwelling species with fission-fusion grouping dynamics. Here, we studied the flexibility of call exchanges in a free-ranging habituated group of black-handed spider monkeys (Ateles geoffroyi). Observations lasted 98 days and were carried out on 41 individuals in the Otoch Ma’ax yetel Kooh protected area (Yucatan Peninsula, Mexico). We sampled all the occurrences of call emissions and vocal interactions in subgroups of individuals travelling together and scored the identities of exchanging partners, the contexts of call emissions and the acoustic structures exchanged (N= 5586 calls sampled in total). Results showed that adult males and females called at higher rates than immatures, and that whinny calls (the call type known to show an individually distinctive acoustic structure) were particularly frequent. Moreover, whinnies could be divided into three acoustic subtypes, based on the frequency range, that seem to play different social functions, one being more frequent at fusion times than the others. Whinnies triggered more vocal responses (more often being also whinnies) than any other call types, and exchanging partners matched whinny subtypes when interacting. These findings suggest a flexibility in both call production and call usage and show the importance of vocal interactions for social coordination in fission-fusion societies. On-going, further analyses will focus on the importance of social bonds and subgroup composition in vocal interaction patterns.