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Why Do Wombats Poo Cubes?

Wombats have poor eyesight, and communicate with each other through smell by piling their faeces in prominent locations, such as on top of rocks and logs. It is important that their faeces do not easily roll away, so the cuboidal shape of their faeces is advantageous.  Credit: Scott Carver

Wombats have poor eyesight, and communicate with each other through smell by piling their faeces in prominent locations, such as on top of rocks and logs. It is important that their faeces do not easily roll away, so the cuboidal shape of their faeces is advantageous. Credit: Scott Carver

By David L. Hu

Wombat faeces stand apart from others you might encounter in the bush. How do they create cubic faeces, and why?

It was 7pm on a Friday night, and University of Tasmania biologist Scott Carver was unwinding with a movie when he took a phone call. “Yes, I’ll be there right away,” he said. He put on his jacket and started the car. A thirty-minute drive to an after-hours veterinary clinic in Tasmania led him to a metre-long brown course-haired lump that had been brought in by a member of the public.

The common (or bare-nosed) wombat had been humanely put down by the veterinarian after being fatally injured in a collision with a motor vehicle. The body was still warm, but Carver didn’t have much time before its blood cooled. He gently wrapped the wombat in a hessian sack and picked it up as he would a sleeping child before sliding it onto the tarpaulin in the trunk of his car before heading back to his laboratory.

For several months Carver had been responding to calls from Bonorong Wildlife Sanctuary, which works with the public and Tasmanian government to coordinate orphaned and injured wildlife rescues, such as wombats that had been struck by cars. For the past 6 years he had been studying the wombat’s susceptibility to sarcoptic mange, a disease caused by mites that kills individual wombats and occasionally wipes out entire populations. To study mange he needed fresh wombat tissues to perform genetic testing. This meant collecting freshly killed wombats and taking them back to his lab to perform a late-night dissection.

Carver performed the dissection on a wooden table in his lab. The wombat had stubby legs, a box-shaped body and a peaty smell. Its fur was course, like overgrown weeds. He cut the initial slit from the mouth of the wombat to the anus. He was after tissue samples from the wombat’s heart, lungs, liver, kidneys, muscle and skin, but on his way there he was struck by the peculiarity of something he was not looking for at all – the wombat’s intestines.

When he pulled the intestines out from the body cavity, he zigzagged them across the tarpaulin on the floor like a garden hose. The intestines were nearly 10 metres long, stretching across ten wombat body lengths. They were longer than other animals of the same size – even a dog’s intestines would only stretch half as far.

The wombat’s intestines are important for its special way of life. Wombats eat grasses with low nutritional value, and must also survive seasonally dry Tasmanian summers. As a result, wombats have one of the lowest metabolisms around.

When we eat, food items travel through our gut in a matter of a day or two. A wombat takes more than ten times longer, up to 2 to weeks, in order to extract all the water and nutritional content possible.

This travel time gives wombats much drier faeces. While humans and dogs have faeces that are 70% water, similar to the water content of our bodies, wombat faeces comprise only 40% water, which may help with their tolerance of drought. The result of this long digestion time and drier faeces is a dropping that is unmistakably wombat.

For years, biologists have tracked animals using their faeces. Their shape, colour and size is like a fingerprint for each animal. A walk through the forest will confirm this: a pile of dark pellets means a deer or elk; a large tubular plop means a bear; and smaller tubules could mean a fox, dog or cat. But the wombat’s faeces don’t have any of these shapes.

The intestines were surprisingly translucent, and as Carver used his surgical scissors to snip through the membrane of the wombat’s intestine, his observations were confirmed. Four shiny dark cubes appeared, the size and shape of ice cubes from a tray.

It was the cubic faeces of the wombat, and it was anyone’s guess how they got that shape.

With the entire intestine stretched out in front of him, Carver could clearly see that the transition to cubism was gradual. Near the stomach, the intestines were amorphous, like mulch bags. Like all animals, wombats eat grass, which becomes mixed with acids in the stomach to make a slurry that fills these bags. As the slurry travels through the intestine, bacteria digest it into smaller components, and water is continually absorbed through the walls of the intestine.

Wombat intestines from roadkill specimens recovered by Scott Carver in Maria Island, Tasmania. Pellets have increasingly sharper corners as they approach the anus. The ruler is 15.2 cm long.

