An experimental analysis of alarm calling behavior in wild tufted capuchin monkeys (<i>Cebus apella nigritus</i>)
Alarm calls are vocalizations given by prey species upon detecting a predator or other threatening stimuli. Because these calls are typically given in moments of potentially significant fitness consequence (i.e., escaping from or falling prey to a predator), they are subject to strong selective pressures. As such, these apparently altruistic calls have received much attention from scientists interested in referential communication, the ontogeny of call production and response, the adaptive significance of the calls, and the potential to use them “deceptively”. While such questions have been addressed in a wide range of taxa including birds, rodents, and ungulates, studies of primate alarm calls have been generally limited to the degree to which they contain referential information. Further, these studies have been primarily limited to Old World monkeys and Malagasy lemurs. This study takes a largely experimental approach to investigate the alarm call system of a New World primate, the tufted capuchin monkey (Cebus apella nigritus), in Iguazú National Park, Argentina.
Specifically, this dissertation addresses four main questions regarding the alarm calling behavior of tufted capuchin monkeys: (1) Are the alarm calls of this species functionally referential signals? (2) Does learning play any role in the ontogeny of alarm call use or response, or are these anti-predator behaviors innate? (3) What is the adaptive significance of the calls? and (4) Do capuchin monkeys use these calls in a functionally deceptive manner to usurp resources? If so, what ecological and social conditions lead to such uses of alarm calls?
To address these questions, experiments using predator decoys, live predators, vocalization playbacks, and provisioning platforms hoisted into the forest canopy were combined with natural observations. The results indicate that: (1) the capuchin monkeys have a functionally referential aerial predator call, but the call most often associated with terrestrial predators is also given in non-predatory contexts; (2) the contexts in which alarm calls should be used and the appropriate ways to respond to alarm calls from others are mostly (but not completely) under-developed in infants, but the degree to which these behaviors are learned or innate requires additional investigation; (3) the adaptive function of alarm calling varies with predator type - it is largely a selfish behavior that reduces the likelihood of predation for the caller, although callers also seem to benefit by warning offspring and collateral kin of the presence of venomous snakes; and 4) terrestrial predator-associated calls are sometimes used “deceptively” by subordinates to elicit anti-predator reactions in call receivers, allowing them to usurp contestable resources from conspecifics.
Taken together, these results indicate that the alarm call system of tufted capuchin monkeys in Iguazú has been largely shaped by several features of their ecology, including the predator guild of the study site, the monkeys’ arboreal habits, and the density of vegetation in their habitat. However, additional research is needed to understand the roles that emotion, cognition, and physiology play as proximate mechanisms driving alarm calling behavior.