| dcterms.abstract | The communication system of a species is the fundamental basis upon which animal social relationships are built and contributes to a species' social organization. Among group-living anthropoid primates that inhabit forested environments, vocal signals are likely to be one of the most important communication channels because reduced visibility limits the use of visual communication. Accordingly, many primates have different vocalizations that convey meaningful information to conspecifics about caller behavior, external objects or events and cues used in recognizing individuals. Thus, in order to better understand the social and ecological factors that have shaped a species' vocal communication system, we must identify the species' repertoire of vocalizations, the behavioral contexts in which they are produced and the functions they serve, as well as the information encoded in their acoustic structure, such as individual distinctiveness, and how it varies across different calls. This dissertation investigates the vocal behavior of wild western gorillas (<italic>Gorilla gorilla</italic>), in particular the use and function of calls used during within-group communication. I address three main questions regarding their vocal behavior: 1) How do changes in ecological and social factors affect the species' vocal repertoire and usage of their vocalizations? 2) What is the role of western gorilla long-distance calls in maintaining group cohesion? 3) What is the adaptive function of individual vocal cues in the western gorillas' vocal repertoire? To address these questions, I collected behavioral data on wild western gorillas and acoustic recordings of their calls at the Mondika Research Center (Central African Republic and Republic of Congo). The results indicate that 1) the vocal repertoire of western gorillas is largely similar to that of mountain gorillas (<italic>Gorilla beringei</italic>) although the two species differ to some degree in call usage and context. For example, most calls used by western gorillas are associated with specific behavioral contexts. Second, group composition and spatial spread determined the type of calls each age-sex class uses as well as the behavioral context in which they are given. Third, dispersed group members in western gorillas use long-distance calls in within-group communication to regain spatial proximity. And finally, all calls displayed acoustic variation and individual distinctiveness, suggesting that individual vocal cues might have evolved to facilitate the functions of all vocal signals used during within-group vocal communication, including those used at both close and at greater distances. Taken together, these results indicate that the vocal behavior of western gorillas has been shaped by several ecological and social factors. Group composition may have influenced the repertoire of each age-sex class, the limited visibility of their environments may have selected for more calls used in specific contexts to facilitate within-group communication, and the greater group spread, compared to that of mountain gorillas, might have selected for using long-distance calls within the group when individuals are separated. Their call types display considerable inter- and intra-individual acoustic variability, suggesting that each of their calls might encode more information than just identity cues. Since all calls were individually distinct, this study suggests that the distance at which communication occurs and the social context in which calls are given might have selected for individual cues in gorilla vocal signals. Finally, providing the first data on western gorilla vocal behavior, this study enlarges the comparative data on ape vocal communication, which is indispensable for the development of theories on the evolution of human language. | |
