Flexibility of Intestine Theory |
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Researchers have recently developed a new theory on the energetic costs of up-regulating the digestive processes. New research suggests that the growth of the small intestine after digestion is not a result of the energetically costly act of tissue production, but instead a result of tissue flexibility. It was found that the increased size of the small intestine is associated with elongation of the intestinal villi and incorporation of lipid droplets into the spaces between the cells instead of rapid cell mitosis as previously thought (Stark and Beese, 2001). As seen in these pictures below, during fasting the small intestine shrinks in size by folding an compacting the cells into multiple layers. After feeding, the cells rearrange so that they are in a single layer and increase in size by incorporating lipid droplets between the cells (Starck and Beese, 2001).
Intestine during fasting |
Intestine after digestion |
Figure 1 used with permission of (Stark and Beese, 2001)
The flexibility of the tissue in the small intestine of pythons is therefore similar to the flexibility of the tissue in the bladder seen in other organisms. This idea accounts for the large size increase of the small intestine in a relatively short period of time which would have been almost impossible due to mitotic activity alone.
Additionally, recent research shows that the fasting duration does not affect the SDA response. As seen on the graph below, the SDA response is the same for digestion after the first meal as it is for digestion after a second meal immediately following the first (Overgaard et al, 2002).
Figure 2 used with permission of (Overgaard et al, 2002)
This data supports the intestinal flexibility theory, because if it is energetically expensive to up-regulate the intestine as the pay before pumping theory suggests, then the SDA response after the second meal should be substantially less because the gut is already up-regulated. However, if the intestine were flexible the SDA response would be the same regardless of fasting duration because gut up-regulation is not dependent on energy expenditure.
The intestinal flexibility theory is also more adaptive because it allows a snake that is energetically exhausted and without fat reserves to still be able to digest and absorb energy from its prey without first expending energy to increase it's intestinal mass. Whereas, in the pay before pumping model, a snake with a full stomach could still die from starvation because it does not have enough energy to up-regulate it's intestine for digestion. Furthermore, the intestinal flexibility model allows the python to increase it's intestine size at a low energetic cost.
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in Animal Physiology