Figure 1. Exchanging tokens through the picture frames in the Test Phase.

Stills from video footage of the Test Phase showing the Subject chimpanzee returning his token through his picture frame. As shown, to avoid cuing the chimpanzees’ responses, the experimenter only reached for the token when the chimpanzee had pushed 50% of the token through the picture frame. Note, although there is a central support bar in the middle of the mesh, the chimpanzees are in the same enclosure and have full visual access to both food rewards outside the cage and also to the actions of their test partner.