Wombats live in Australia, and their poop is shaped like small cubes. Many people think this is because their bottoms are square, but that is not true.

Scientists studied the guts of wombats that died in road accidents. They found that the gut wall has hard parts and soft parts. When the gut squeezes, these parts make corners in the poop. This study won an Ig Nobel Prize in 2019.

Wombats are marsupials, like kangaroos, and they are famous for producing cube-shaped poop. They often leave it on rocks or fallen logs, perhaps as a sign for other wombats. Many people imagine that a wombat's bottom must be square, but it is actually round.

Researchers in the United States and Australia, including Patricia Yang and David Hu, examined the intestines of wombats killed on roads. They discovered that the poop stays soft for most of its journey and only becomes cube-shaped in the last 17 percent or so of the intestine. In that section, the wall has parts that are about twice as thick and four times as stiff as others. These differences create flat sides and corners.

Of all the oddities in the animal world, few are as memorable as the cube-shaped feces of the wombat, an Australian marsupial that deposits dozens of these small blocks each night, often on rocks and logs where they may serve as scent marks. The popular explanation, a square anus, is simply wrong.

In a study that earned the 2019 Ig Nobel Prize in Physics and was published in full in 2021, Patricia Yang, David Hu, and Scott Carver dissected wombats killed in road accidents. They found that the intestinal contents remain soft until roughly the final 17 percent of the gut, where they acquire corners. Tissue tests revealed that, around the circumference, the wall alternates between regions about twice as thick and four times as stiff and regions that are softer.

A computer model showed that stiff regions contract faster than soft ones, carving flat faces and corners. The team suggests that the principle could inspire new manufacturing methods. However, only a few animals were dissected, the model offers one possible explanation, and other factors such as drying may contribute. The idea that cubes help with scent marking because they do not roll also remains untested.