Fracture Mechanics of Pelleted Feces Within Mammals

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Magondu, Benjamin W.
Hu, David L.
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In this combined experimental and theoretical study, we test the hypothesis that feces length is set by the dynamics of drying within the intestine. With previous measurements of the water contents of both pelleted and cylindrical feces at Zoo Atlanta collected, we find a transition from cylindrical to pelleted feces when the fecal water content drops below 65%. We thus focus our study on the drier pellet feces, whose length is dictated by crack formation. Using previous measurements for feces length, food intake, and intestinal dimensions, we find that pellet length scales inversely with the flux of water absorbed during digestion. This relationship suggests that the drying is similar to the formation of hexagonal columnar jointing, found in cooling lava beds. We build a mimic of the intestine using drying corn starch cakes in open troughs and confirm that feces length scales with water flux^−0.61, giving us qualitative confirmation of the scaling of feces length in mammals. Our study shows new similarities between geological rock formations and the formation of pellet feces within the intestine. The physical picture shown here may be of use to physiologists and veterinarians interested in using feces length as a marker of intestinal health.
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