RECOGNITION OF ORDER EFFECTS IN SQUIRREL-MONKEY ANTIPHONAL CALL SEQUENCES

Squirrel monkeys exchange chuck vocalizations in antiphonal sequences that suggest a request for information, and a response to that request. Chucks are characterized by measurable acoustic differences, notably in peak frequency, related to their position in a sequence. To determine the cues by which animals might identify a chuck that is a request for information from one that is a response, first-in-sequence (alpha) and second-insequence (beta) chucks, differing in peak frequency, were recorded from familiar individuals and strangers. These chucks, and a sham control, were played back, in the temporal context of a first-in-sequence call, to a target group of socially housed captive squirrel monkeys. Animals responded more strongly to a chucks than to beta chucks for calls originating from familiar individuals. No distinction was made between the alpha and beta chucks of strangers, and all strangers' chucks got a weaker response than did familiar alpha chucks. Squirrel monkeys probably use acoustic differences, rather than context, to discriminate the ordinal significance of a chuck, but cannot do so unless they are familiar with the caller.