In 1881, Charles Darwin spent hours observing earthworms in his garden, mesmerized by the neat, spiral-shaped mounds they left behind. He never understood why their fecal castings formed these elegant tower-like shapes. Now, more than a century later, physicists have finally cracked the puzzle—and the answer, delightfully, also explains the shape of the poo emoji on your phone.
A team of researchers from Wageningen University and Research, CNRS/Université Paris-Saclay, and the University of Amsterdam discovered that both the classic spiral-collected earthworm feces Darwin marveled at and the ubiquitous poo emoji follow the same fundamental physics. Their study, published in Nature Communications, reveals that the shape of feces depends on a surprisingly simple factor: the direction of gravity relative to extrusion.
Most animals defecate downward. As their feces coil, the pile grows higher and each new coil has a shorter distance to fall, making it progressively smaller. The result is the characteristic tapered mound—broad at the base, pointy at the top—exactly like the spiral swirl of soft-serve ice cream that inspired the iconic emoji.
Lugworms, however, defy gravity. These creatures extrude their feces upward from their burrows, and their castings form uniform spirals with constant coil radii. The researchers, led by Mehdi Habibi, Neil M. Ribe, and Daniel Bonn, found that lugworms operate in a previously unknown physics regime where coil size remains independent of height—the exact pattern Darwin observed but couldn't explain.
The team confirmed their theory by testing it across a range of materials: living lugworms, extruded pea dough (a mixture of chickpea flour and water mimicking the wet sand-like texture of worm feces), and even pasta. Across all materials, the direction of gravity consistently determined the final shape.
"Evolution, it turns out, doesn't design waste disposal. Physics does," the researchers noted.
As a playful extension of their work, the team is now designing a second poo emoji—representing the upside-down variety—and preparing to officially propose it to the Unicode Consortium. It's a small reminder that even the most everyday phenomena, from a child's drawing to the pile left by an earthworm, can hold surprising depths of scientific wonder.