RESUMEN
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 (ß) 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 ß chucks for calls originating from familiar individuals. No distinction was made between the α and ß 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. © 1993 Wiley-Liss, Inc.
RESUMEN
Play by young squirrel monkeys (Saimiri boliviensis) may put them and other troop members at risk for predation because youngsters are noisy, separated from adults, and not vigilant when at play. In a study using separated groups of adults and 1-year-old juveniles caged outdoors, we found that adult female squirrel monkeys become more vigilant during periods of spontaneous play among juveniles. This behavioral response could be obtained with auditory cues (play vocalizations) alone. Five times as much vigilance activity was directed toward an area from which threat or disturbance was likely to come as was directed toward the juveniles themselves. These results suggest both an adaptive, compensatory increase in adult vigilance during play and a function for play vocalizations. Additional functions for play vocalizations remain to be investigated.