Age and social affinity effects on contact call interactions in free-ranging spider monkeys

Abstract: Nonhuman primates’ vocal repertoire has shown little plasticity, with immatures producing adult-like acoustic structures. Yet, the use of different call types shows a degree of socially dependent flexibility during development. In several nonhuman primate species, group members exchange contact calls respecting a set of social and temporal rules that may be learned (e.g., overlap avoidance, turn-taking, social selection of interacting partners, and call type matching). Here, we study the use of contact calls in free-living adult and immature (old and young) spider monkeys (Ateles geoffroyi). We focused our study in two contact call types of the species’ repertoire: whinnies and high-whinnies. Our results suggest that individuals in all age classes produced both call types, with immatures producing less frequently the whinny call type. Immature individuals exchanged calls less often than adults, although their contribution increased with age. Conversely, mature individuals regulated their emissions by (1) exchanging more calls with their preferred affiliative partner and (2) matching the call type, while immatures did not. Our results show that contact call usage changes during development and suggest that adult rules might be learned. We argue that call matching is a “conversational rule” that young individuals acquire with apparent call-type-dependent variations during development. Our findings support the idea that social factors influence vocal development in nonhuman primates. Significance statement: We studied the social rules underlying vocal interaction patterns in free-ranging spider monkeys. We found that, while both immature (old and young) and mature individuals were able to produce the two species contact call types, they differed strongly in the way they used them. Matures called more often and exchanged more, while the vocal response rates of immature individuals increased with age. Also, mature individuals exchanged preferentially with their close associates and matched their call types while immatures did not. As in other species, we predict that these exchange patterns serve as a social rule to maintain and strengthen social bonds between individuals. We discuss our findings in light of the probable role of social learning during acquisition of the appropriate context of calling and of the response to others’ calls. These findings support the idea that social influences guide vocal development in nonhuman primates.