Scientists unravel secret of cube-shaped wombat faeces

Of all the many mysteries that surround the common wombat, it is hard to find one as baffling as its ability – broadly acknowledged as unique in the natural world – to produce faeces shaped like cubes.

Why the pudgy marsupials might benefit from six-faced faeces is generally agreed upon: wombats mark their territorial borders with fragrant piles of poo and the larger the piles the better. With die-shaped dung, wombats boost the odds that their droppings, deposited near burrow entrances, prominent rocks, raised ground and logs, will not roll away. That, at least, is the thinking.

But quite how the animals produce the awkward-shaped blocks – and they can pass up to 100 per night, presumably with some trepidation – has proved a harder one to work out. Scientists who find themselves intrigued by the phenomena have made little progress beyond ruling out the nagging suspicion that the animals possessed square anuses.

“My curiosity got triggered when I realised that cubical feces exist,” said Patricia Yang, a postdoctoral fellow in mechanical engineering at Georgia Institute of Technology in Atlanta. “I thought it was not true in the first place.”

In a new study, Yang and her colleagues have had a fresh crack at the problem. To gain new insights into the mystery, they studied the digestive tracts of common wombats that had been euthanised after being struck by cars and trucks on roads in Tasmania.

Close inspection revealed that the wombat’s excrement solidified in the last 8% of the intestine, where the faeces built up as blocks the size of long and chunky sugar cubes. By emptying the intestines and inflating them with long modelling balloons, of the sort used to make balloon animals at children’s parties, the researchers measured how the tissue stretched in different places.

In work to be presented at a meeting of the American Physical Society’s fluid dynamics division in Georgia, the team explain how the last section of the wombat intestine does not stretch evenly, unlike the rest of the intestine. When measured around the circumference, some parts give more than others. This allows the intestine to deform in such a way that packs faeces into 2cm-wide cubes rather than the usual sausage shapes. The findings were buoyed up by tests on pig intestines which found no such irregularities in how those stretched.

“Wombat intestines have periodic stiffness, meaning stiff-soft-stiff-soft, along the circumference to form cubical faeces,” Yang said.

But the researchers have yet to finish the job. To produce a poo with a square cross-section, the circumference of the intestine would need four stretchy regions interspersed with four stiff regions. That way, the stiff regions form the flat faces, while the stretchier parts allow corners to form. The balloon tests revealed only three stretchy parts and two stiffer ones. In an upcoming paper, the scientists suggest that the other stiff and stretchy bits may only become apparent when they can inflate the intestines to a larger size. In other words, strain a little harder.

Yang believes that the revelation will have implications beyond the small community of researchers who admit an interest in wombat scat. Today, engineers have only two methods for making cubes: either moulding them or cutting them, she points out. The wombat’s intestines suggest a third route is possible. “It would be a cool method to apply to the manufacturing process,” she said.

“We can learn from wombats and hopefully apply this novel method to our manufacturing process. We can understand how to move this stuff in a very efficient way.”