Why do feces form that sharp spiral everyone recognizes — and that the poop emoji faithfully reproduces? By studying marine worms that defecate upward rather than downward, physicists have found that two variables are enough to explain all shapes of animal waste: the rigidity of the material and the direction of gravity. Charles Darwin had observed this mystery in 1881 without ever solving it.
What you will learn
- Why most animal feces form a pointed mound — and what creates the characteristic spiral
- How upward-defecating marine worms produce radically different shapes
- Which known physical principle governs these two forms — and how researchers tested it with pasta
A mystery Darwin never solved
In 1881, Charles Darwin marveled at the spiral, regularly structured shape of earthworm feces. He found them fascinating but never understood why these excretions took the form of a tower. One hundred forty-five years later, three physicists have published the answer in Nature Communications — and it applies just as well to both marine worms and the poop emoji.
Gravity as the architect of form
Most animals defecate downward. The extruded feces fall onto themselves, curling, each successive coil being slightly smaller than the previous one as height increases and the drop shortens. Result: a conical mound, wider at the base and pointed at the top — exactly the shape of the poop emoji, and of most piles of excrement found in nature.
The arenicoles — marine worms — defy this logic. Buried in the sand, they eject their waste upward. There, researchers observed something remarkable: the radius of the spiral remains constant no matter the height, creating uniform towers rather than cones. A physical regime previously unknown.
The elastic-wrapping theory of cords
The two shapes obey the laws of the same mathematical framework: the elastic-wrapping theory of cords, which describes how slender materials unwind and wrap themselves. Only two variables differ between the two cases: the rigidity of the extruded material and, above all, the direction of gravity relative to the extrusion direction.
Downward, gravity progressively amplifies each coil. Upward, it stabilizes it by maintaining a constant radius. That’s all there is to it. The final form is not the product of evolution or the animal’s specific anatomy — it’s pure physics.
Verification with chickpea paste and spaghetti
To validate their equations, the researchers did not settle merely for observing worms. They reproduced the experiments with extruded chickpea paste — a texture similar to that of the arenicoles’ waste — and even with pasta, systematically testing different orientations. In every case, the mathematical predictions matched the observed shapes.
Darwin might have been disappointed by the conclusion: evolution did not design the shapes of waste disposal. Physics took care of them instead.
