Abstract
There is ongoing debate about the extent to which nonhuman animals, like humans, can go beyond first-order perceptual information to abstract structural information from their environment. To provide more empirical evidence regarding this question, we examined what type of information great apes (chimpanzees, bonobos, and orangutans) gain from optical effects such as shadows and mirror images. In an initial experiment, we investigated whether apes would use mirror images and shadows to locate hidden food. We found that all examined ape species used these cues to find the food. Follow-up experiments showed that apes neither confused these optical effects with the food rewards nor did they merely associate cues with food. First, naïve chimpanzees used the shadow of the hidden food to locate it but they did not learn within the same number of trials to use a perceptually similar rubber patch as indicator of the hidden food reward. Second, apes made use of the mirror images to estimate the distance of the hidden food from their own body. Depending on the distance, apes either pointed into the direction of the food or tried to access the hidden food directly. Third, apes showed some sensitivity to the geometrical relation between mirror orientation and mirrored objects when searching hidden food. Fourth, apes tended to interpret mirror images and pictures of these mirror images differently depending on their prior knowledge. Together, these findings suggest that apes are sensitive to the optical relation between mirror images and shadows and their physical referents.
Highlights
A central debate in the field of comparative cognition is to what extent nonhuman animals go beyond the perceptual input to discriminate between appearance and reality (e.g., Carruthers 2008; Krachun et al 2009) and to abstract higherorder relations (Penn et al 2008)
We hypothesized that apes would perform significantly better in the mirror and shadow condition compared to the control condition if they were sensitive to the relation between these optical effects and their physical referents
We found that some individuals of all examined ape species used shadows and mirror images of hidden food items as a cue to locate them
Summary
A central debate in the field of comparative cognition is to what extent nonhuman animals go beyond the perceptual input to discriminate between appearance and reality (e.g., Carruthers 2008; Krachun et al 2009) and to abstract higherorder relations (Penn et al 2008). This is a pivotal question that transcends different areas of research including picture recognition (e.g., Fagot et al 2010), object permanence. Thereafter, the appearance of the items is manipulated such that the preferred item looks like the less-preferred one and
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