An often-stated ecomorphological assumption that has the status of ‘textbook knowledge’ is that the dimensions of the digestive tract correlate with diet, where herbivores – consuming diets of lower digestibility – have longer intestinal tracts than faunivores – consuming diets of higher digestibility. However, statistical approaches have so far failed to demonstrate this link. Here, we collated data on the length of intestinal sections and body mass of mammal species, and test for various relationships with trophic, climatic and other biological characteristics. All models showed a strong phylogenetic signal. Scaling relationships with body mass showed positive allometry at exponents >0.33, except for the caecum, which is particularly large in smaller species. Body mass was more tightly linked to small intestine than to large intestine length. Adding a diet proxy to the relationships increased model fit for all intestinal sections, except for the small intestine when accounting for phylogeny. Thus, diet has a main effect on the components of the large intestine, with longer measures in herbivores. Additionally, measures of habitat aridity had a positive relationship with large intestine length. The small intestine was longer in species from colder habitats at higher latitudes, possibly facilitating the processing of peak intake rates during the growing season. This study corroborates intuitive expectations on digestive tract anatomy, while the dependence of significant results on large sample sizes and inclusion of specific taxonomic groups indicates that the relationships cannot be considered fixed biological laws
