Of these 54 kin pairs, 19 had kinships around 0.25, consistent with a first-degree relationship (sibling, or parent/offspring).
Request was also observed significantly more frequently in the mother-offspring pairs than in the non-kin pairs.
Tool-transfer events occurred significantly more frequently in the mother-offspring pairs than in the non-kin pairs.
The non-kin pairs were not tested in experiment 2, since targeted helping in the non-kin pairs as revealed in experiment 1 was already unilateral, which suggested that reciprocity was not the main mechanism for perseverance of tool transfer in the non-kin pairs.
Each adult female in non-kin pairs was paired with two different partners; for example, Pendesa was paired with Puchi and also with Mari.
Dominant individuals requested significantly more frequently than did subordinates (Chi-square test for the pooled data from the three non-kin pairs: χ2 (1) = 42.8, p<0.001).
In contrast, in the non-kin pairs, tool transfer occurred predominantly from the subordinate individual to the dominant individual and request was mostly observed in dominant individuals.
A similar negative linear regression of dominance rank and pro-social tendency was found within the kin-pairs (t = −2.893, β = −0.715, n = 10, p = 0.02)(Figure 3).
Participants exhibited request on average 88.9% (SD = 12.3) of trials during the mismatched condition in the mother-offspring pairs, while 37.5% (SD = 42.7) in the non-kin pairs (random permutation test: Nmother-offspring = Nnon-kin = 6, T = 51.4, p = 0.034).
Moreover, the absolute dominance rank of the subject, and not its rank position relative to that of its partner, determines its pro-social behavior, both among kin and in non-kin pairs.
