Recognition of order effects in squirrel monkey antiphonal call sequences

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Abstract
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 (α) and second‐in‐sequence (β) 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 α chucks than to β chucks for calls originating from familiar individuals. No distinction was made between the α and β chucks of strangers, and all strangers' chucks got a weaker response than did familiar α 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.