“You Are What You Eat” OR “You Eat What You Are?” Comparative Mammalian Gastrointestinal Anatomy

As mammals occupy a wide array of ecological niches and exploit various sources of nutrition, so too do they exhibit great functional and morphological diversity in the gastrointestinal tract (GIT). The GIT is a metabolically expensive organ and its associated adaptations are essential to maintaining energy balance. Balancing homeothermy and lactation require a faster metabolism among mammals relative to those of reptiles given their substantial energetic cost. This study documents morphological analogies among the GIT's of three mammalian omnivores: squirrels, pigs and humans. Similarities included a monogastric (or nonruminant) template to accommodate a dietary staple of nuts, seeds, fruits, fungi, green vegetation, and animal matter. All species presented a mesentery that serves as a conduit for arterial supply and venous and lymphatic drainage of the regional gut except that, in the squirrel, the mesentery was thinner and appeared more translucent. Further differences included a relatively large caecum (probably a vestige from an ancestor that engaged in hindgut fermentation as modern squirrels cannot digest cellulose) and a curious loop in the squirrel large intestine. Pigs and human both exhibited small intestines that appeared longer and with more intricate networks of vasae rectae. They also had considerably greater amounts of adipose tissue in their messenteries with humans possessing the most. Based on our study, the pig is remarkably similar to the human condition. Both are large‐bodied mammals with similarly wider dietary preferences than that of the squirrel. In spite of the closer phylogenetic relationship among squirrels and humans (Rodentia and Primates are both within the superorder Archonta), the GIT of pigs and humans have been subjected to similar selective pressures as a likely function of similar body mass.