Gross intestinal morphometry and allometry in ruminants

While some descriptions of ruminants' dietary adaptations suggest that the length of the intestinal tract reflects the proportion of grass or browse in the diet, this assumption has been questioned. We collated data on body mass (BM), as well as small intestine, caecum, colon/rectum, large and total intestine length in 68 ruminant species, and, while accounting for the phylogenetic structure of the dataset, evaluated both allometric scaling and the potential influence of diet, digestive physiology or climate proxies on measures of intestine length. Intestinal length generally scaled to BM at an exponent higher than the 0.33 expected due to geometry. Diet or digestive physiology proxies did not have an influence on any intestinal length measures, though some proxies indicating more arid natural habitats were positively correlated with measures of the large intestine. The relative size of a forestomach compartment, the omasum, was negatively correlated with intestine length. The results indicate that intestine length measures provide little indication of feeding type or digestive physiology, but rather indicate adaptations to aridity. Higher‐than‐geometry scaling of intestinal length may be related to the necessity of maintaining geometric (or metabolic) scaling of intestinal surface area while keeping gut diameter, and hence the diffusion distances, small. The way in which space trade‐offs determine the macroanatomy of different organs in the abdominal cavity, such as the omasum and the intestine, deserves further investigation. Compared to carnivores, ruminants have distinctively longer large intestines. This is not only linked to extensive microbial fermentative digestion at this site as such, but also to the high moisture content of digesta required for efficient microbial fermentation: The moisture needs to be reabsorbed, by a long large intestine, to avoid excessive faecal fluid losses.