Wild gelada monkeys detect emotional and prosocial cues in vocal exchanges during aggression

Abstract

Recognizing vocal behaviours intended to benefit others is a crucial yet understudied social skill. Primates with rich vocal repertoires and complex societies are excellent models to track the evolution of such capacity. Here, we exposed wild geladas (Theropithecus gelada) to vocal exchanges between unfamiliar female victim screams and male affiliative calls. The stimuli were arranged in sequences either simulating vocal affiliation towards victims (scream-affiliative call) or violating such order (affiliative call-scream), with varying emotional arousal conveyed by the affiliative call type. Measuring gazing activity towards the loudspeaker and the interruptions of feeding, we show that monkeys were sensitive to the sequential order in vocal exchanges as well as to the emotional arousal conveyed by affiliative calls. Our field study suggests a prosocial use of vocalizations in wild monkeys and reveals that foundational cognitive elements for processing vocal exchanges as meaningful third-party interactions may have existed in our common ancestors with monkeys.

Fig 1. A study animal and examples of vocal exchanges used as stimuli.

a) Picture of an adult gelada male in the study area (picture by EP); b) graphical representations of the four conditions of simulated vocal exchanges used as stimuli during the experiments are represented, with four examples of spectrograms (obtained on Audacity© v. 3.3.2); c) Schematic representation of the different steps of the experimental procedure. See also S1 Fig.

Fig 2. The effect of stimulus sequential order on gelada gazing behaviour.

Influence of the Sequential order of the simulated vocal exchange on a) Duration of first interest towards the loudspeaker (measured in seconds) (Model 1: χ² = 11.646, P < 0.001), b) Duration of total gazing towards the loudspeaker (measured in seconds) (Model 2: χ² = 17.36, P < 0.001), and c) Mean duration per gaze towards the loudspeaker (measured in seconds) (Model 3: χ² = 14.718, P < 0.001). Subjects’ ID are represented with different colours and the Type of affiliative call of the stimulus is indicated by the point shape (triangle = grunts; circle = moans). The boxes display the median value and first and third quartiles, whiskers are extended to the most extreme value inside the 1.5-fold interquartile range.

Fig 3. The effect of the emotional arousal of the affiliative call in the sequence on gelada gazing behaviour and feeding interruption.

Influence of the Type of affiliative call of the stimulus (grunts vs moans) on a) Duration of total gazing towards the loudspeaker (seconds) (Model 2: χ² = 3.994, P = 0.046), b) Duration of stop in feeding activity (seconds) (Model 5: χ² = 5.65, P = 0.017). Subjects’ ID are represented with different colours and the Sequential order of the vocal exchange is indicated by the point shape (triangle = scream-affiliative call; circle = affiliative call-scream). The boxes display the median value and first and third quartiles, whiskers are extended to the most extreme value inside the 1.5-fold interquartile range.

Fig 4. Graphical abstract of the playback experiment.

Geladas showed more interest towards stimuli violating a positive sequential order as well as after stimuli with calls of high emotional arousal. Gelada monkeys seem to show cognitive ability to recognize vocal affiliation directed at victims.