Dietary Specialisation Shapes Gut Bacterial Diversity in Dung Beetles: Insights From Coprophagy to Millipede Carnivory.

Dung beetles are ecosystem engineers, providing ecosystem services like nutrient cycling, waste degradation and parasite suppression. Their gut microbiome is essential for exploiting specialised diets, yet the eco-evolutionary factors driving microbial composition across diverse feeding strategies remain ambiguous. Here, we show that diet strongly influences gut bacterial composition across seven dung beetle species specialising in coprophagy, necrophagy, detritophagy, fungivory and carnivory. Most dietary specialisations grouped separately, though fungivores clustered with carrion and millipede feeders. The millipede-feeding species, Sceliages brittoni and S. hippias, hosted the most distinct and least diverse gut microbiomes. Taxonomically, differences were driven by distinct marker taxa, many of which are consistently isolated across taxonomic orders with similar diets. For example, the indicative bacterial species I. indica has been identified in various flesh-feeding insect taxa. Crucially, this pattern of shared bacterial communities suggests that diet is a dominant structuring factor which promotes community convergence regardless of host phylogeny. This study highlights the role of diet in shaping the dung beetle gut microbiome and provides the first characterisation of the gut microbiota in millipede-feeding dung beetles. Our findings underscore the critical role of diet, laying the foundation for functional studies into the eco-evolutionary significance of these host-microbe interactions.