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The structural and motivational role of the unique lip‐flip movement in the gelada ( Theropithecus gelada ) facial display repertoire
Author(s) -
Lazow Stefanie P.,
Bergman Thore J.
Publication year - 2020
Publication title -
american journal of physical anthropology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.146
H-Index - 119
eISSN - 1096-8644
pISSN - 0002-9483
DOI - 10.1002/ajpa.24031
Subject(s) - psychology , repertoire , graphical display , function (biology) , facial expression , facial action coding system , communication , cognitive psychology , developmental psychology , social psychology , biology , computer science , evolutionary biology , art , computer graphics (images) , literature
Objectives Human language represents an extreme form of communicative complexity. Primate facial display complexity, which depends upon facial mobility, can be used as a model for the study of the evolution of communicative complexity. The gelada ( Theropithecus gelada ) is the only primate that can produce a lip‐flip eversion. This study investigates the role of the lip‐flip relative to the bared‐teeth display to understand its role in generating communicative complexity. Materials and methods We reviewed videos of gelada social interactions. We utilized the facial action coding system (FACS) to define structural component action units (AUs) of each display. We inferred display motivation from the behaviors of the display sender. Results The lip‐flip was used only in combination with the essential AUs of the bared‐teeth display, serving as an optional structural element added to produce a structural variant. Both the bared‐teeth display with and without a lip‐flip occurred most frequently with nonaggressive, submissive behaviors. The lip‐flip was more frequently preceded by approach than the bared‐teeth display, especially in males. The lip‐flip was also present in the majority of structurally blended facial displays though the motivation of the non‐lip‐flip parent display often dominated. Discussion 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.