Are bonobos anxious when others are ignorant of a threat? Using an affect measure to assess Theory-of-Mind abilities in bonobos

Previous studies have shown that bonobos, along with other great apes, possess certain theory-of-mind (ToM) abilities, such as the ability to attribute states of knowledge and belief to others. Our study is the first to assess ToM abilities in bonobos using scratching behavior, a widely used affective measure in great apes. Subjects watched two videos; one video showed a group member ignorant of a snake threat while the other video showed a group member knowledgeable of a snake threat. The ToM hypothesis predicted greater frequency of scratching in subjects watching the ignorance video compared to subjects watching the knowledge video. Results were consistent with this prediction. All but one subject scratched more while watching the ignorance video, and subjects scratched nearly 4 times more often on average while watching the ignorance video. Small sample size prevents any strong conclusions to be reached; however, the experimental design has the potential to be used to assess the relation between affective states and ToM abilities in great apes and other species of nonhuman animals with ToM abilities in which there are reliable behavioral indexes of affective states.