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Intestines of non-uniform stiffness mold the corners of wombat feces †

Patricia J. Yang, ‡a Alexander B. Lee, ‡b Miles Chan,a Michael Kowalski,c Kelly Qiu,c Christopher Waid,c Gabriel Cervantes,a Benjamin Magondu,c Morgan Biagioni,a Larry Bird's Nest,d Alynn Martin,e Ashley Edwards,e Scott Carver *e and David L. Hu *from  
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* Corresponding authors

a School of Mechanical Engineering, Georgia Institute of Technology, 801 Ferst Drive, MRDC 1308, Atlanta, GA 30332, USA
E-mail:[email protected]
Tel: +1 404 894-0573

b School of Biological Science, Georgia Institute of Technology, 801 Ferst Drive, MRDC 1308, Atlanta, GA 30332, USA

c School of Biomedical Engineering, Georgia Institute of Technology, 801 Ferst Drive, MRDC 1308, Atlanta, GA 30332, USA

d Taronga Conservation Society, Mosman, NSW 2088, Australia

e Department of Biological Science, University of Tasmania, Hobart, Tasmania 7005, Australia
E-mail:[email protected]
Tel: +61 3 6226 2794

Abstract

The bare-nosed wombat (Vombatus ursinus) is a fossorial, herbivorous, Australian marsupial, renowned for its cubic feces. However, the ability of the wombat's soft intestine to sculpt flat faces and sharp corners in feces is poorly understood. In this combined experimental and numerical study, we show one mechanism for the formation of corners in a highly damped environment. Wombat dissections show that cubes are formed within the last 17 percent of the intestine. Using histology and tensile testing, we discover that the cross-section of the intestine exhibits regions with a two-fold increase in thickness and a four-fold increase in stiffness, which we hypothesize facilitates the formation of corners by contractions of the intestine. Using a mathematical model, we simulate a series of azimuthal contractions of a damped elastic ring composed of alternating stiff and soft regions. Increased stiffness ratio and higher Reynolds number yield shapes that are more square. The corners arise from faster contraction in the stiff regions and relatively slower movement in the center of the soft regions. These results may have applications in manufacturing, clinical pathology, and digestive health.

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Soft matter, 2021,17, 475-488

Intestines of non-uniform stiffness mold the corners of wombat feces

PJ Yang, AB Lee, M. Chan, M. Kowalski, K. Qiu, C. Waid, G. Cervantes, B. Magondu, M. Biagioni, L. Vogelnest, A. Martin, A. Edwards, S. Carver and DL Hu, Soft matter, 2021, 17, 475

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