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

Zhuli Cheng, André S. Pereira, Cayo Biobank Research Unit NA, Josué E. Negron-Del Valle, Daniel Phillips, Delphine De Moor, Erin R. Siracusa , Lauren J. N. Brent

Members of social groups often form social relationships, which can 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 relationships exist between non-kin of the same sex. 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 and stability of non-kin bonds in a mix-related society, along with the

extent to which kin availability predicts their formation. We found that in line with kin selection

theory and previous work on this population, there was a clear kin bias in social bond partners.

However, bonds with non-kin were nevertheless 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 that had fewer kin group mates were more likely to have 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.