Evolution of regulatory responses to feeding in snakes

Do animal species that normally consume large meals at long intervals evolv e to down-regulate their metabolic physiology while fasting and to up-regul ate it steeply on feeding? To test this hypothesis, we compared postfeeding regulatory responses in eight snake species: four frequent feeders on smal l meals and four infrequent feeders on large meals. For each species, we me asured factorial changes in metabolic rate, in activities and capacities of five small intestinal brush border nutrient transporters, and in masses of eight organs that function in nutrient processing after consumption of a r odent meal equivalent to 25% of the snake's body mass. It turned out that, compared with frequent feeders, infrequent feeders digest that meal more sl owly; have lower metabolic rates, organ masses, and nutrient uptake rates a nd capacities while fasting; have higher energy expenditure during digestio n; and have higher postfeeding factorial increases in metabolic rate, organ masses, and nutrient uptake rates and capacities. These conclusions, which conform to the hypothesis mentioned above, remain after phylogeny has been taken into account. The small organ masses and low nutrient transporter ac tivities during fasting contribute to the low fasting metabolism of infrequ ent feeders. Quantitative calculations of partial energy budgets suggest th at energy savings drive the evolution of low mass and activities of organs during fasting and of large postfeeding regulatory responses in infrequent feeders. We propose further tests of this hypothesis among other snake spec ies and among other ectotherms.