SECOR, S.M; University of Alabama, Tuscaloosa: Evolutionary and proximate mechanisms of intestinal adaptation

Vertebrates display a broad spectrum of adaptive responses of their digestive system to feeding and fasting. This is exemplified among amphibians and reptiles from species that feed relatively frequently and experience modest fluctuations in intestinal performance to those species that feed infrequently and widely regulate intestinal performance with feeding and fasting. The evolutionary incentive to narrowly or widely regulate digestive performance with feeding, respectively, for frequent and infrequent feeders apparently lies in reduced rates of energy expenditure compared to the alternative digestive strategy. The proximate mechanisms underlying the regulation of intestinal performance are divided between morphologic and functional responses. A morphologic postfeeding response is fairly universal, generating at least a 50% increase in small intestinal mass. In contrast, the upregulation of intestinal nutrient uptake rates with feeding is extremely variable among species, from no changes to 20-fold increases in uptake rates. From dozens of amphibian and reptile species with varied feeding habits, I shall illustrate the interplay between the morphologic and functional mechanisms of intestinal response and discuss those mechanisms in the context of feeding ecology and phylogeny.