Abstract
The alarm calls of vervet monkeys (Chlorocebus pygerythrus) constitute the classic textbook example of semantic communication in nonhuman animals, as vervet monkeys give acoustically distinct calls to different predators and these calls elicit appropriate responses in conspecifics. They also give similar sounding calls in aggressive contexts, however. Despite the central role the vervet alarm calls have played for understanding the evolution of communication, a comprehensive, quantitative analysis of the acoustic structure of these calls was lacking. We used 2-step cluster analysis to identify objective call types and discriminant function analysis to assess context specificity. Alarm calls given in response to leopards, eagles, and snakes could be well distinguished, while the inclusion of calls given in aggressive contexts yielded some overlap, specifically between female calls given to snakes, eagles and during aggression, as well as between male vervet barks (additionally recorded in South Africa) in leopard and aggressive contexts. We suggest that both cognitive appraisal of the situation and internal state contribute to the variation in call usage and structure. While the semantic properties of vervet alarm calls bear little resemblance to human words, the existing acoustic variation, possibly together with additional contextual information, allows listeners to select appropriate responses.
Highlights
Monkey alarm calls were seen as the first example of symbolic communication in nonhuman animals
For female East African vervet calls, the analysis identified a 4-cluster solution (Fig. 1A) with a silhouette coefficient (SC) = 0 .5, indicating a fairly good solution
Cluster 1 corresponded to broadband “chutter” calls typically produced in response to snakes and during intergroup encounters, cluster 2 to “chirp” calls, typically given in response to terrestrial predators, while cluster 4 corresponded to the low frequency “rraup” calls typically produced in response to raptors, and during escalated between and within-group aggression
Summary
Monkey alarm calls were seen as the first example of symbolic communication in nonhuman animals. Macedonia and Evans[8] pointed out that field observations and playback experiments provided only limited information about the mechanisms underlying call production In other words, it remained unclear whether the production of alarm calls could be likened to speech production in humans, where speakers voluntarily modify the vocal output to adhere to the conventions of their respective language community, to refer to events or objects (or ideas). It remained unclear whether the production of alarm calls could be likened to speech production in humans, where speakers voluntarily modify the vocal output to adhere to the conventions of their respective language community, to refer to events or objects (or ideas) Irrespective of this limitation in terms of identifying the mechanisms supporting vocal production, the experiments did demonstrate that listeners responded to calls as if the calls provided information about, for example, events in the environment. Because the calls of males and females differed substantially, we ran separate analyses for the two sexes, reducing the overall variation and allowing for a clearer picture within each sex
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