Group size modulates kinship dynamics and selection on social traits

An individuals relatedness to its group may change with age due to demographic processes, and such kinship dynamics can shape age-linked social behaviours. 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 social mammal contexts, followed by exploring predictions under broader demographic contexts to clarify how demographic conditions, including local group size, influence patterns of female and male kinship dynamics. We show an individuals age-specific relatedness to others is higher, and changes faster (especially when younger), in smaller social mammal groups; consequently, smaller groups favour more extreme helping/harming (especially when younger) -- in the context of bi-sexual philopatry with non-local mating (e.g., whales) that favours female shift from harming to helping with age (which explains the evolution of menopause and post-reproductive helping), such shifts are earlier in smaller groups. Further explorations suggest that under group size local variation, kinship dynamics in a group reflect not only its own 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 be linked to observed variations in age-linked social traits, such as the timing of menopause in social mammals.