Multimodal yawns signal emotional arousal in geladas

Abstract

Group living requires individuals to make their internal states perceptible to sustain coordination and cohesion. Multimodal signals may efficiently convey such socioemotional information. Although often considered reflexive, yawning exhibits structural variability and can trigger contagion, suggesting communicative potential. In humans and geladas, Theropithecus gelada , yawns can include a vocal component that alone triggers yawn contagion, yet the reasons behind its emergence remain underexplored. We conducted the first systematic study of vocalized yawns in geladas, analysing over 2500 yawns from nearly 200 wild and captive individuals to test whether multimodal yawning reflects affective states. Using Bayesian modelling, we tested hypotheses linking yawn vocalizations to emotional valence, arousal or motor by-products. Multimodal yawns were more frequent in adult males, who face greater socioemotional demands, and were strongly associated with high-arousal contexts: they occurred most often after aggression, copulations and greetings, less during grooming and least during rest. Moreover, yawn vocalizations often co-occurred with other species-specific affiliative calls, which are known to index arousal and maintain cross-sex bonds, indicating integration within the species' broader communicative system. Our findings indicate that yawn vocalizations may act as socioemotional signals, amplifying arousal and integrating into the species’ vocal repertoire. By externalizing internal states, they could enhance emotional attunement, synchrony and social cohesion in complex social groups.