In many social mammals, early social life and social integration in adulthood largely predict individual health, lifespan and reproductive success. To date, research has mainly focused on chronic stress as a physiological mediator between social environment and fitness. Here, we propose an alternative, non-exclusive mechanism based on microbe-mediated effects: social relationships with conspecifics during early life and into adulthood could strongly contribute to the diversification of host microbiomes and the transmission of beneficial microbes. In turn, more diverse and valuable microbiomes would promote pathogen resistance and optimal health, translating into lifelong fitness benefits. This mechanism is based on recent findings showing that microbiomes are widely transmitted through social pathways and play a pervasive role in host development, physiology and susceptibility to pathogens. We suggest that social transmission of microbes could explain the link between sociality and fitness to a similar or greater degree than chronic social stress, and merits empirical studies in social mammals.