Fig. 2.

Pattern of pulling across conditions. (a) Tamarins pulled one of the two tools in almost 100% of the trials in all conditions except the Other Only trials in both the Partner Absent/Barrier and Partner Present conditions. Pulling did not differ across these two conditions. (b) To investigate whether the subjects preferentially pulled one of the tools, a ‘correct’ pull was defined differently depending on the condition. In the Partner Absent/Barrier condition, there is no partner present and the subject cannot access either adjacent compartment. Since there is no correct response in this condition, pulling the right tool was randomly chosen as a correct response. In the Partner Absent/No Barrier condition, no partner was placed in the adjacent cage, but one of the barriers was removed to give access to the adjacent compartments. Thus, a correct pull was one which delivered food to the side with no barrier. In the Partner Present condition, a partner was present in one of the two adjacent compartments. Here, the trials in which the subject pulled the tool delivering food to the partner was scored as a correct choice. Error bars represent 95% confidence intervals.