Devouring an unsuspecting victim poses a serious challenge for pythons' guts. During the long gaps between meals, pythons cut their energy costs by keeping their metabolic rate low. But when they guzzle a large meal, their metabolic rate soars as they fire up their digestion. This post-feeding increase in metabolic rate is called specific dynamic action (SDA). All animals pay this `tax' on food processing, but the degree of metabolic upregulation depends on the meal. In mammals, digesting fats and carbohydrates requires minimum upregulation, while the highest SDA levels accompany diets rich in protein. Thus, pythons eager to gorge on protein are perfect models to identify the causes of metabolic responses to feeding.

Marshall McCue, Albert Bennett and James Hicks from the Department of Ecology and Evolutionary Biology in Irvine, California were intrigued by the metabolic tax caused by different diets, and set out to quantify the effect of meal composition on SDA in pythons. They fed 18 Burmese pythons (Python molurus) pureed meals with identical energy content but varying amounts of carbohydrates, lipids and proteins. To compare the metabolic costs of digesting prepared meal mixtures with those of processing an entire carcass, they also offered the snakes whole mice. To monitor how pythons' metabolic rate adjusted to cope with different meals, the team served these dishes to food-deprived snakes while continuously measuring the reptiles' oxygen consumption. A large increase in metabolic rate (a large SDA response) would reveal which part of the pythons' meal carries the highest metabolic cost.

McCue and his colleagues found that python SDA is highly dependent on the meal's chemical composition. Because pythons are carnivorous, the Californian group was not surprised to find that fat and carbohydrate meals passed through the snakes' intestines without considerable SDA response; they weren't being digested, except for glucose and sucrose, which induced relatively high SDA. Instead, the group discovered that de novo protein synthesis, the new formation of proteins from prey amino acids, causes the greatest increase in pythons' metabolic rate. To demonstrate the influence of de novo protein synthesis on SDA, the group chemically prevented the process by administering a protein synthesis inhibitor to the amino acid mixture that they fed the snakes. They found that SDA dropped over 70%. The importance of protein synthesis in metabolic upregulation was also supported by the group's discovery that digesting complete protein mixtures, which consisted of all the amino acids found in whole mice, induced higher metabolic rates than snake menus with an `incomplete' mix, deficient in certain amino acids. In addition, they found that digesting simple proteins such as gelatin and collagen caused lower levels of metabolic upregulation than the complex proteins in whole-mice dishes. The team concluded that, due to protein synthesis after eating prey amino acids, pythons pay the biggest metabolic price for digesting protein, not for digesting fat or carbohydrates.

But pythons don't normally eat pureed meals, so the team wanted to find out if a python's habit of cramming a whole animal into its gut carries a cost. McCue and his co-workers were astonished to find that pureed python meals induced 36% greater SDA than wolfing down an intact mouse. They concluded that mechanical digestion of prey doesn't bear metabolic costs. Since snakes fuel SDA with the energy stored within their prey, pythons apparently mobilize nutrients more effectively from a prey soup than from a complete carcass.