Gibbon Gestures: Hylobatid Facial Signals and Insights Into Language Evolution

The evolution of human language is a subject of ongoing debate. Some researchers suggest that language developed gradually from earlier forms of communication, akin to the manual gestures of great apes. Great ape manual gestures are produced intentionally and flexibly to achieve specific goals. Recent research on chimpanzees has demonstrated that they also use facial signals as gestures, raising the possibility that such facial gesturing may be present in other species. In our present study, we examined whether hylobatid facial signals also qualify as gestures.

We studied the facial signaling behavior of 20 captive hylobatids. We observed 484 facial signals spanning six types. We assessed all coded facial signals based on four key gesture properties: communicative, intentional, flexible, and goal-associated, along with their 12 related variables identified in prior research.

Our findings suggest that hylobatid facial signals may be used in a communicative and flexible manner. However, hylobatid facial signals do not meet the essential criteria for intentionality or goal association. We found statistically significant differences were observed in intentional and goal-oriented facial signaling among genera of hylobatids, with members of Hylobates being significantly more likely to exhibit variables associated with these two traits compared to Hoolock.

Our study suggests that some properties of great ape manual gesturing (and language) are present in hylobatid facial signals. However, there are numerous interpretations of our results. We therefore suggest directions for future research on the facial gesturing capabilities of hylobatids and monkeys to clarify the ambiguities our data have raised.