Mechanisms of inbreeding avoidance in a wild primate

Inbreeding often imposes net fitness costs,^1-5 leading to the expectation that animals will engage in inbreeding avoidance when the costs of doing so are not prohibitive.^4-9 However, one recent meta-analysis indicates that animals of many species do not avoid mating with kin in experimental settings,⁶ and another reports that behavioral inbreeding avoidance generally evolves only when kin regularly encounter each other and inbreeding costs are high.⁹ These results raise questions about the processes that separate kin, how these processes depend on kin class and context, and whether kin classes differ in how effectively they avoid inbreeding via mate choice-in turn, demanding detailed demographic and behavioral data within individual populations. Here, we address these questions in a wild mammal population, the baboons of the Amboseli ecosystem in Kenya. We find that death and dispersal are very effective at separating opposite-sex pairs of close adult kin. Nonetheless, adult kin pairs do sometimes co-reside, and we find strong evidence for inbreeding avoidance via mate choice in kin classes with relatedness ≥0.25. Notably, maternal kin avoid inbreeding more effectively than paternal kin despite having identical coefficients of relatedness, pointing to kin discrimination as a potential constraint on effective inbreeding avoidance. Overall, demographic and behavioral processes ensure that inbred offspring are rare in undisturbed social groups (1% of offspring). However, in an anthropogenically disturbed social group with reduced male dispersal, we find inbreeding rates 10× higher. Our study reinforces the importance of demographic and behavioral contexts for understanding the evolution of inbreeding avoidance.⁹.