Why wombat droppings can come out cube-shaped

The short answer

Wombat droppings sometimes come out cube-shaped because the shape appears to form inside the distal colon, where the intestinal wall has non-uniform mechanical properties. The leading explanation is that some parts of the colon are stiffer or contract differently than others, molding the material into relatively flat faces and corners before it exits.

Is the shape caused by a square anus?

How researchers built the explanation

1. Earlier observations

   The 2020 paper notes that angular wombat feces had been documented decades earlier, including references from 1960 and 1979.

2. 2018 public result

   At an American Physical Society Division of Fluid Dynamics meeting in November 2018, researchers presented evidence linking the shape to elastic differences along the intestine.

3. 2020 peer-reviewed study

   On 8 October 2020, the full study combined dissection, mechanical testing and modeling to argue that non-uniform intestinal stiffness can mold corners.

Which explanation best fits the evidence?

Compare these documented possibilities using the dossier evidence.

A square sphincter shapes the poop at the very end

This does not fit the reported observations well. The lab materials say the sphincter is round, and shaping is placed earlier in the distal colon.

The distal colon has uneven stiffness and contractions

This best matches the paper and lab summaries. The sources describe periodic differences in thickness or stiffness around the colon and support this as a mechanism for forming faces and corners.

Pelvic bones simply compress the material into cubes

This appears in earlier speculation, but the 2020 paper treats such ideas as unverified and instead supports a colon-based biomechanical mechanism.

This comparison reflects the dossier only. The 2020 paper explicitly presents one supported mechanism, not necessarily the only possible factor, and it does not provide a complete in vivo visualization of the whole process.

What is still uncertain

The evidence is strong enough to reject the simple 'square anus' myth and support a colon-based explanation, but the paper is cautious: it presents one plausible mechanism supported by anatomy, mechanical tests and modeling. That means the story is well grounded, yet not every biomechanical detail has been observed directly in a living wombat.