Seth D. Newsome

Herbivory is a dominant feeding strategy among animals, yet herbivores are often protein limited. The gut microbiome is hypothesized to help maintain host protein balance by provisioning essential macromolecules, but this has never been tested in wild consumers. Using amino acid carbon (δ 13 C) and nitrogen (δ 15 N) isotope analysis, we estimated the proportional contributions of essential amino acids (AA ESS ) synthesized by gut microbes to five co‐occurring desert rodents representing herbivorous, omnivorous and insectivorous functional groups. We found that herbivorous rodents occupying lower trophic positions ( Dipodomys spp.) routed a substantial proportion (~40%–50%) of their AA ESS from gut microbes, while higher trophic level omnivores ( Peromyscus spp.) and insectivores ( Onychomys arenicola ) obtained most of their AA ESS (~58%) from plant‐based energy channels but still received ~20% of their AA ESS from gut microbes. These findings empirically demonstrate that gut microbes play a key functional role in host protein metabolism in wild animals.