Digestive flexibility during fasting in the characid fish Hyphessobrycon luetkenii.

Digestive flexibility is a widespread phenomenon among animals, and the congruence between empirical data and optimal digestion models strongly supports the idea that it has evolved by natural selection. However, current understanding of the evolution of this amazing flexibility is far from being comprehensive. Evidence from vertebrate tetrapods suggests that there are two major mechanisms for intestinal down-regulation during fasting periods: a decrease in the number of enterocytes in the mucosal epithelium in endothermic species, and a transitional epithelium in concert with a marked hypotrophy of enterocytes in ectothermic species.