Chimpanzees (Pan troglodytes) show more understanding of human attentional states when they request food in the experimenter's hand than on the table

Although chimpanzees have been reported to understand to some extent others' visual perception, previous studies using food requesting tasks are divided on whether or not chimpanzees understand the role of eye gaze. One plausible reason for this discrepancy may be the familiarity of the testing situation. Previous food requesting tasks with negative results used an unfamiliar situation that may be difficult for some chimpanzees to recognize as a requesting situation, whereas those with positive results used a familiar situation. The present study tested chimpanzees' understanding of others' attentional states by comparing two requesting situations: an unfamiliar situation in which food was put on a table, and a familiar situation in which chimpanzees requested food held by an experimenter. Chimpanzees showed evidence of understanding the experimenter's attentional variations and the role of eye gaze only in the latter task. This suggests that an unfamiliar requesting situation may keep subjects from expressing their understanding of others' attentional states even though they are sensitive to them.