AbstractThe gut microbiota is critical for host function. Among mammals, host phylogenetic relatedness and diet are strong drivers of gut microbiota structure, but one factor may be more influential than the other. Here, we used 16S rRNA gene sequencing to determine the relative contributions of host phylogeny and host dietary guild in structuring the gut microbiotas of 11 herbivore species from 5 families living sympatrically in southwest Kenya. Herbivore species were classified as grazers, browsers, or mixed-feeders. We found that gut microbiotas were highly species-specific, and that host family accounted for more variation in the gut microbiota (35%) than did host dietary guild (14%). Overall, similarity among gut microbiotas increased with host phylogenetic relatedness (r=0.73), yet this relationship was not apparent among seven closely related Bovid host species (r=0.21 NS). In bovids, host dietary guild explained twice as much variation in the gut microbiota as did host species. Lastly, we found that the gut microbiotas of herbivores residing in southwest Kenya closely resemble those of conspecifics from central Kenya, suggesting that regardless of variability in host local habitat, hosts consistently provide microbes with similar niches for colonization. Overall, our findings suggest that host phylogeny may structure the gut microbiota at broad taxonomic scales, but that host ecology may be more influential in shaping the gut microbiotas of closely related host species.