Western Gorilla Vocal Repertoire and Contextual Use of Vocalizations

Despite considerable interest in the vocal communication of non-human primates and its possible relevance to theories of language evolution, we know surprisingly little about how vocal communication varies between closely related species inhabiting differing environments. Here, we provide the first quantitative description of the vocal repertoire, calling rates, and call usage in wild western gorillas and compare it to the previous work on mountain gorilla vocal behavior. During 1572 h of focal follows (n = 533), we collected behavioral data on and recorded vocalizations (n = 2163) of eight individuals in one group at Mondika Research Center (Republic of Congo). We supplemented these data with opportunistic recordings of an additional adult male in a second group. We used discriminant function analysis to test how well calls can be categorized by their acoustic structure and used behavioral data to determine the typical usage of western gorilla call types. The vocal repertoire comprised of 17 call types. Twelve of 17 call types were given primarily in a single context. Our results were similar to previous studies of mountain gorillas in that grunts, and grumbles were used most frequently and the silverback male vocalized more frequently than other group members. However, compared to mountain gorillas, western gorillas used an additional call type (sex-whinny), used a second call type (hoot series) in a completely different context and by all age–sex classes, and used many more call types in a more context-specific fashion. Our study suggests that although vocal production is highly constrained by morphology and phylogeny, differing social and ecological conditions can yield differences in the use and function of calls, even between two closely related species such as western and mountain gorillas.