How wombats poo cubes

You can’t fit a square peg through a round hole, as English writer Sydney Smith said in around 1805. But now it’s 2021, and scientists have shown how wombats can - using their cube-shaped poo! Eva Higginbotham spoke to scientist David Hu...

David - Wombats are marsupials: the size of an obese toddler, the face of a teddy bear, and the nose of a koala. And they don't like each other; they like to live in sort of separate territories. What people most know them for is the way they defend their territories, and they do it with little flags of faeces. They make latrines as tall as a wombat can climb with its short stubby legs, which is not very tall: usually a stump or a rock. And they'll get on top of this rock and defecate. They defecate about a hundred cubes a day and they'll leave about ten or so as a calling card.

Eva - I cannot believe that! A hundred times a day? And they're building essentially a tower of poo outside their house?

David - Yeah, they're separate latrines, and they'll dump a hundred cubes dispersed among the various latrines. For years people had known that these wombat faeces are different from all the other mammals, that they're cubic, but no one knew exactly how an animal can make anything that's this strangely shaped. They're kind of the size and colour of a Godiva chocolate or an Almond Joy minibar with one nut, but they smell like grassy poop, and are probably not very tasty.

Eva - Ha! Thank you. So what did you do?

David - Our first task was finding a good collaborator. So we sought out Scott Carver who's a wombat expert and works with wombats. And he shipped us intestines - full, intact intestines, and wombat faeces - through the mail. It was around Christmas time, so it was one of the best Christmas presents I've ever gotten, wombat intestines. We opened them up and they had tiny little presents inside. I was very happy to see them.

One of the first discoveries we made was that the cubes happen inside the wombat. They start out as a yoghurt-like slurry, and they eventually solidify, dry. In the last metre of the intestines or so they were just a factory line of cubes. And so it was amazing to see inside the body, going from a sort of amorphous and sort of strangely shaped solid, to something that had edges and flat faces.

The other thing that we noticed is that the cubes were arranged very nicely. When we hung the intestines from the ceiling, we noticed that after they finished swinging, all the corners and edges of the intestines aligned. And that meant that the cubes... they had a clock in the intestines that was telling them where to make the corners and where to make the flat faces. So we knew there was something in the intestines themselves that was communicating where to put the different parts of the cube. We performed these materials tests and measured how much it stretches, and we found that there are certain stripes on the intestines that stretch less than the others. So some parts of the intestines are four times as stretchy as the stiff parts. The rest we had to turn to mathematical modeling to basically simulate oscillation of the intestines, try to simulate the properties of faeces, and see how the two would interact until we got corners and flat faces.

伊娃——所以你知道进入模型,“好吧ay, so the intestines have a more stretchy bit and a less stretchy bit." You input that into a computer algorithm. And then what happens?

David - We wrote the equations for how the intestine should move if they're contracted like a muscle. And over many, many contractions, we saw that the stiff sections would produce corners at their midpoint.

Eva - How long is wombat faeces inside the wombat?

David - So when we eat something, it's basically out of our bodies in one to two days, and a wombat is three to five days. And in part that's because they're very drought tolerant; they want to capture as much water as possible from the faeces before it leaves. And it's also that time that allows the intestines to do their sculpting work. Faeces, as it gets drier, gets very, very solid-like. The longer time it takes allows the corners to get formed a little bit more like a square.