Why wombat poop is shaped like a cube

Today I learned that a) the poop of wombats is shaped like a cube, and b) scientists have figured out how they poop like that.

These are both excellent things!

Maybe you were already aware of the geometry of wombat excrement? Goddamn hipsters. Anyway, it was a surprise to me. And apparently the precise mechanism by which it assumes that shape has been a mystery to wombat scientists for a long time too.

But as Science reports …

To get to the bottom of the mystery, scientists dissected a wombat that had died after being hit by a car. They examined the intestines and found that they contain two grooves where the guts are more elastic, which the team first reported in 2018.

In the new study, the researchers dissected two further wombats and tested the guts' layers of muscle and tissue, finding regions of varied thickness and stiffness. They then created a 2D mathematical model to simulate how the regions expand and contract with the rhythms of digestion. The intestinal sections contract over several days, squeezing the poop as the gut pulls nutrients and water out of the feces, the team reports today in the aptly titled journal Soft Matter.

The stiffer portions are "like a stiff rubber band—[they're] going to contract faster than the soft regions," says David Hu, a biomechanics researcher at the Georgia Institute of Technology and author on the study. Softer intestinal regions squeeze slowly and mold the final corners of the cube, the team found. In other mammals, the wavelike peristalsis of the intestinal muscles are consistent in all directions. But in the wombat, the grooved tissue and the irregular contractions over many cycles shape firm, flat-sided cubes.

The full study is here, albeit paywalled; that fine, fine photo of wombat poop above comes from the paper.