Animals
How Do Wombats Poop Cubes? The Answer to a Scientific Mystery
Nature has almost no cubes. Stones get rounded, fruit is round, and most animal droppings are round or elongated. Yet one animal leaves behind small, neat cubes: the Australian wombat. For years this was a puzzle. Only recently did a team of engineers explain how it happens.
Meet the wombat
The wombat is a marsupial that lives in Australia. It looks a bit like a small, stocky bear with short legs, and it digs long burrows. It eats mostly grasses, roots and bark, spends much of the day underground, and is active mainly at night.
How many cubes, and how big?
A wombat can produce up to about a hundred cubes a day. Each one is roughly two centimetres across, and they come out in groups of four to eight at a time. They are not perfect dice, but the flat faces and corners are very clear.
Ideas that turned out wrong
Before the mystery was solved, there were several theories. One was that the wombat's anus is square. It is not: it is round, like that of most mammals. Another was that the wombat shapes its droppings deliberately. That was not supported either. The answer turned out to be deeper inside the gut.
The solution: stiff and stretchy patches in the gut wall
The research was led by Patricia Yang, a mechanical engineer at the Georgia Institute of Technology, together with fluid-mechanics professor David Hu. The team examined the digestive tracts of wombats that had been killed in road accidents. The intestine turned out to be very long, around ten metres, roughly ten times the animal's body length.
The shape forms only at the very end of digestion. In that part of the intestine, the wall is not uniform: it has stiffer regions and more elastic ones. When the muscles contract, these regions contract at different speeds. The stiff parts squeeze harder and form the corners, while the stretchy parts form the flat faces. The researchers compared a cross-section of the gut wall to a rubber band with two taut ends and a slack middle.
Diet helps too: wombat droppings are very dry, because the long intestine absorbs almost all the water. Without that dryness, the shape would not hold.
The team presented its first findings in 2018 and published a more detailed study in the journal Soft Matter in 2021. In 2019 the work also won an Ig Nobel Prize in physics, a humorous award for research that "first makes people laugh, and then makes them think".
Why would a wombat need cubes?
The usual explanation is communication. Wombats have poor eyesight and rely heavily on smell. They leave their droppings on rocks, logs and small mounds to mark territory and signal to other wombats. A cube does not roll away easily, so it stays on the high spot where it was left. There is no definitive proof that this is the reason, but it is the explanation most researchers accept.
Why engineers care
This may sound like a purely amusing piece of research, but there is a serious side. In industry, shapes with corners are usually made by cutting or by casting in a mould. The wombat shows another way: shaping something with a soft wall that contracts unevenly. That insight could one day inspire new manufacturing methods, for example for soft materials.
Test yourself
The wombat question is one of the favourites in Bullshit. Most players are sure somebody made it up. Now you know it is true, and you even understand how it works.