In the last five years, several studies have shown that an animal’s social context can influence the composition of their microbiomes across body sites. These effects are potentially important because inter-host differences in the composition of the microbiome can affect host physical functioning and health. To date, the mechanisms driving socially structured microbiomes and the functional consequence of these patterns remain largely unknown.
Working in the well-studied Amboseli baboon population in Kenya, we find that socially structured gut microbiomes could be caused by direct and indirect transmission among group members and may affect the functional capacity of individual microbiomes. Specifically: (1) each baboon social group harbors a distinct gut bacterial community; (2) baboons in larger social groups have more diverse gut bacterial communities than baboons in smaller groups; (3) the longer a male baboon has lived in a given social group, the more his gut microbiome comes to resemble those of his fellow group members; and (4) within social groups, grooming bonds predict microbiome similarity, even controlling for genetic relatedness or similar diets. Importantly, some of these social signatures extend to the genic content of the gut microbiome, suggesting that social context affects to the microbiome’s functional capacities. We discuss these results in the context of the functional consequences of socially structured microbiomes and important future directions for this line of research.