The gut microbiota does not promote host sociality in the European earwig

Among the many effects of gut microbes on their hosts, a recent hypothesis suggests that they may have played a key role in host social evolution. By increasing the level of host aggregation, gut microbes could increase their own transmission to new hosts. While this hypothesis has been addressed in some theoretical work, it remains untested empirically. Here, we addressed this knowledge gap using the European earwig, an insect that naturally exhibits both solitary and social life. We first tested whether variation in earwig aggregation levels reflected variation in their gut microbiota, and then performed a gut content transplant experiment to test whether acquiring a gut microbiota from social individuals augments host aggregation. Contrary to our predictions, naturally high or low aggregation levels were not associated with gut microbial communities, and low-aggregation individuals did not become more gregarious by ingesting gut content from high-aggregation conspecifics. Overall, these results reveal that the aggregation level of earwigs is neither driven by a specific group of manipulative microorganisms nor by their gut microbial diversity. More generally, these experimental data indicate that the gut microbiota may only have limited influence on its host’s behavioural transition from solitary to group living.