Social engagement modulates wild monkeys' vocal expressions and the behavioral response to that of others

Animal vocal communication relies on the dynamic interaction between emitter and receiver, with signals shaped within a social and embodied context. To fully understand how such interactive processes operate, we used yawn vocalizations of geladas (Theropithecus gelada), a species showing exceptional yawning variability. We first examined yawn calls produced in three contexts: high-intensity social, low-intensity social, and non-social context and found clear acoustic differences among them, revealing context-dependent modulation in a typically stereotyped behavior. We conducted field playback experiments exposing wild geladas to unfamiliar male yawn vocalizations emitted in the three contexts. During playbacks, monkeys gazed more at the loudspeaker when yawns originated from a social rather than a non-social context, indicating that animals perceive the stimuli's differing nature. Although yawn responses did not vary across contexts, contagion was higher when geladas were grooming during test, suggesting that positive social engagement enhances, rather than reduces, susceptibility to contagion.