Biological interactions, including symbiotic ones, have vital roles in ecological and evolutionary processes. Microbial symbionts in the intestinal tracts, known as the gut microbiome, are especially important because they can fundamentally influence the life history, fitness, and competitiveness of their hosts. Studies on the gut-resident microorganisms of wild animals focus mainly on vertebrates, and studies on species-rich invertebrate taxa, such as ground beetles, are sparse. In fact, even among the species-rich genus Carabus, only the gut microbiome of two Asian species was studied, while results on European species are completely missing. Here, we investigated the gut bacterial microbiome of a widespread European Carabus species, targeting the V3 and V4 regions of the 16S ribosomal RNA genes by next-generation high-throughput sequencing. We identified 1138 different operational taxonomic units assigned to 21 bacterial phyla, 90 families, and 197 genera. Members of the carbohydrate-degrading Prevotellaceae family, previously not detected in ground beetles, were the most abundant in the gut microbiome of the carnivorous C. convexus. Presumably, individuals from the studied wild populations also consume plant materials, especially fruits, and these carbohydrate-degrading bacterial symbionts can facilitate both the consumption and the digestion of these supplementary foods.

Urbanization is a major and rapidly expanding form of land-use change worldwide and is one of the main drivers of the decline in arthropod diversity. Within urban matrices, remnants of natural or semi-natural habitats serve as important refuges for native organisms. However, these urban fragments are typically small, isolated, and strongly affected by various forms of disturbance. Therefore, connectivity among urban remnant patches may enhance population persistence and resilience. Increased tendencies to explore novel environments, tolerate human disturbance, and exploit unpredictable resources can be advantageous in urban environments. Accordingly, in this study of a flightless ground beetle species, we hypothesized that individuals from urban habitats—especially from isolated ones—would be bolder and more exploratory than their rural conspecifics, that sexes would differ in behavior, and that these behaviors would be temporally consistent, indicating animal personality. Activity-, exploration-, and boldness-related behavioral traits were significantly repeatable, providing evidence for animal personality, particularly in females and rural beetles. Contrary to our hypothesis, no behavioral differences were detected between rural and urban individuals. Furthermore, no significant sex-dependent differences in behavior were observed. Overall, our findings highlight the importance of species-specific traits and ecological context in shaping behavioral variation.