• Macaca
• lipsmacking
• tonkeana
• complexity
• flexibility

The aim of this study was to evaluate the main contexts of occurrence and the possible functions attributed in literature to lipsmacking in one of the most tolerant species within the Macaca genus, the Tonkean macaque (M. tonkeana). Additionally, the communicative complexity referred to this facial display was investigated by using a relatively new tool for measuring facial activity, the MaqFACS . As it is based on the musculature responsible for the facial movements, this methodology would be employed to deeper understand facial expressions’ use and modulation, providing quantitative and more objective data to describe facial expressions, thus making a comparative study far more reliable. Since lipsmacking is used in several contexts in Tonkean macaques, this expression represents a good model to investigate how external factors may change facial appearance, and how these changes alter the displays’ meaning. Finally, as lipsmacking is a dynamic facial expression, duration could be meaningful in influencing the social outcome of the interaction and indicative of the communicative complexity of this facial expression: this study investigated this aspect as well.