This process is similar to how a cake is baked in the oven. The dough releases its moisture, and the remaining particles link up. Just like a fully-formed cake coming out of the oven, the end result of wombat digestion is a cuboidal stool that resembles a brick. It’s quite sturdy.

In Australia, cubic wombat faeces can be a common sight, just like dog droppings are in the USA. As with all everyday things in nature, like the clouds and the rain, folklore has a number of theories for why wombat faeces are so oddly shaped.

The primary theory is the square anus theory. This states that a wombat pushes a dough-like stool through a square die, like making pasta. However, Carver’s discovery of fully formed cubic faeces in the intestine put an end to this theory. Examination of the anus also showed a normal-looking sphincter.

The next theory is that the faeces are pushed past a pubic bone, which moulds the faeces flat. However, Carver could put his hand between the pubic bone and the intestine. They were nowhere near each other. Cross out this theory.

What could the answer be?

Halfway around the world, in 2015, my graduate student Patricia Yang had just presented a mathematical theory for defecation, showing that animals defecate for on average of 12 seconds. A scientist raised his hand and said that his 8-year old children were fascinated by cubic wombat faeces. Could our theory account for this shape? This is the first time we heard of such a thing, so we searched for the faeces on our phones and were amazed.

After the conference, Yang recruited undergraduates Alice Zhang and Miles Chan, who followed up with this question and tracked down Carver, who at the time was one of a handful of the world’s wombat experts. Before long, Carver’s prize intestines were shipped around the world to my laboratory in Atlanta.

The first thing that I asked my students to check was if the cubes were aligned or not. I had a hunch that the cubes would all be oriented the same way rather randomly oriented on their way through the intestine.

Chan let the intestines hang freely from the ceiling, like a gruesome Christmas ornament. By letting it hang without external forces, we could see how the cubes were likely aligned in the body. Indeed, the edges of the cubes were aligned in columns, as if there were imaginary bands like highways running down the intestine, forming all the edges at once. Such bands could affect how the intestinal walls moulded the faeces.

When you squeeze a water balloon, water shoots out in a circular jet. This is because the walls of the balloon uniformly compress the contents. But what if the intestine was made up of a series of vertical bands of alternating stiff and soft zones?

As brown slurry fills the intestine, a stiff zone would resist bending in that particular region. Four such stiff zones could create the tell-tale four walls of the cube. The corners of the cube would be a consequence of the intermediary soft zones.

Chan tested this idea in two ways. He squeezed the cubes of faeces out and inflated the intestine with a long party clown balloon. He then used dissection markers to label stripes on the outer walls of the intestine. By measuring the position of the stripes before and after the balloon was inflated, he could measure how much the walls stretched due to the constant pressure of the balloon, which would imitate the input of faecal slurry. This experiment indeed showed that such bands of stiffness existed, at least in the last part of the intestine.

This means that a wombat making faeces is like baking a cake. When cake batter is poured into a pan, it initially forms a shapeless mass. As the cake bakes, it solidifies, assuming the pan’s shape. Similarly, the bands of the intestine can do their work only when the faeces are soft and wet.

We tested this idea further with Yang’s pantyhose, which was a great mimic for an intestine. When filled with small Styrofoam packing beads, the intestine assumed round shapes when pressure was applied. However, when we sewed in bands of thicker pantyhose, we found that the squeezed beads assumed cubic shapes. This is because of the property of the Styrofoam beads, which are a granular material that can flow like a fluid or hold its shape depending on the nature of the applied forces.

The wombat is the only animal to evolve cubic faeces, possibly because of its unique lifestyle. It is a solitary animal, spending most of its daytime in burrows that the young inherit from their mothers. It avoids contact with other wombats, and even leaves faeces in conspicuous spots like atop or alongside tree stumps, logs and rocks to act as communication markers to any nearby wombats. The cubic shape might help these faeces to stay in their position, like a pile of stone markers in the forest.

The formation of square shapes from a round intestine might also inform new ways to extrude different shapes in the food industry. Cubic shapes are easier to pack and store than round ones.

Until that occurs, Carver and my team intend to celebrate our discovery with a bottle of champagne and a plate of warm cubic brownies.

David L. Hu is Associate Professor of Mechanical Engineering and Biology and Adjunct Professor of Physics at Georgia Institute of Technology. He has won an Ig Nobel Prize in Physics, and is the author of How to Walk on Water and Climb up Walls: Animal Movement and the Robots of the Future (Princeton, 2018).