The structural and motivational role of the unique lip-flip movement in the gelada (Theropithecus gelada) facial display repertoire.

The lip-flip may potentially function as an indicator of benign intent after an approach or as an intensifying component of nonaggressive intent. Adaptations to increase facial mobility in geladas via facilitating the lip-flip may promote increased communicative complexity through increased conspicuousness and motivational signaling specification or intensification.