  In many social mammals, early life social adversity and social integration
largely predict individual health, lifespan and reproductive success. Efforts
in identifying the physiological mechanisms mediating the relationship between
the social environment and individual fitness have so far concentrated on
socially-induced stress, mediated by alterations in neuroendocrine signaling
and immune function. Here, we propose a much-needed alternative mechanism
relying on microbially-mediated effects: social relationships with
conspecifics, both in early life and adulthood, might strongly contribute both
to the transmission of beneficial microbes and to diversifying host
microbiomes. In turn, more valuable and diverse microbiomes would promote
pathogen resistance and optimal health and thus translate into positive fitness
outcomes. This mechanism relies on two emerging findings from empirical
studies, namely that microbiomes (i) are largely socially transmitted via
vertical and horizontal routes, and (ii) play a pervasive role in host
development, physiology, metabolism, and susceptibility to pathogens. We
suggest that the social transmission of microbiomes has the potential to
explain the sociality-fitness nexus, to a similar - or even higher - extent
than chronic social stress, in ways that have yet to be studied empirically in
social mammals.
