Evolutionary and cellular mechanisms regulating intestinal performance of amphibians and reptiles

SYNOPSIS. Vertebrate intestinal tracts possess an array of structural and functional adaptations to the wide diversity of food and feeding habits. In addition to well-described differences in form and function between herbivores and carnivores, the intestine exhibits adaptive plasticity to variation in digestive demand. The capacity to which intestinal performance responds to changes in digestive demands is a product of evolu-tionary and cellular mechanisms. In this report, I have taken an integrative approach to exploring the mechanisms responsible for the regulation of intestinal performance with feeding and fasting among am-phibians and reptiles. Intestinal performance is presented as the total small intestinal capacity to absorb nutrients, quantified as a product of small intestinal mass and mass-specific rates of nutrient uptake. For sit-and-wait foraging snakes and estivating anurans, both of which naturally experience long episodes of fasting, the dramatic downregulation of intestinal morphology and function with fasting reduces energy expenditure during extended fasts. In contrast, frequently-feeding species modestly regulate intestinal per-formance with fasting and feeding, trading higher basal rates of metabolism during fasting for the frequent expense of upregulating the gut with feeding. Surveying the magnitude by which intestinal uptake capacity is regulated among 26 families of amphibians and reptiles has revealed potentially five lineages that have independently evolved the capacity to widely regulate intestinal performance. The extent to which intestina