The socioecology of wild chimpanzee vocal behavior in the Kibale Forest, Uganda.

Abstract

The vocal behavior of unprovisioned wild chimpanzees was studied between May, 1988, and December, 1989, in the Kibale Forest, Uganda. Data were gathered on patterns of association, grooming, and dominance interactions among female and male chimpanzees. Data were collected to determine if individual rates of vocal production varied in different social contexts, and to perform acoustic and contextual analyses of the pant hoot vocalization. Patterns of female and male social relationships among the Kibale chimpanzees, a rain-forest population, were similar to those reported at the woodland sites of Gombe and Mahale in Tanzania. This result differed from earlier findings on other rain-forest chimpanzee populations. Vocal output differed between male and female chimpanzees. Females tended to be quieter than males, except when in large, mixed-sex associations. Males, by contrast, had relatively high vocalization rates in all party types. This difference is interpreted to reflect the greater sociability of chimpanzee males compared with females. The possibility is discussed that females are more vocal in large associations to deter intercommunity encounters. Acoustic and contextual analyses of pant hoots given in the food arrival context yielded no evidence that a distinct pant hoot variant is used uniquely in this context. Thus no evidence emerged that a pant hoot variant referred semantically to features of the physical environment. That pant hoot variants might broadcast information about social context is discussed. Pant hoot production upon arriving at unoccupied food sources was restricted to high-ranking males, and did not appear to depend on relative food availability at food sources. There was no indication that individuals who did not give food arrival pant hoots were punished, and parties from which food arrival pant hoots were heard were not joined more frequently than quiet parties. Thus food arrival pant hoots did not appear to reflect feeding excitement only, or to be altruistic in broadcasting information about the location of food. Food calling did not depend on the presence or absence of other high-ranking males, and thus did not appear to attract allies. No evidence was found against the hypothesis that food arrival pant hoots functioned to attract estrous females.