Social bonds between nonkin are common, but less stable, in a mixed-related society

Members of social groups often form social relationships, which are known to carry important fitness benefits. Kin selection predicts that these relationships should be prevalent between kin, yet there is increasing evidence that, in societies that feature a mixture of related and unrelated individuals, social bonds are also formed with non-kin. Nevertheless, quantitative research on non-kin social relationships remains rare, hampering our understanding of their nature and adaptive value. Here, we combined long-term social and pedigree data from semi-free-ranging adult female rhesus macaques (Macaca mulatta) to quantify the prevalence, stability, and extent to which kin availability predicts the formation of non-kin bonds in a mix-related society. We found that in line with kin selection theory and previous work on this population, there was a clear kin bias in the formation of social bonds. However, bonds with non-kin were still more common than those with kin. We also found that bonds between non-kin were less stable: they were shorter in duration and varied more in strength across years in comparison to bonds with kin. Finally, we found that individuals who had fewer kin group mates were more likely to form social bonds with non-kin. Together this suggests that kin bonds might provide individuals with stable and predictable benefits, whereas non-kin bonds might be formed more opportunistically, to access specific or volatile resources and to compensate for a lack of kin. Future efforts to quantify and characterise social bonds between non-kin in different societies will yield a better understanding of the proximate and ultimate causes of social bonds, including how they are formed and maintained and what functions they serve.