Facial expressions and pair bonds in hylobatids

Area of Science:

• Primate communication
• Ethology
• Evolutionary biology

Background:

• Facial expressions are key to primate social communication.
• While studied in chimpanzees and macaques, hylobatid facial expression origins and functions remain unclear.
• The social complexity hypothesis posits that increased social complexity drives the evolution of complex communication signals.

Purpose of the Study:

• To test the link between facial expressions and sociality in hylobatids, specifically pair-bond strength.
• To investigate the role of facial expressions in primate social complexity.

Main Methods:

• Compared 206 hours of video and 103 hours of focal animal data from ten hylobatid pairs.
• Analyzed 5,969 facial expressions across repertoire use, breadth, and facial expression synchrony (FES).
• Correlated facial expressiveness dimensions with pair bond strength and behavioral synchrony.

Main Results:

• Hylobatids possess a large repertoire of at least 80 unique facial expressions.
• Facial repertoire composition did not correlate with pair bond strength or synchrony.
• Facial Expression Synchrony (FES) was the strongest measure of expressiveness and positively correlated with sociality index scores.

Conclusions:

• Facial expressions play a significant role in maintaining hylobatid pair bonds and coordinating activities.
• The findings support the hypothesis linking facial expressions and sociality in primate evolution.
• This study reinforces the social complexity hypothesis as a driver for the evolution of complex communication.