Hair and feathers are composed of keratin and are indigestible, inalimental and unpalatable for carnivores. However, carnivores often ingest hair and feathers during feeding or when grooming. We hypothesized that ingestion of hair and feathers changes species diversity and relative abundance of bacteria in the gut of carnivores. To test this hypothesis, we added disinfected poultry down feathers to the normal diet of captive Arctic foxes (Alopex lagopus). We then used fluorescently labeled terminal restriction fragments (T-RFs) to examine changes in fecal bacterial diversity and abundance. The results showed that the number of bacterial species increased significantly after feather ingestion, but that total abundance was unchanged. This demonstrated that addition of disinfected feathers to the diet stimulated increased production among less abundant bacteria, resulting in a balancing of relative abundance of different bacterial species, or that some newly-ingested microbial species would colonize the gut because a suitable microhabitat had become available. This implies that the overall production of bacterial metabolites would be made up of a greater range of substances after feather ingestion. On one hand, the host's immune response would be more diverse, increasing the capacity of the immune system to regulate gut microflora. On the other hand, the animal's physiological performance would also be affected. For wild animals, such altered physiological traits would be subjected to natural selection, and, hence, persistent geographic differences in the character of ingested feathers or fur would drive speciation.