Social Basis of Vocal Interactions in Western Lowland Gorillas (Gorilla g. gorilla)

Authors have raised the possibility of identifying primitive forms of conversational rules in monkeys: temporally ruled vocal interactions, call overlap avoidance, and socially based calling partner preferences. The question as to how these abilities have evolved in the primate lineage remains open to debate, particularly because studies based on apes are scarce and controversial. We studied a captive group of western lowland gorillas and tested the influence of caller characteristics and the type of bond between calling partners on vocal behavior based on the following: age, dominance, spatial proximity, sociopositive contact, and gaze. Four calling patterns that are call type dependent were identified: vocal interaction with and ( more frequently) without call overlap, isolated calling, and repeated calling. Adult calls and grunts (contact calls) were predominant during vocal interactions, and the "response" delay was most often around half a second. The frequency of grunt dyadic exchanges was found to be linked to spatial proximity, gaze exchanges, and age proximity between calling partners. The dominance rank of callers determined the rate of contribution to these exchanges. These results show that some apes use rule-governed call exchanges and that these socially guided vocal interactions are more widespread than previously believed.