A mammal's intestines stamp waste into perfect cubes—which?

The Cube Factory

Wombats defecate cubes, and for more than a century nobody knew how. The Australian marsupials produce 80 to 100 individual pieces per night, each a roughly two-centimeter brownish-green block with slightly rounded edges. Early naturalists assumed the shape was a fluke or an optical illusion. It is neither. In 2018 a team led by Patricia Yang at the Georgia Institute of Technology dissected the digestive tracts of wombats killed in vehicle strikes in Tasmania, inflated the intestines like balloons, and discovered that the final eighth of the colon has two stiff grooves running lengthwise and two highly elastic regions between them. As rhythmic contractions push fecal matter through, the stiff bands resist stretching while the elastic zones bulge outward, carving flat faces and sharp edges into what begins as a soft cylinder. The process takes two weeks, and by the time the cubes emerge the wombat has extracted so much moisture—the feces are nearly twice as dry as a human's—that the shape holds. Yang's team published their findings in the journal *Soft Matter* in January 2019, and the research earned an Ig Nobel Prize that November.

The wombat's intestine operates like a slow biological die-cutter. Peristalsis, the wave of muscle contraction that moves food through every mammal's gut, normally produces smooth, rounded shapes because intestinal walls squeeze evenly. Wombats broke that rule by evolving regions of differential stiffness. Patricia Yang's group measured the elastic modulus—the resistance to stretching—at points along the final 23 centimeters of colon and found fourfold variation. The stiff grooves hold their ground while the stretchy sections balloon out by up to 50 percent during each contraction, molding corners into the passing material. Because wombats are hindgut fermenters like horses, their colons are unusually long, giving the process time to work. The fecal matter spends roughly 14 days in the colon, losing water steadily through absorption, and exits as firm, stackable blocks. Yang's team replicated the geometry by inflating the intestine with a balloon and showed that the shape appeared even without live muscle contractions—the anatomy itself imposes the geometry. No other mammal is known to produce feces with corners, let alone six flat faces. The cube is a signature.

Wombats use the cubes as three-dimensional billboards. They are territorial, and their home ranges—typically 5 to 25 hectares depending on food availability—overlap with neighbors'. To advertise ownership they defecate on elevated surfaces: logs, rocks, stumps, even footpaths. A cube won't roll, so it stays put where placed, unlike the cylindrical or spherical droppings of most herbivores. Each night a wombat might deposit 80 to 100 cubes across its territory, often stacking four or five in a single latrine. The feces carry scent compounds from anal glands, and the flat surfaces maximize contact with air, broadcasting the message farther. Researchers observing bare-nosed wombats (*Vombatus ursinus*) in Tasmania noted that latrines cluster near burrow entrances and along territorial borders, suggesting the cubes serve as both property markers and social résumés. Because wombats are poor climbers—they are stocky, low-slung diggers with short legs—placing scent markers at nose height on a rock is far easier than climbing a tree. The cube's stability makes the system work. A rolling pellet would tumble into leaf litter and vanish; a cube stays visible for weeks, a durable flag in a slow-motion argument over boundaries.

Why It Still Matters

The cube mystery took 120 years to solve, and the answer has implications beyond marsupial bathroom habits. Engineers at Yang's lab are now exploring whether the principle of differential stiffness could shape soft materials—plastics, hydrogels, medical implants—without molds or cutting. If a tube with varying elasticity can mold corners into a passing substance, the same logic might manufacture cubic pills, stackable biodegradable packaging, or internal stents that self-assemble into non-round cross-sections. The wombat's intestine is a proof of concept for geometric manufacturing at the microscale. Meanwhile, the cubes themselves have become tools for conservation. Researchers collect and analyze wombat feces to monitor population health, diet, and stress hormones, and the distinctive shape makes field collection straightforward—no risk of confusing wombat scat with that of kangaroos or wallabies. In 2020, veterinarians at Taronga Zoo in Sydney sequenced DNA from cube samples to track genetic diversity in declining northern hairy-nosed wombat populations, using the geometry as a reliable species marker. The cube, once a curiosity, is now a window into a threatened lineage.