Temporal Rules Regulating Affiliative Vocal Exchanges of Squirrel Monkeys

All contact calls (chuck vocalizations) occurring in a group of ten captive squirrel monkeys were recorded over 200 minutes. The temporal distribution of inter-chuck intervals was bimodal, with one peak at short intervals (0.1-0.2 sec) and another peak at relatively longer intervals (1.4-1.5 sec). Sequalae analysis revealed that in two consecutive chucks separated by intervals longer than 0.5 sec, the following call took place independently of the preceding chuck. If two chucks occurred at intervals of 0.5 sec or less, on the other hand, the following caller vocalized in response to the preceding vocalization. The results indicate that if squirrel monkeys intend to respond to chucks given by others, they should do so within the next 0.5 sec. Otherwise, they should not vocalize during that period. This temporal rule allows the animals to determine whether a caller of the following chuck really responds to the preceding call or not, when two chucks are heard consecutively.