The social microbiome: the missing mechanism mediating the sociality-fitness nexus?

TL;DR

This paper proposes that social transmission of microbiomes is a key mechanism linking social behavior to health and reproductive success in mammals, potentially surpassing stress-related explanations.

Contribution

It introduces the hypothesis that microbiome transmission mediates the sociality-fitness relationship, highlighting its importance over traditional stress-based mechanisms.

Findings

Microbiomes are socially transmitted via vertical and horizontal routes.

Microbiomes influence host development, physiology, and pathogen resistance.

Social microbiome transmission may explain fitness benefits of sociality.

Abstract

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.