Because maintaining an acidic pH environment is costly, acidic stomachs should be present primarily in those cases where it is adaptive (or where it was adaptive in a recent ancestor). The cost of stomach acidity is twofold. The host must invest significant energy for both acid production and protecting the stomach from acid-related damage [17]. In addition, the acidity of the stomach may preclude, or at least make more difficult, chance acquisition of beneficial microbes. At the opposite extreme are those specialized herbivores in which stomach morphology is derived to include an alkaline chamber (forestomach or pre-saccus) that house microbes critical for fermenting a plant diet [18–22]. In these animals, an acidic stomach is not only of limited value (because the risk of foodborne pathogens in plant material is low), it may also remove those microbes that aid in the breakdown of plant material. Broadly then, we expect stomach acidity to mirror animal diets in ways that reflect pathogen risk. We expect that animals feeding on carrion will have the most restrictive filter, i.e. higher stomach acidity. Carrion has the potential to sustain high pathogen loads because the dead host’s body has stopped suppressing bacterial growth. Similarly, carnivores and omnivores would be expected to have higher stomach acidities than herbivores with specialized fermenting forestomachs because pathogens found in prey are more likely to be capable of infecting the predator than plant-associated microbes [23]. However, we would also expect the acidity of the carnivore and omnivore stomach to also depend on the phylogenetic distance between predator and prey. Pathogens are far more likely to be able to infect related hosts [23], such that a bird consuming an insect should face a lower risk of a foodborne infection than a bird consuming a bird. To test these hypotheses, we compare the stomach acidity of mammals and birds across a diversity of diet types.