Affiliation and aggression among adult female rhesus macaques: A genetic analysis of paternal cohorts

Kin selection promotes the evolution of social behaviour that increases the survival and reproductive success of close relatives. One prerequisite for kin selection is that individuals have the ability to discriminate between kin and nonkin. Studies incorporating data on paternal kinship are still rare, but in species with a high male reproductive skew, many adult females will be paternal half siblings. Using both microsatellites and DNA-fingerprinting, we here analyse data on paternal relatedness in order to compare the influence of maternal and paternal kinship on rates of affiliative and aggressive interactions among semi free-ranging adult female rhesus macaques (Macaca mulatta). Because paternal half siblings tend to be peers whereas maternal half siblings are almost always nonpeers, we also examine the interactions between age proximity and genetic relatedness on social behaviour. Genetic analyses show that male reproductive success is strongly skewed with 75% of infants within the troop having a paternal half sibling in the same age cohort. The highest rates of both affiliation and aggression occur among maternal half sisters. Adult females are significantly more affiliative, but not more aggressive, with paternal half sisters than with nonkin. Affiliative relationships declined in conjunction with increasing age difference among paternal half sisters, but the reverse effect was found for affiliative relationships among maternal half siblings. No association emerged among nonkin. Among both maternal and paternal kin, rates of affiliative and aggressive interactions increase as the degree of relatedness increased, thereby questioning the concept of a relatedness threshold as a mediator of social interactions in rhesus macaques. The asymmetry in affiliation and aggression between maternal and paternal half siblings, and the effect of age proximity on partitioning social interactions suggests that context-dependent kin discrimination characterises rhesus macaques. Paternal kin discrimination probably results from an interaction between phenotype matching and familiarity, rather than from one process or the other