Looking into the eyes of gibbons: New noninvasive eye-tracking study features these little-studied apes

When observing a fellow primate, humans typically focus on their face as a primary source of social information. Despite the importance of face perception in primates, previous research has primarily concentrated on humans and other great apes, with minimal investigation into gibbons. Gibbons share a unique social structure, being the earliest to diverge from the ancestral line leading to humans 17 million years ago, characterized by small family groups and enduring pair bonding.

Consequently, understanding the relationship between social organization and the evolution of social cognition in gibbons is crucial for comparative studies across the ape lineage. To address this gap, researchers at Kyoto University utilized a noninvasive eye-tracking method, which had previously been applied in similar studies, to examine the eye movements of three adult male gibbons while they viewed photographs of both gibbon and human faces.

Conducted within the gibbons' usual living environment, the experiment allowed the apes to engage or disengage as they pleased. The resulting data revealed that gibbons spent more time focusing on the eyes than other facial features, such as the nose, mouth, or peripheral regions, when viewing both gibbon and human faces. Furthermore, the researchers discovered that gibbons exhibited a stronger fixation on the eyes of their own species compared to human eyes, suggesting a heightened focus on eye-focused face viewing specific to their conspecifics.

These findings not only shed light on the evolutionary foundations of face perception and social information processing in primates but also offer new insights into the relationship between social organization and the evolution of social cognition across the primate lineage.