Snakes are exothermic animals so they require much less energy than endothermic animals, thus they have relatively low metabolic rates during fasting periods. Most snakes are periodic feeders consuming anywhere from 6 to 30 meals per year, that are 55 to 300 percent of their body mass. Most feeding occurs during warmer months because digestion cannot occur at low temperatures, in boa constrictors the majority of feeding occurs during late summer through fall.
Snakes have the ability to up-regulate and down-regulate their gastrointestinal activity to increase or decrease their metabolic rate and energy expenditure. Up-regulation occurs within 6-72 hours, during which there is an increase in digestive enzymes, hydrochloric acid, and transport molecules. The mass of the small intestines increases by 70% and the length of microvilli increases by 400 percent. The pancreas, liver, and kidney double in mass. The mass of the heart increases by 40 percent and the heart rate increases by 300 to 400 percent. This causes the metabolic rate of the snake to increase by 500 to 4,400 percent (687% for pythons). After 4 to 15 days the metabolic rate begins decreasing until fasting metabolic levels are restored, entering down-regulation.
Cycling between these two metabolic states is not energetically efficient if the snake is eating more frequently than every five weeks due to the energetic demands of boost starting metabolism. Snakes that feed periodically typically in four to six-week intervals (pythons, boas, vipers) cycle between up and down-regulated metabolic activity, while snakes that feed frequently typically in one to two-week intervals (colubrid and elapids) do not experience large variations in metabolic rate or intestinal mass.
The specific dynamic action (SDA) is the increase in metabolism above fasting levels resulting from the total expended energy of ingestion, digestion, and absorption. SDA varies between snake species, ranging from 13 to 33 percent of the ingested energy of the prey. SDA is higher in infrequent feeders and lowers in infrequent feeders. Compared to endothermic animals, the digestive efficiency of snakes is relatively high averaging between 85 to 95 percent.
Undigested and unabsorbed energy is expelled as feces. The time it takes feces to be expelled from the body differs between terrestrial and arboreal snakes. Arboreal snakes quickly pass fecal matter, expelling it within days, this reduces their weight to increase their agility and stability. In contrast, terrestrial snakes retain feces until the cloaca reaches maximum capacity, thus defecating every several months. At any given moment in terrestrial snakes, fecal matter represents 5 to 20 percent of their total mass. This retention and storage of fecal matter can result in obstruction if the snake does not receive adequate water, this is harmful and can be fatal.