Why do wombats poop cubes?

Short answer: wombats make cubic feces because of what happens inside the last part of their intestine. A combined anatomical and mathematical study found regions of the gut wall with different thickness and stiffness. As the intestine contracts around increasingly dry fecal material, those differences can produce flatter faces and sharper corners.

The anus is not a square cookie cutter

Researchers studying bare-nosed wombats found that the cube shape develops before the feces leave the body. In their dissections and models, the characteristic corners formed in roughly the final 17% of the intestine. That makes the mechanism a property of gut mechanics rather than the shape of the exit.

Different stiffness creates different deformation

The intestinal cross-section was not mechanically uniform. The study reported regions that differed strongly in thickness and stiffness. During contractions, the stiffer and softer sections deform the material at different rates. Repeating that process along the gut can turn an initially rounder mass into the familiar block-like pellets.

But why did evolution favor cubes?

That part is less certain. Wombats use feces in scent marking, and cubes are often said to be useful because they are less likely to roll away from prominent rocks or logs. That is a plausible functional idea, but the research that explains how the corners form does not by itself prove the evolutionary reason. The honest answer separates the well-tested mechanics from the still-debated purpose.