Syntactic Structures in the Vocalizations of Wedge-Capped Capuchin Monkeys, Cebus Olivaceus

Abstract

Capuchin monkeys, Cebus olivaceus, combine different types of calls to form compound calls. These compound calls are syntactically organized (i. e. , there is a predictable ordering of call types in the compound call). The analysis of the syntax of animal communication necessarily includes: (a) Accurate classification of types of calls, and a demonstration that calls that occur both alone and in compound calls are structurally similar. (b) Description of the syntactic rules generating compound calls or call sequences, and classification of types of compound calls or call sequences. (c) Examination of the social circumstances or contexts in which both single and compound calls occur. I followed these steps using a large sample of Cebus olivaceus calls, recorded in riparian gallery forest in central Venezuela. The calls were initially screened using a real-time spectrum analyzer, and a group of structurally related call types was selected for further analysis. Temporal and frequency characteristics of 868 of these calls were measured from sound spectrograms produced on a sound sonograph. Call classification involved first defining a very large number of possible call types on the basis of these characteristics, and then using a discriminant analysis to identify which types should be lumped together. A stepwise procedure was followed, using only calls which occurred singly, until five call types (squaws, chirps, trills, whistles, screams) were statistically separable. Two of these types (trills, whistles) showed considerable within-type variation and were further subdivided into four variants each. Discriminant analysis was then used to demonstrate structurally similarity between calls produced singly and those in compound calls. Social circumstances were defined using similarities in the vocalizer's actions, arousal, orientation to and distance from the presumed receiver. Use of call types, when given singly, covaried predictably with social circumstances and presumably with the internal state of the vocalizer. Different calls expressed different internal states on a continuum from contact-seeking to contact-avoiding. Use of trill and whitle variants also covaried with social circumstances. Different variants expressed different states on a continuum from affiliation or submission to aggression. Combinations of internal states, in theory, might be expressed as intergradations or intermediates between different call types. However, such intermediate states are often coded syntactically. Syntactically organized compound calls accounted for 38% of the total sample. The distribution of social circumstances in which compound calls are given was intermediate between the distributions of the constituent call types, which presumably indicates an intermediate internal state. Compound calls are generated by syntactic rules closely analogous to lexical rules of human language. Specifically they act like compounding rules that combine two lexical entries to form a third. They are not analogous to the grammatical rules that generate human sentences.