Irvine, Calif., August 26, 2002
Gary Larson, creator of "The Far Side," is noted for morphing animal scientific attributes into human behavior in his comic strips. Consider the sketch of a family of pythons lying about after Thanksgiving dinner. The snakes that consumed a mouse, some chicken and glucose are ready to go out and play football shortly after dining. But the pythons that indulged on the starchy foods cannot budge from the couch, still trying to digest their meal.
A scene from a Larson cartoon? Perhaps. But it is also a notion based in scientific fact.
A team of UC Irvine researchers has found that pythons use significantly more energy to digest proteins than they do carbohydrates, revealing that metabolic rates needed for digestion are based on the content of the food instead of the volume. The findings also provide more information on understanding how other animals—and humans—metabolize food and the importance of their diet.
Marshall McCue, Albert Bennett and James Hicks, researchers in the Department of Ecology and Evolutionary Biology at UCI, will present these findings at the American Physiological Society intersociety meeting on Tuesday, Aug. 27, in San Diego, Calif. The researchers tested pythons to determine the reptiles' specific dynamic action (SDA), which is the metabolic increment associated with a python's digestion, assimilation and excretion of specific foods. SDA is determined not by how much a python eats, but what it eats.
Moreover, the energy required for a certain level of SDA accounts for a large energy expenditure that may reduce the energy available for other activities. The researchers used pythons because their metabolic rates vary drastically from when they are at rest, to when they are digesting.
Hatchling Burmese pythons were raised in the laboratory on a diet of mice and rats for four months prior to experiments. The pythons were then fed various meals of proteins, carbohydrates and lipids. Protein meals consisted of lean chicken breast meat, casein, collagen and gelatin. Carbohydrate meals included two complex (wheat starch, cellulose) and two simple carbohydrates (D-glucose, sucrose). Lipid meals consisted of lard and beef suet. Meal volumes were varied and feeding treatments were randomly assigned to each individual python.
The post feeding metabolic responses for each of the 11 experimental meals were combined and compared with their standard metabolic rate.
The key findings from this experiment were:
Mean masses of eight pythons before and after experiment did not change significantly.
Mouse, chicken, casein, collagen, glucose and sucrose meals induced an SDA response; gelatin, suet, lard, cellulose and starch did not induce SDA.
SDA responses induced by meals with the largest and smallest volumes were not statistically different; intermediate meals induced the greatest SDA.
Gelatin appeared to be assimilated but did not cause a significant SDA response.
Collagen caused an SDA response; however it was always completely regurgitated several days following ingestion.
Protein meals that induced the greatest SDA (casein, mouse, and chicken breast) were also those highest in essential amino acids.
The results revealed that single and dual element sugars caused the pythons' metabolic rate to increase two-fold. However, complex carbohydrates were unable to elicit a significant metabolic response and were not assimilated by the snakes. Protein meals caused variable SDA responses that appeared to be related to the amino acid composition of the specific meals. Casein caused a four-fold increase in metabolism, while gelatin caused no detectible changes and was not assimilated. Various lipid meals did not cause any significant change in oxygen consumption and were generally not assimilated.
The findings suggest that a large serving of one particular type of food probably does not trigger the large SDA increment well known in this species. The researchers' next steps are to investigate SDA induced by specific amino acids and amino acid mixtures and digestive assimilation efficiency of meals that induce SDA.
Essentially, pythons are governed by physiological principles that encourage consumption of specific foods to optimize their metabolic rate and to allow energy expenditure for activities other than digestion.
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