Rank differences in the production of vocalizations by wild chimpanzees as a function of social context.

Rank differences in the production of vocalizations by wild, semihabituated, unprovisioned chimpanzees were investigated during a 10-month study in the Kibale Forest, Uganda. Vocalization rates were calculated from data collected during 230 hours of focal-animal sampling on adult females, adult males, and subadult males. Rates were calculated according to whether individuals were alone, with adult females only, or in mixed parties, and the results were compared with published data collected at the Gombe provisioning area. Adult females and low-ranking adult and sub-adult males were generally quiet except when they were in mixed parties, whereas high-ranking males vocalized in all social contexts. These results were in partial contrast to data collected at Gombe, which indicated that vocal production was similar across all age and sex classes. Vocal production at Gombe did, however, resemble that from mixed parties at Kibale, suggesting that the provisioning area at Gombe was comparable to a natural socioecological context occurring at large fruiting trees. It is suggested that low-ranking chimpanzees refrain from loud vocalizing when they are alone or with females only in order to avoid attracting feeding competition and/or potentially aggressive males. These individuals may vocalize when they are associating with high-ranking males in order to advertise the presence of large parties and to deter other individuals from joining them. The use of loud, interparty calls by high-ranking males, when alone or with others, is consistent with the greater sociality of adult male chimpanzees. Loud calling might be advantageous for adult males in attracting mates or allies. © 1993 Wiley-Liss, Inc.