Date of Award


Degree Name

Master of Arts



First Advisor

Reichard, Ulrich


Among primates, humans have the largest and most complex facial repertoires, followed not by their closest living hominid relatives but by hylobatids. Facial behavior is an important component of primate communication that transfers and modulates intentions and motivations. However, why great variation in primate facial expressions evolved and why hylobatid facial repertoires seem to be more similar to humans than other apes is unclear. The current study compared 206 hours of video and 103 hours of focal animal data of facial expression repertoires, measures of pair bond strength, and behavioral synchrony of ten hylobatid pairs from three genera (Nomascus, Hoolock, and Hylobates) living at the Gibbon Conservation Center, Santa Clarita, CA. This study explored whether facial repertoire breath or frequency were linked to social parameters of pair-bonds, how facial expressions related to behavioral synchrony, and if facial feedback (i.e., the transfer of behaviors and intentions by mimicking observed facial expressions) were important between pair-partners. Intra-pair facial repertoires correlated strongly with repertoire composition and rate of use, suggesting that facial feedback was important, while behavioral synchrony showed no correlation with facial behavior. The results of this study suggest that larger facial repertoires contribute to strengthening pair bonds, because richer facial repertoires provide more opportunities for facial feedback which effectively creates a better ‘understanding’ between partners through smoother and better coordinated interaction patterns.




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