Alarm calls of white-faced capuchin monkeys: an acoustic analysis

Abstract

Capuchin monkeys (Cebus capucinus) are threatened by several types of predators, including various aerial, arboreal and terrestrial predators. Whenever they encounter one of these threats, capuchin monkeys show different escape strategies and utter one of several types of alarm calls. These alarm calls constitute two acoustic categories, each of which is composed of several subtypes: the first category is composed of alarm calls given in response to aerial predators, humans and other capuchin monkeys, and the second is composed of alarm calls given in response to terrestrial predators and snakes. However, within these two broad categories the different subtypes of alarm calls appeared to show a graded continuum with transitions in between. Therefore, we performed an acoustic analysis to determine whether alarm calls between and within categories are characterized by distinct acoustic features. First, the two broad alarm call categories differed in call structure. Second, alarm calls of the first category, given in response to aerial predators, humans and other monkeys, differed in several acoustic variables and thus may convey context-specific information about the type of threat to listeners. Third, alarm calls of the second category, given in response to terrestrial predators and snakes, also differed in several acoustic variables; however, variants of these alarm calls are also given in response to caiman as well as during highly aggressive interactions between conspecifics and thus may not convey specific information about the type of predator or the threat to listeners. Capuchin monkey alarm calls may therefore have the potential to be functionally referential, general and/or urgency-based signals. Future research using detailed playback experiments with alarm calls and analyses of listeners' responses is required.