Ecology and evolution of the social microbiome

Animals with close social relationships often have similar microbiomes. These socially structured microbiomes can arise through multiple mechanisms that are often difficult to disentangle, including transmission between social partners or via socially structured, shared environments. Here, we review evidence for socially structured microbiomes and propose methods to differentiate the mechanisms that given rise to them. We discuss the evolutionary implications of these mechanisms for both hosts and their microbiomes, including the possibility that social transmission selects for host-specialized microbiomes. We conclude by identifying outstanding questions related to social microbiomes and their implications for social evolution. We identify new or underutilized approaches like longitudinal study designs, strain sharing analysis, and culture-based characterization to address these outstanding questions.