Why Do Wombats Poop Cubes? The Science Explained

How does a wombat make cube-shaped poop?

For a long time nobody knew. The answer came from a team of engineers and biologists from the United States and Australia, led by Patricia Yang and David Hu, together with wombat researchers at the University of Tasmania. They examined the intestines of common wombats that had died after road accidents in Tasmania.

Their findings, published in the journal Soft Matter in 2021, explained three key points:

• The cubes form late. Dung travels through most of the intestine as a soft, paste-like mass. The cube shape appears only in roughly the last 17% of the intestine.
• The gut wall is uneven. In that final section, cross-sections of the intestine show regions that are about twice as thick and about four times as stiff as the rest of the wall.
• Contractions do the molding. Using a mathematical model, the team showed that when a ring with alternating stiff and soft zones squeezes repeatedly, the stiff zones move faster and press flat faces, while the corners form where the softer zones are.

In other words, the wombat does not need a square opening. Its intestine works more like a set of flexible molds that slowly press each piece of drying dung into shape.

Why does dryness matter?

Wombat dung is unusually dry, and that dryness is part of the trick. Wombats eat tough, fibrous grasses and digest them very slowly; food takes about 8–14 days to pass through the gut. Along the way the long colon pulls out a great deal of water.

Firm, low-moisture dung can hold sharp edges, much as damp sand holds a shape better than either dry sand or mud. Softer feces would simply slump back into rounded lumps. This also explains why a wombat’s droppings can look less cube-like when it eats very lush green grass.

Why would cube poop be useful?

Wombats use their droppings to communicate. They are solitary, nocturnal animals, and each one advertises its presence by leaving dung in conspicuous places: on top of rocks, fallen logs, tree stumps and the soil mounds outside burrows. Other wombats can smell who left it and when.

Many researchers suggest that cubes are less likely to roll off these raised spots than round pellets would be. That idea is reasonable and widely repeated, but it is a hypothesis rather than a proven reason. The cube shape may simply be a by-product of having a dry diet and an unevenly stiff gut, which wombats then happen to use well.

Do all wombats poop cubes?

Yes. All three living wombat species, the common wombat and the southern and northern hairy-nosed wombats, produce cube-shaped droppings. The species differ in size and range, but they share the same slow, dry digestion.

No other animal is known to produce cubes regularly. Rabbits, deer and goats make neat pellets, and many other herbivores produce round or oval droppings, but true flat faces and corners appear to be a wombat specialty.

The Ig Nobel Prize and why scientists care

In 2019 Patricia Yang and David Hu received the Ig Nobel Prize for Physics for their early results on wombat cubes. The Ig Nobel Prizes are a light-hearted award for research that “makes people laugh, then think.” The full study appeared two years later.

The research is more than a curiosity. Understanding how a soft tube can make sharp corners might help engineers design new ways to shape soft materials. The authors also noted possible links to medicine, because the shape and texture of feces can reveal how the intestine is working.

Myths and facts about wombat poop

Myth: wombats have a square bottom. The anus is round. The cube shape is set before the dung reaches it.

Myth: wombats stack their cubes like bricks. Wombats do leave several cubes on the same rock or log, and photos sometimes show small piles. There is no evidence that they deliberately build towers; any stacking is simply where the cubes fall.

Fact: wombat scat helps scientists. Field researchers use cube-shaped scats to confirm that wombats are present in an area, map their ranges and count signs of activity without disturbing the animals.

Wombat burrows and droppings are often the easiest signs to spot on a walk in southeastern Australia or Tasmania. To learn more about the animal behind them, read the common wombat profile, find out how fast a wombat can run, or see why wombats have backward pouches.