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It’s the wombat’s strange intestines, not its anus, that produces cubed poo

cube shaped wombat dropping
Enlarge / Look at this lovely cube-shaped piece of poo, courtesy of the Australian bare-nosed wombat.

Patricia J. Yang et al., 2021

Scientists have been puzzling for decades over how the Australian bare-nosed wombat poops out neat little cubes of feces instead of tapered cylinders like pretty much every other animal. According to a new paper published in the journal Soft Matter, the secret lies in their intestines, which have varying stiff and soft regions that serve to shape the poo during the digestive process. Earlier preliminary findings by the same group won the 2019 Ig Nobel Physics Prize.

“Bare-nosed wombats are renowned for producing distinctive, cube-shaped poos. This ability to form relatively uniform, clean cut feces is unique in the animal kingdom,” said University of Tasmania wildlife ecologist Scott Carver, a co-author of the paper. “They place these feces at prominent points in their home range, such as around a rock or a log, to communicate with each other. Our research found that these cubes are formed within the last sections of the intestine—and finally proves that you really can fit a square peg through a round hole.”

Zoologist Eric Guiler first noted the unusual shape of wombat droppings in 1960, and to date, wombats are the only known animals to produce six-sided cube-shaped poo. It’s one of several examples of naturally occurring pattern formation, such as the columns of Ireland’s Giant’s Causeway (formed by cooling lava), or how vibrating membranes can make grains of sand form “Chladni figures.” But naturally occurring cube shapes are extremely rare. The Australian bare-nosed wombat (Vombatus ursinus) can pump out as many as 100 cube-shaped droppings a day.

The intrepid team of researchers from Georgia Tech and the University of Tasmania were able to figure out why the wombat poops out cubes with the help of a dissected wombat carcass. They presented preliminary findings at a 2018 meeting of the American Physical Society’s Division of Fluid Dynamics, concluding that it all comes down to the shape and flexibility of the wombat intestines, combined with the relatively dry environments in which the animals live. Specifically, they found that the feces started out in a liquid-like state and then changed to a solid state in the last 25 percent of the intestine. In the last eight percent, the team noted two grooves that were more elastic than the rest of the intestine, which they believed accounted for the unusual cube-shaped droppings.

This new paper follows up on those earlier findings, adding the results of two additional dissections. (All three carcasses were those of wombats hit by cars, an unfortunately common occurrence in Australia, according to the authors.) The researchers combined this analysis with CT scans of a live adult female wombat, which showed a round anus, much like other animals. This enabled them to conclude that the wombats do not form the cube-like shape through extrusion, like pasta or injection-molded plastics—one of the proposed potential mechanisms. Rather, the cubes are the result of “a combination of unique material properties and muscular contractions,” the authors wrote.

Wombats have unusually long and spacious intestines to compensate for their relatively low-nutrient diet of grasses and sedges, resulting in longer digestion times of between 40 and 80 hours. Their low metabolic rate is critical to helping them survive droughts, for example. The intestine has four sections: the stomach, a short small intestine, a long proximal colon, and a distal colon. The feces typically takes on the consistency of a yellow-green slurry the consistency of yogurt, with water gradually being removed as it travels through the intestine. The fecal matter becomes darker and harder within the last 17 percent of the intestine, and the trademark edges of a cube begin to form as it reaches the distal colon.

By combining their laboratory test with 2D mathematical modeling, the researchers found two regions around the circumference of the intestine that were stiffer than the rest and two that were more flexible. Other mammals have intestinal muscles that are consistent in all directions. “The non-uniformity in the wombat intestines cause amplified contractions in distinct pre-set locations,” the authors wrote, and this in turn, over many cycles, “encourage the preferential movement of feces and the sculpting of the corners.”

While their model found that it takes less than 10 contraction cycles for corners to form, they acknowledge that this time frame is not realistic. In live wombats, contractions would occur every couple of seconds over the course of five days—some 100,000 contractions in all. The model also focuses on the feces cross-section—four faces of the six. They believe that taking the effects of feces dryness into account in future models might account for the remaining two faces of the cube, based on preliminary testing with drying corn starch. While the model is imperfect, “coming up with this absolutely new mechanism on how you can form these corners took lots of iterations, and we’ve now managed that without using the wombat itself,” said co-author David Hu of Georgia Tech.

This new knowledge could one day prove useful to manufacturing industries keen to expand their techniques for producing cube-shaped products or to detect colon cancer (an early symptom is the stiffening of parts of the colon). It could also help track the digestive health of wombats in captivity.

“Cube formation can help us understand the hydration status of wombats, as their feces can appear less cubed in wetter conditions. It also shows how intestinal stiffening can produce smooth sides as a feature of pathology,” Carver said. “Now we understand how these cubes are formed, but there is still much to be learned about wombat behavior to fully understand why they evolved to produce cubes in the first place.”

DOI: Soft Matter, 2021. 10.1039/D0SM01230K  (About DOIs).

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