Understanding visual access in common marmosets, Callithrix jacchus: perspective taking or behaviour reading?

We tested common marmosets' understanding of visual access in a series of experiments. In experiment 1, we investigated whether marmosets know what conspecifics do and do not see, using a food competition paradigm originally developed for chimpanzees, Pan troglodytes. Subordinate marmosets consistently chose a piece of food only visible to them and not one that was visible to a dominant competitor as well, indicating a simple mentalistic understanding of visual access (level-1 perspective taking). In experiment 2, we aimed to validate these results by using a different experimental approach, building on the marmosets' ability to use gaze as a cue. Although the marmosets precisely extrapolated gaze direction, no understanding of visual access could be found in this task. In experiment 3, we tested an alternative mechanism to account for the positive outcome in experiment 1. The results suggested that the marmosets had a predisposition to treat a piece of food that was looked at by another individual as belonging to that individual and to avoid it. The marmosets overall showed high proficiency in exactly extrapolating gaze direction but failed to show consistent and context-independent perspective-taking abilities. The findings are discussed with regard to results from other primate species and, from a broader evolutionary perspective, with regard to their implications for the emergence of sophisticated forms of gaze understanding in humans.