Influence of Social Context on the Use of Blended and Graded Facial Displays in Chimpanzees

Our understanding of social communication and emotional behavior in nonhuman primates has advanced considerably through research over the past half century. Chimpanzee facial displays have typically been described as highly graded communicative signals, but we propose an additional distinction: blended displays. They appear to be morphologically and acoustically similar to the expressions in 2 prototypical/parent categories. We describe the facial and vocal communicative repertoire of chimpanzees and examine how they use graded and blended signals in different social contexts. Data from behavioral observations revealed that they used facial displays differently depending on the social context. Specifically, the variability can be explained by 7 factors representing nervousness and distress, agonism, contact reassurance, excitement, greetings, play, and vigilance. Additionally, the use of blended displays was not simply divided between the contexts that elicited the parent types, nor were they used in totally unique contexts. Instead, the data showed that the contextual use of blended displays is primarily correlated with the social contexts that elicited only one of the parent expressions. Thus, the blended displays appeared to reflect conflicting internal motivational states in the sender, instead of expressing features of the external environment. We proffer several possible explanations for how the blended signals may be interpreted by receivers and why they would be contextually associated with only one parent group.