Visual preference for previously familiar faces in Chimpanzees (Pan troglodytes).

This study investigated how chimpanzees' visual preferences for familiar faces are influenced by the temporal status of social relationships (specifically, whether the familiar face was a past or present groupmate) and by the duration of cohabitation. Using eye-tracking technology, the visual behavior of six chimpanzees was recorded as they viewed paired facial photographs consisting of one familiar and one novel individual. Familiar faces were categorized into two conditions: previously cohabitating individual's face (past-cohabitation) and currently cohabitating individual's face (present-cohabitation). A significant looking bias toward familiar faces was observed only in the past-cohabitation condition, indicating a visual preference for faces no longer present in the group. In contrast, no such preference was found in the present-cohabitation condition. Furthermore, within the present-cohabitation condition, longer cohabitation duration was associated with reduced visual preference for the familiar face, whereas no such relationship was observed in the past-cohabitation condition. These findings suggest that chimpanzees' visual attention toward familiar faces is modulated by both the temporal context of the relationship and the duration of cohabitation, and that long-term social memory may persist across age and even after extended periods of separation.