......... there has been a coevolution between brain size and gut size in humans and other primates. The logical conclusion is that no matter what is selecting for brain-size increase, one would expect a corresponding selection for reduction in the relative size of the gut. If it was necessary for the primate to have a large gut, that primate would also be expected to have a relatively small brain. The gut is the only one of the expensive metabolic tissues that could vary in size sufficiently to offset the metabolic cost of the encephalized brain. The reason for this is that, although gut size is related to body size, its size & proportions are also strongly determined by diet. Gut size is associated with both the bulk and the digestibility of food. Diets characterized by large quantities of food of low digestibility require relatively large guts characterized by voluminous and elaborated fermenting chambers. An extreme example is the ruminants, which are folivores, usually subsisting almost entirely on grasses. Conversely, diets characterized by smaller quantities of food of high digestibility require relativity smaller guts and are characterized by simple stomachs and proportionately long small intestines. Carnivores typify this pattern. ..........
Reference:
Aiello LC, Wheeler P: The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution. Current Anthropology 1995;36:199-221.