The role of herbivory in the macroevolution of vertebrate hormone dynamics

Vertebrates have high species-level variation in circulating hormone concentrations, and the functional significance of this variation is largely unknown. We tested the hypothesis that interspecific differences in hormone concentrations are partially driven by plant consumption, based on the prediction that herbivores should have higher basal hormone levels to 'outcompete' plant endocrine disruptors. We compared levels of glucocorticoids (GCs), the hormones with the most available data, across 166 species. Using phylogenetically informed comparisons, we found that herbivores had higher GC levels than carnivores. Furthermore, we found that the previously described negative relationship between GC levels and body mass only held in herbivores, not carnivores, and that the effect of diet was greatest at extreme body sizes. These findings demonstrate the far-reaching effects of diet on animal physiology, and provide evidence that herbivory influences circulating hormone concentrations. We urge future direct testing of the relationship between phytochemical load and GC levels.