Why does the "poop emoji" look the way it does? Physics has the answer: as most animals defecate downward, each new coil falls a shorter distance, naturally forming the familiar tapered swirl.

A new study reveals that when this process is reversed, as in certain worms, the same laws create completely different shapes, proving that even our most humorous icon is shaped by gravity and material science.

The poo emoji mimics a typical mound of faeces you might see in nature; it features a coil, wider at the base than at the top, and looks rather like a spiral of soft-serve ice cream (albeit it nowhere near as appetising).

This is because most animals do their business downwards, so as the coils of faeces grow higher, the fall height decreases and so does the size of each successive coil.

But some worms, like the lugworm, defy gravity and poo upwards. This creates a rather different shape – a tower whose coil radius stays constant and uniform, rather than tapering into a mound.

“Lugworms show an unusual mechanical regime: they extrude soft material upward against gravity, so their casts probe the anti-gravitational buckling of soft solids rather than the gravitational coiling seen in most animals, where the coil radius is given by the poo height,” explains Daniel Bonn, corresponding author from the Institute of Physics, University of Amsterdam.

But why? This is a question that puzzled even Charles Darwin. The answer lies in the laws of elastic rope-coiling, which mathematically describe how ropes and other materials coil.

Bonn, together with Mehdi Habibi from Wageningen University and Research and Neil M. Ribe from CNRS/Université Paris-Saclay, used this principle to explore the causes of the two different shapes.

They discovered that the material's stiffness and the direction of gravity relative to the direction of extrusion were important. That means physics trumps biology and evolution in determining the shape of a single poop.

“Our theory and the experiments show that the coil radius is given by the poo diameter (which of course depends on the species) and the poo elasticity and density. The latter two may vary somewhat (but not much) with diet,” explained Bonn. “So, Darwin's rainworms make very small coils, as the poo diameter is small, the lugworms are actually much bigger and make much bigger coils. I think that is the main biological influence.”

And their observations hold for many other materials, including extruded pea dough and even pasta.

“There are loads of foods made by extrusion, think of pastas and noodles, all types of sweets,” said Bonn.

But what applications might this research have beyond food, like engineering, manufacturing, or materials science?

“For engineering, this might be a very simple way of making springs. A wide variety of structures have been created by moving the substrate while the coiling happens, leading to beautiful and sometimes useful patterns,” Bonn said.

Bonn and his colleagues are now planning to design a second poo emoji and officially propose it to the Unicode Consortium, so you might yet see a different poo emoji on your phone.

The paper has been published in Nature Communications.

Fact-checked by Mike McRae.

Editor's note: This story was originally published on our dedicated Health & Science site Refractor.io a week ago.