Many studies highlight the correlation between social ties and fitness, yet often cannot reveal how ties influence fitness. This review is aimed to facilitate the formulation and testing of hypotheses in this area on non-human primates. I outline fitness-relevant measures of social ties and 6 potential pathways from ties to fitness. Pathways include communal care; group level cooperation for resources; monopolizing resources within groups; attaining social status; reducing risk and allostatic load; and developing behavioural competence. Hypotheses for further evaluation include (1) fitness increases sociality, not vice versa; (2) early life experience influences both ties and fitness, (3) ties are actually costly; (4) short term costs of ties are outweighed by long term benefits. With the advance of theoretical and methodological approaches to evaluate the costs and benefits of social ties, and monitor them at multi-generational field sites, primate behavioural ecologists are poised to test several of these hypotheses.