Does intestine length explain digesta retention times in birds and mammals?

Previous studies have indicated that across birds and mammals, body mass is a comparatively poor predictor of the time digesta is retained in the digestive tract (mean retention time, MRT). Rather, MRT might be determined by gastrointestinal anatomy, which can differ considerably within and between trophic guilds. Here, we used two recent literature compilations on the intestine length and the MRT in birds (n = 33 species) and mammals (n = 149) and applied comparative statistical approaches to assess whether intestine length is more closely correlated with MRT than body mass. Regardless of the statistical model used, whether small species (< 120 g and any larger bats) were included or not, or whether birds and mammals were assessed together or separately, intestinal length generally yielded a better model fit to MRT than body mass, supporting the general concept. Nevertheless, data scatter was substantial, indicating that intestine length, though better than body mass, is still a limited explanatory factor for MRT. The intestine length-MRT relationship is an example of the direct statistical assessment of a generally presumed form-function relationship that is typically represented as a narrative. In the comparative literature, such assessments are rare.