Department of Anthropology, Dartmouth College, Hanover, New Hampshire.
Department of Surgery, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts.
Am J Phys Anthropol. 2020 Jun;172(2):280-290. doi: 10.1002/ajpa.24031. Epub 2020 Feb 26.
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.
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.
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.
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.
人类语言是一种极端形式的交际复杂性。灵长类动物的面部表情复杂性依赖于面部的灵活性，因此可以作为研究交际复杂性进化的模型。狒狒（Theropithecus gelada）是唯一能够产生唇外翻的灵长类动物。本研究调查了唇外翻相对于露齿展示的作用，以了解其在产生交际复杂性中的作用。
我们回顾了狒狒社交互动的视频。我们利用面部动作编码系统（FACS）来定义每个展示的结构成分动作单元（AU）。我们从展示发送者的行为推断展示动机。
唇外翻仅与露齿展示的基本 AU 结合使用，作为添加的可选结构元素，以产生结构变体。有和没有唇外翻的露齿展示最常与非攻击性、顺从性行为一起发生。唇外翻比露齿展示更频繁地被接近行为所伴随，尤其是在雄性中。唇外翻也存在于大多数结构混合的面部展示中，尽管非唇外翻的父展示的动机通常占主导地位。
唇外翻可能在接近后潜在地作为善意意图的指标，或者作为非攻击性意图的强化成分。通过促进唇外翻来增加狒狒面部灵活性的适应可能会通过增加显著性和动机信号的特异性或强化来促进交际复杂性的增加。
