A scene from a Larson cartoon? Perhaps. But it is also a notion based in scientific fact.
A team of California researchers have tested pythons, determining the reptiles' specific dynamic action (SDA), or the metabolic increment associated with a python's digestion, assimilation, and excretion of specific foods. SDA is determined not by how much it eats, but what it eats. Moreover, the energy required for a certain level of SDA accounts for a large energy expenditure, which may reduce the metabolic scope available for other activities.
The authors of these findings will discuss their results in a presentation entitled, "Effects Of Meal Type On Postprandial Calorigenesis In Python Molurus." The investigators are M.D. McCue, A.F. Bennett, and J.W. Hicks, all of the Department of Ecology and Evolutionary Biology, University of California, Irvine. They will appear during the upcoming scientific conference, "The Power of Comparative Physiology: Evolution, Integration and Application" an American Physiological Society intersociety meeting being held August 24-28, 2002, at the Town & Country Hotel, San Diego, CA. Further information about the conference and the speakers can be found at: http://www.
Hatchling Burmese pythons were obtained and raised in the laboratory on a diet of mice and rats for four months prior to experiments. The pythons were then fed various 500 and 250 kJ meals (protein, carbohydrate, lipid) using rigid acrylic feeding tubes. 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. Slurries were created with dehydrated chemicals by adding water until a consistency similar to maple syrup was achieved. Meal volumes were not controlled and varied and feeding treatments were randomly assigned to each individual python.
Following experimental feedings, snakes were placed inside metabolic chambers and oxygen consumption was measured over a 62-96 hour period, using flow-through respirometry. Trials where snakes regurgitated meals during metabolic measurements were excluded from the study. Snakes were given an experimental feeding every 14-21 days. Between experimental feedings, snakes were allowed to voluntarily consume one mouse or rat.
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.
- All meals had heats of combustion of 500 kJ except for simple carbohydrates, which were found to be lethal to these snakes at 500 kJ.
- 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 is 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 does probably 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 allow energy expenditure for activities other than digestion.
The American Physiological Society (APS) is one of the world's most prestigious organizations for physiological scientists. These researchers specialize in understanding the processes and functions by which animals live, and thus ultimately underlie human health and disease. Founded in 1887 the Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals each year.
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