How gravity determines feces shape—and why Darwin never figured it out
Ever notice how children always draw poop the same way—as a coiled spiral, wider at the bottom and tapering to a point? It’s the same shape as the poo emoji, and it matches what we actually see in nature. But there’s an intriguing exception: certain worms defy this pattern entirely, producing feces that spiral upward instead of downward. Behind these seemingly trivial differences lies elegant physics that scientists have only recently understood.
Darwin’s unanswered question
Back in 1881, naturalist Charles Darwin was captivated by a puzzle. Earthworms, particularly lugworms, produced remarkably neat, tower-like spiral castings—yet he couldn’t explain why their shape differed so dramatically from other animals’ waste. For over a century, this observation remained a mystery. That is, until a team of physicists decided to investigate.
Gravity does the designing
As explained here, the answer, it turns out, comes down to one simple variable: the direction of gravity relative to where the material exits the body.
Most animals defecate downward. As each loop of feces falls and coils onto the pile below, the falling distance decreases with each cycle, making each coil progressively smaller. This creates the characteristic cone shape—precisely what we see in the poo emoji.
Lugworms operate differently. They extrude their waste upward from the ground, working against gravity. In a study published recently in Nature Communications, physicists Mehdi Habibi, Neil M. Ribe, and Daniel Bonn discovered something remarkable: when material is extruded upward, a completely different physics regime takes over. The coil radius remains constant throughout, creating uniform, tower-like spirals rather than tapered mounds—exactly what Darwin observed.
One theory explains it all
Both patterns follow the same mathematical framework: elastic rope-coiling theory, which describes how flexible materials naturally coil when extruded. The researchers confirmed this wasn’t just true for worm feces. They tested the same principles on extruded pea dough (a mix designed to mimic the consistency of worm waste) and even on pasta. Every time, gravity’s direction produced the predicted shapes.
The humble conclusion
The takeaway is humbling: the shape of feces isn’t the result of millions of years of evolutionary fine-tuning. It’s simply physics at work. Evolution may design the plumbing, but gravity designs the output.