Figure 1.

Performance in the cooperative rope-pulling task by partner status. (a) Illustration of the cooperation apparatus. The ‘rope-pulling game’ was adapted from previous studies on chimpanzees and children [44,45]. A photograph of the apparatus is also provided in electronic supplementary material figure S1 in the supplemental information (SI). (b) Proportion of successful first trials of kin (i.e. siblings) and non-kin friend and non-friend dyads in cooperative rope pulling. Error bars indicate 95% bootstrapped confidence intervals. N values indicate the number of dyads in each category (i.e. kin, friends, non-friends). Kin partners were less successful than non-kin in the rope-pulling task. (Online version in colour.)