Few studies have focused on mechanisms of developmental change during the prelinguistic period. The lack of focus on early vocal development is surprising given that maternal responsiveness to infants during the first two years has been found to influence later language development. In addition, in a variety of species, social feedback is essential for vocal development. Previous research demonstrated that maternal feedback to prelinguistic vocalizations influenced the production of more developmentally advanced vocalizations, suggesting that effects of maternal responsiveness on vocal development may start during the prelinguistic phase; however, because mothers were instructed how and when to respond to their infants' vocalizations, the timing and type of typical maternal feedback is unknown. In the present study, we analyzed unstructured play sessions for 10 mother-infant dyads to explore the relationship between prelinguistic vocal production and maternal responsiveness. Mothers responded contingently to prelinguistic vocalizations over 70% of the time. Mothers responded with more vocal responses compared to interactive responses (e.g., gazes, smiling, physical contact). Investigation of specific types of vocal responses revealed that mothers responded mainly with acknowledgments to both vowel-like sounds and consonant-vowel clusters. Mothers also showed differential responding to vocalizations that varied in quality. Mothers responded with play vocalizations to vowel-like vocalizations significantly more than to consonant-vowel clusters, whereas they responded with imitations to consonant-vowel clusters more than to vowel-like sounds. Mothers, therefore, appeared to regulate their contingent feedback relative to the speech-like quality of infants' vocalizations which may provide relevant stimulation to guide communicative development.

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Researchers have identified a variety of cross-site differences in the foraging behavior of free-ranging great apes, most notably among chimpanzees (Pan troglodytes) and more recently orangutans (Pongo pygmaeus), that are not due to obvious genetic or ecological differences. These differences are often referred to as "traditions." What is not known is whether this high level of interpopulation variation in behavior is limited to hominoids. In this study, we use long-term data from three Costa Rican field sites that are geographically close and similar ecologically to identify potential foraging traditions in white-faced capuchins (Cebus capucinus). Foraging traditions are predicted in Cebus because of many behavioral and morphological convergences between this genus and the great apes. The processing techniques used for the same food species were compared across sites, and all differences found were classified as present, habitual, or customary. Proximity data were also analyzed to determine if social learning mechanisms could explain variation in foraging behavior. Of the 61 foods compared, we found that 20 of them are processed differently by capuchins across sites. The differences involve pound, rub, tap, "fulcrum," "leaf-wrap," and "army ant following." For most of the differences with enough data to analyze, the average proximity score of the "matched" dyads (two individuals within a group who shared a "different" processing technique) was statistically higher than the average proximity score of the remaining "unmatched" dyads.

Chimpanzees emit a loud, species-typical long distance call known as the pant hoot. Geographic variation between the pant hoots of chimpanzees living in two neighboring populations, the Mahale Mountains and Gombe Stream National Parks, Tanzania, was examined. Analysis of six acoustic features revealed subtle differences in the way chimpanzees from the two populations called. Individuals from the Mahale study site uttered one section of their pant hoots at a faster rate and with shorter elements than animals from Gombe Stream. In addition, individuals at Mahale delivered broader-band, higher pitched "climax" elements than males from Gombe. While genetic factors, anatomical differences, variations in the use of calls at the two sites, and changes in calling over time may account for the variability between populations, we suggest the additional possibility that differences in pant hooting may be due to learning. o 1992 Wiley-Liss, Inc.

Male chimpanzees produce a species-typical long distance call known as the pant hoot. Males give acoustically similar pant hoots when calling together during choruses, but the process by which this vocal convergence takes place is unclear. Three potential mechanisms might account for call matching. First, vocal convergence could represent the passive effect of male chimpanzees sharing a common affective state during choruses. Second, matching could result through the active modification of calls. Such modification might occur if males modeled their calls after those produced by their current chorusing partner. Alternatively, actively mediated convergence would result if males mimicked the calls given by high-ranking males. We examined acoustic variation in calls given during choruses and while alone to test these three hypotheses. Results showed that a similar call type is not given repeatedly during successive choruses. In addition, low-ranking individuals did not produce calls that matched those of the alpha male. Chorused calls produced by two dyadic pairs were more similar to each other than they were to calls delivered during other choruses. The higher ranking, yet dependent, member of one of these dyads appeared to give calls that converged on those produced by the other individual. These data are consistent with the hypothesis that male chimpanzees accomodate each other vocally through the active alteration of their calls during choruses. We suggest that a similar form of accomodation takes place in animals from a wide range of other taxa and that the process serves to maintain and strengthen social bonds between individuals.