Wombats have a distinctly cube-shaped shit and scientists finally know how they do it

Of all the poop in the world, only wombats have the shape of cubes.

The varied elasticity of the wombat's intestines helps marsupials sculpt their cube-like chip fragments, instead of round pellets, messy piles, or tubular turns made by other mammals, researchers have reported. November 18th at the Fluid Dynamics meeting of the American Physical Society in Atlanta.

The wombats mark their territories with small piles of dung. Cuboid hoops stack better than circles and do not roll as easily.

But cubic forms in nature are very unusual, says David Hu, a mechanical engineer at Georgia Institute of Technology in Atlanta. Making and preserving flat facets and sharp corners takes energy. It is therefore surprising that the intestines of the wombat – which are very similar to those of any other mammal – create this form.

When an Australian colleague sent Hu and his colleague Patricia Yang the intestines of two wombats killed on the road, accumulating frost in his freezer, "we opened them as if it were Christmas", a Hu said.

The intestines were full of poop, Yang said. In humans, a gut filled with poo stretches slightly. In wombats, the bowel stretches on two to three times its normal width to accommodate all feces.

Yang used skinny balloons – carved in animals at carnivals – to inflate the intestines and measure their extensibility in different places. Some regions were more scalable; some were stiffer. The stiffer regions probably help to create distinct edges on the wombat droppings when the waste moves into the intestine, suggests Yang.

Sculpting the poo in cuboid nuggets seems to be a finishing touch for the womb's digestive tract. On a typical 30-meter-long wombat gut, the dung does not have distinct edges until the last meter, Hu said. Until then, the waste solidifies as it moves through the intestine.

Finished torments are particularly dry and fibrous, which can help them retain their characteristic shape when they are squeezed out, Yang suggests. They can be stacked or rolled like dice, standing on any face. (She knows, she tried.)

In the wild, wombats deposit their droppings on rocks or logs as a marker of territory, sometimes forming small piles. They seem to prefer to poop in raised places, Hu says, but they are also limited by their fleshy legs.

To confirm that the variation of elasticity really forms the cubes, Yang and Hu are now trying to model the wombat's digestive tract with the help of tights.

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