The function of loud calls (Hoot Series) in wild western gorillas (Gorilla gorilla)

The use of loud vocal signals to reduce distance among separated social partners is well documented in many species; however, the underlying mechanisms by which the reduction of spacing occurs and how they differ across species remain unclear. Western gorillas (Gorilla gorilla) offer an opportunity to investigate these issues because their vocal repertoire includes a loud, long-distance call (i.e., hoot series) that is potentially used in within-group communication, whereas mountain gorillas use an identical call exclusively during intergroup encounters. First, we tested whether the hoot series functions as a contact/separation call. Second, we examined which individuals were more likely to reply and which party was more responsible for decreasing distance to identify the underlying mechanisms and cognitive implications of hoot series. We collected behavioral, spatial, and acoustic data on five adult gorillas over 15 months at the Mondika Research Center (Republic of Congo and CAR). Hoot series are individually distinct calls and given by both male and female gorillas when separated from each other. Following hooting, the distance between separated group members decreased significantly; thus we concluded that western gorillas use this call to reestablish group cohesion. The way in which proximity was achieved depended upon listeners replying or not to the caller. Replies may indicate a conflict between callers about intended travel direction, with vocal interchanges serving to negotiate a consensus. Although the acoustic features of vocal signals are highly constrained in closely related species, our results demonstrate that the function and usage of particular calls can be flexible.