Group size modulates kinship dynamics and selection on social traits.

An individual's relatedness to its group may change with age due to demographic processes, and such kinship dynamics can shape age-linked social behaviors. Existing models, however, have focused almost exclusively on dispersal and mating, leaving the consequences of group size local variation unexplored. Here, we extend these models to incorporate such variation within a single, genetically connected meta-population. We then contrast predicted kinship dynamics and age-linked selection for helping/harming in coexisting smaller and larger groups in three typical social mammal systems characterized by distinct patterns of dispersal and mating, followed by exploring how group size local variation impacts female and male kinship dynamics across broader demographic contexts to clarify the population-genetic mechanisms. We show that, within the adult reproductive lifespans of these social systems, an individual's age-specific relatedness to others is higher, and changes faster (especially when younger), in smaller groups; consequently, smaller groups favor more extreme helping/harming (especially when younger)-in social system with bisexual philopatry with nonlocal mating (e.g., whales) that favors female shifts from harming to helping with age (which explains the evolution of menopause and postreproductive helping), such shifts are earlier in smaller groups. Further explorations suggest that, in a genetically connected population with group size local variation, group size effects on local relatedness are shaped by the relative strengths of ancestry dilution versus lineage coalescence, and kinship dynamics in a group reflect not only its local demographic conditions, but also those in coexisting groups of different size. Collectively, our study generates new insights into how female and male kinship dynamics emerge and vary under within-population group-size heterogeneity, and how such locally dynamic, varying kinship environments may help to explain variation in age-linked trends in social traits, such as the timing of menopause in social mammals.