Response of Fecal Bacteria and Fungi to Tannin‐Rich Diets in Sika Deer (Cervus nippon): Evidence from Both Feeding Experiments and Field Investigations

The rumen microbes of large mammalian herbivores (LMHs) play a critical role in resisting the chemical defenses of plants, such as tannins, but whether fecal microbes in LMHs function in this adaptive process remains unclear. Despite ample studies over the past decade have focused mainly on bacteria, the role of fungi in the response to tannins requires more attention. Additionally, most previous studies have been restricted to captive herbivores, and we still do not know much about those in the wild. To understand the role of fecal bacteria and fungi in plant‒herbivore interactions, we first tested the response and variation in the response of fecal bacteria and fungi to different concentrations of tannin‐rich food using 16S/ITS rRNA data in feeding experiments with sika deer ( Cervus nippon ). For wild sika deer in Northeast Tiger and Leopard National Park, we observed seasonal differences in the tannin content of their diets, which was significantly higher in spring than in autumn. Interestingly, the pattern of the fecal bacteria in wild sika deer response to tannin‐rich diets was similar to that observed in the feeding experiments but showed variation in fungi. Feeding with tannin‐rich diets altered the fecal microbial composition and increased the relative abundance of tannin‐degrading microbes. We hypothesize that fecal bacteria and fungi may play important roles in helping LMHs adapt to tannin‐rich diets. We need to further explore the function of microbes to provide important insights into the role of fecal microbes in plant chemical defenses and herbivore co‐evolution.