Why is wombat poo cube-shaped?

Wombats are the only animals in the world that produce cube-shaped scat. But why—and how—do they do it? Scientists now have an idea why.

By Tik Root
Published 19 Nov 2018, 18:55 GMT
A young common wombat ('Vombatus ursinus tasmaniensus') at the Healesville Sanctuary. These animals produce some of ...
A young common wombat ('Vombatus ursinus tasmaniensus') at the Healesville Sanctuary. These animals produce some of the animal kingdom's strangest-shaped scat.
Photograph by Joël Sartore, National Geographic Photo Ark

Wombats are a burrowing animal native to Australia and perhaps best-known for being, well, podgy—and quite cute. But there’s something you might not know about these adorable marsupials: Wombats are the only animals in the world that produce cube-shaped poo.

While this peculiarity has sparked much interest and debate, actual research into the intricacies of wombat scat has been scant. That’s left scientists largely in the dark about the phenomenon – until recently.

Earlier this year, Patricia Yang, a researcher at the George Institute of Technology who specialises in bodily fluids, started to look into the topic more closely after hearing about it at a conference.

“I didn't really believe it,” Yang says. But after confirming that it is, indeed, a fact, she began trying to figure out why, and how, wombats poo in cubes.

“People have had all sorts of theories,” says Mike Swinbourne, a wombat expert at the University of Adelaide in Australia. One popular postulate is that wombats make cubes so that they can stack them to mark their territory, without the pieces rolling away. But Swinbourne says that’s a misconception.

While wombats do use their scat to mark territory, “it's not like they're trying to build little brick pyramids,” he says. “They just poop where they poop.”

Instead, Swinbourne says the cubic shape is more likely related to the dry environments that most wombats live in. “They have to really squeeze every drop of moisture out [of their food],” he said. And sometimes, in zoos, where the animals have readier access to hydration, Swinbourne says their scat is less cubic. Being dry helps the scats form more rigid shapes with sharper angles.

Moisture plays a role, but “it's also a factor of the primary digestive tract,” adds Bill Zeigler, senior vice president for animal programs at Brookfield Zoo in Chicago, which has had wombats since 1969. Peter Clements, the president of the organisation Wombats SA in Southern Australia, concurs, speculating that it’s a combination of the two.

Finding a more concrete answer, however, hasn’t been easy. It took Yang and her colleagues months just to get hold of wombat innards for their study. No zoos in North America had any, so Yang had the intestines of two roadkill wombats shipped from Australia. She wasn’t sure what to expect when they arrived.

“At first I thought they maybe have square anus, or maybe [the cube] forms right around the stomach,” she said. But neither of those hypotheses turned out to be the case. What she found to be more important was how the wombat intestines’ stretched.

As food is digested it moves through the gut, and pressure from the intestine helps sculpt the faeces – meaning that the shape of the intestine will affect the shape of a dropping. So Yang and the team expanded both wombat and pig intestines with a balloon to measure and compare their elasticities (or stretchiness).

The pig intestine had a relatively uniform elasticity, which would explain the animal’s rounder poo. The wombat intestines, however, had a much more irregular shape. Yang observed two distinct ravine-like grooves, where the intestine is stretchier, which she believes helps shape wombat faeces into cubic scat.

“It's really the first time I've ever seen anybody come up with a good biological, physiological explanation,” said Swinbourne, who reviewed the draft. Clements, who also read the early study, added, “I think this is a useful contribution but more explanation of a possible mechanism would be helpful.”

Yang agrees that there are still a host of questions to answer and says her research is ongoing. Her next task is to figure out why only two groves produce a cube, as opposed to needing four. But even the initial findings imply broader implications for sectors such as manufacturing.

Cubes, Yang says, are very rare in nature. “We currently have only two methods to manufacture cubes,” she said, explaining that humans either mold cubes from soft materials, or cut them from harder objects.

“Wombats have a third way.”


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