A single falling cubic wombat positioned by researchers on top of a rock
Yang et al. 2021
By Tess Joosse
Humans may be fascinated by cubes, but only one animal makes them fall: the bare-nosed wombat. This furry Australian marsupial squeezes nearly 100 six-sided poop every day – a skill that has long baffled scientists. Now, the researchers say they have discovered how the wombat gut creates this exceptional excrement.
“This study is really good,” says Sunghwan Jung, a biophysicist at Cornell University who studies the mechanics of animal movements and was not involved in the research. This shows, he says, that the entrails of these animals “are very special”.
The bare-nosed wombat (Vombatus ursinus), weighing up to 35 kilograms, lives on Australia’s grassy plains and eucalyptus forests, where he spends his nights grazing on plants and days in underground tunnels. It is a territorial animal, leaving its unusual excrement as a business card. But how does this scattering of pointed sides come from a round anus?
A female wombat with her joey
Yang et al. 2021
To get to the bottom of the mystery, scientists dissected a wombat that died after being hit by a car. They examined the intestines and found that they contain two grooves where the intestines are more elastic, which the team first reported in 2018.
In the new study, the researchers dissected two more wombats and tested the layers of muscles and tissues in the intestines, finding regions of varying 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 while the intestine draws nutrients and water from the stool, reports the team today in the properly titled newspaper Soft Matter.
The stiffer portions are “like a rigid rubber band -[they’re] it will contract faster than the soft regions, ”says David Hu, a biomechanics researcher at the Georgia Institute of Technology and author of the study. The softer intestinal regions compress slowly and shape the final corners of the cube, the team found. In other mammals, the wave peristalsis of the intestinal muscles is consistent in all directions. But in the womb, the ridged tissue and irregular contractions over many cycles form firm, flat cubes.
This leaves only one mystery: why wombats developed cubic poop in the first place. Hu speculates that, as animals climb rocks and logs to mark their territory, flattened stools are unlikely to come out of these high perches.
As for what the world should do with this new information, Hu admits that “it will not replace the way we manufacture plastic”. But wombat’s strategy can help engineers design better ways to shape valuable or sensitive materials, he says.
In the meantime, Hu also believes that this knowledge can help researchers create wombats in captivity. “Sometimes, your stools are not as cubic as [wild] ones, ”he says. The more square the poop, the healthier the wombat.