Wednesday, June 6, 2012

Facial display complexity in geladas

Stefanie Lazow '12 recently completed a senior honors thesis in anthropology. Her project is the first systematic investigation of facial display structure and motivation in wild geladas (abstract below). Stefanie will begin medical school next year at Weill Cornell Medical College in New York.

My thesis explores mechanisms for the generation of communicative complexity within the facial display repertoire of the gelada baboon (Theropithecus gelada). Firstly, this project provides a systematic investigation of the unique lip-flip movement of the gelada baboon in order to explore the relationship between facial mobility and communicative complexity in primate facial communication systems. I applied the Facial Action Coding System (FACS) to facial events present in video footage of social interactions between wild geladas. In order to determine the role of the lip-flip, I clearly defined the structure of displays containing a lip-flip movement and inferred the lip-flip’s motivational basis from associated sender behaviors. I found that the lip-flip is an optional expression element in the non-aggressive and mostly submissive gelada bared-teeth display, and appears to add signaling information about benign intent during or after an approach. My conclusions about the possible communicative roles of the lip-flip movement as a specifier, intensifier, and/or alerting component suggest that there are various ways that increased facial mobility can facilitate the generation of communicative complexity within primate communication systems. Secondly, I present evidence of the blending of facial displays within the gelada repertoire, and discuss how the capacity for display blending might contribute to the diversification of possible signals within a communication system. Thirdly, I report sex differences in the gelada facial expression communication system, which suggest that social pressures influence the development of communicative complexity. My study has implications not only for the study of primate facial expressions, but also for the fields of animal communication and human language more broadly.