The experimental set-up and procedure for Experiment 1.

<p>At the beginning of each session, a chimpanzee watched as Experimenter 1 (E1) entered the test area and baited a metal bucket with a large preferred food item at the back of the test area. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0112015#s5" target="_blank">Experiment 2</a> (E2) then entered and sat in a chair between the two matching trays at the front of the test area and began the ongoing matching task by placing 3 unique photographs on each tray. E2 then passed the chimpanzee one lexigram token at a time, and the chimpanzee placed the token in the appropriate bowl (the one attached to the matching photograph) to earn small food rewards. But, when the chimpanzee received the token matching the previously hidden food item, it should instead carry that token to E1 to obtain the target food item.