Temporal and Structural Analysis of Affiliative Vocal Exchanges in Squirrel Monkeys (Saimiri Sciureus)

Affiliative, aggressive, and vocal behaviors were observed in a group of five adult female squirrel monkeys. Affiliative partner preferences were correlated with use of the chuck vocalization. Mutually preferred partners exchanged chucks more often and with shorter latencies of response. Animals with few affiliative contacts were more likely to repeat themselves, and to obtain responses at longer latencies. Chuck exchanges neither follow nor precede changes in nonvocal behavior. Structural differences associated with the position of a chuck within a sequence were found. The peak frequency attained by several acoustic features of chucks was higher in chucks that followed closely another chuck compared with chucks that followed a silent period. We have termed these vocalizations "answer" and "question" chucks respectively. Among answer chucks, those that terminate an exchange have higher peak frequencies than those that become part of a longer sequence. These results are viewed as evidence for flexible use of signals in an exclusively linguistic context, and may be an example of categorical prosody.