ETD

• affective neuroscience
• chimpanzee play
• ChimpFACS
• emotional communication
• full play face
• play face

Facial expressions, central to primate emotional communication, offer an evolutionary window into the neural systems that coordinate affect and social behavior. This study examined how chimpanzees (Pan troglodytes) express positive affect during social play through facial and vocal signals, focusing on the play face (PF) and full play face (FPF). Over 50 hours of video recorded at the Chimfunshi Wildlife Orphanage Trust (Zambia) yielded 3,594 play-related facial expressions, 341 of which were suitable for coding with the Chimpanzee Facial Action Coding System (ChimpFACS). Action units (AUs) were analyzed using NetFACS to assess structural specificity and co-activation patterns, and generalized linear mixed models (GLMMs) to test contextual and demographic predictors of AU recruitment. PF and FPF differed systematically in muscular composition while sharing a common mouth-opening core action, supporting the view that they represent graded variants along a single expressive continuum. FPF exhibited higher specificity and stronger integration of upper and lower face AUs (notably AU6, AU10, AU12, AU25+AU27, AU43), whereas PF primarily recruited lower face components (AU25+AU26). Laughter occurred with both expressions at similar rates overall, but GLMM results showed that laughter predicted activation of AU25+27, implying functional coupling between facial and vocal systems. Physical contact, sex, and age also modulated AU recruitment, indicating that expression structure varies with both situational and demographic context. Interpreted within an affective neuroscience framework, these findings support graded coordination between subcortical affective generators and cortical motor control systems, highlighting an evolutionarily conserved affective–motor architecture underlying socio-emotional communication in primates.