Associations between lifetime fitness and social bonds in female red deer

Abstract Social connections affect important components of fitness above and beyond environmental or morphological effects. In some primates and carnivores, females live in stable groups, supporting each other in competitive interactions and breeding within their birth groups. Social status and breeding success are linked to close social connections. However, it is unclear how important social bonds are in open membership fission–fusion societies. Recent work suggests that the strength of social bonds and the centrality of individuals within groups in these types of societies might be equally beneficial and important as in closed-membership groups, influencing population dynamics on par with morphological or environmental effects. Using social network analysis, we studied free-ranging adult female red deer (Cervus elaphus) on the Isle of Rum, confirming that they preferentially associate with relatives while forming open groups. It turned out that the strength of close social bonds in female red deer is a vital fitness correlate. Females’ lifetime breeding success and lifetime reproductive success positively correlated with the strength of their associations, as did female survival. Although home range overlap, matriline size and spatial measures were included in models of social network measures and fitness, the strength of social bonds was more crucial for fitness than spatial variables, except for calf survival, which was not linked to the strengths of associations between females. This study suggests that social bonds among female red deer are essential for reproductive success and survival, highlighting the importance of social network analysis in understanding fitness correlates in species with open group structures.