What shape are wombat droppings?

Answer

Bare-nosed wombats leave cube-shaped droppings, up to about 100 a day. Researchers who studied the intestines of two wombats think the stretchiness of the intestinal wall shapes the corners.

Context

The leading mechanical explanation comes from a Georgia Institute of Technology team led by Patricia Yang, working with David Hu. They inflated sections of intestine from two wombats that had died on Australian roads and compared them with pig intestine. Pig intestine stretches fairly evenly and yields rounder droppings, while wombat intestine has regions of different stiffness, including two groove-like areas that stretch more. The 2021 paper, titled around non-uniform intestinal stiffness molding the corners of wombat feces, argues that this uneven stretch shapes the cube's corners.

Moisture appears to matter too. Wombat biologist Mike Swinbourne links the shape to dry habitats, where wombats squeeze out much of the water from their food. Drier droppings hold sharper angles, and zoo wombats with easier access to water reportedly produce less cubic scat. Some experts therefore favour a combination of gut anatomy and dryness.

Wombats are the only animals known to make cube-shaped droppings. They do use droppings to mark territory, and a popular idea says the cubes stay put on logs and rocks. The popular picture of wombats neatly stacking cubes as buildings is a misconception, though: they simply drop them where they are.