A new study points out that the seemingly funny "poop emoji" actually hides a deep law of physics: most animals defecate downward, and when the poop is stacked in circles, the distance of each circle gradually becomes shorter, thus naturally forming a classic cone-shaped coil shape with a wide bottom and a narrow top, similar to a "soft-serve ice cream vortex." The research team found that when this process is "reversed," such as when certain worms excrete feces upwards, the same laws of physics produce a completely different "tower-like" structure, which means that even one of the most humorous icons on our phones is shaped by a combination of gravity and material mechanics.
The study, led by Daniel Bonn of the Institute of Physics at the University of Amsterdam, and his collaborators analyzed why poop shapes vary in nature and traced the answer to the physical law of "elastic rope coiling" - a mathematical model used to describe how ropes and other flexible materials coil under the influence of external forces and gravity. Research shows that the key factors are the stiffness of the material itself and the direction of gravity relative to the direction of extrusion, so that physical factors even outweigh biological and evolutionary factors in determining the shape of a single bowel movement.
The common "poop emoji" in daily life is highly similar to the shape of a typical poop pile in nature: it is wider at the bottom and narrower at the top, and the whole is stacked in a spiral shape, quite like soft ice cream squeezed out in circles, but it is far less "palatable". The reason is that the defecation direction of most animals is downward. When the soft feces is excreted and piled high, the height difference of each new circle gradually decreases, and the corresponding coil size also becomes smaller. Eventually, a conical spiral pile that is narrow at the top and wide at the bottom is naturally formed.
But there are exceptions. Studies have pointed out that some worms, such as lugworms, defecate upwards "against gravity". When their feces are accumulated on the surface, they form a structure similar to a small tower. The radius of the coil remains basically constant along the height direction, rather than gradually tightening like a common dung pile. Bang explained that lugworms exhibit a special mechanical state of "anti-gravity buckling": they extrude soft materials upward in the direction opposite to gravity, and the fecal casts they form no longer follow the "gravity coiling" pattern of ordinary animals, but embody the buckling and stacking behavior of soft solids under anti-gravity conditions.
This phenomenon once confused even Charles Darwin, but current research has given a clearer explanation: as long as the elastic rope coiling theory is used to describe the extrusion and coiling process of soft materials, it can uniformly explain the differences in the morphology of different animal feces. Bang collaborated with Mehdi Habibi from Wageningen University and Research Center and Neil M. Ribe from the French National Center for Scientific Research/University Paris-Saclay. Through theoretical analysis and experimental verification, it was clear that under different combinations of gravity and extrusion directions, soft materials will naturally evolve completely different stacking structures.
The study found that the stiffness (elasticity) and density of the material, as well as the diameter of the feces itself, were key variables in determining the coiled structure. In short, the radius of the coil is determined by a combination of the diameter of the excrement and its elasticity and density, which vary slightly but not much with diet, so the main difference between species is "how thick the excrement itself is." For example, the excrement diameter of the rain earthworm is smaller, forming a thinner coil; on the contrary, the excrement of the larger Nereis worm has a larger diameter, thus forming significantly larger coils and tower-like accumulations. This is considered to be the main influence of "biological factors" based on physical laws.
Interestingly, these observations are not limited to feces. The research team pointed out that the same coiling and buckling principles also apply to pea dough, pasta and other foods made through the extrusion process. Bang said that in reality, a large number of foods and materials are formed by extrusion, such as various noodles, pasta and candies. The spiral, curved or wavy shapes they exhibit during the production process can all be described by similar physical models.
The research also hints at potential applications from an engineering and manufacturing perspective. Bang said that similar coiling mechanisms can be used to prepare structures such as springs in an extremely simple way. By moving the substrate or adjusting the relative motion trajectory when the soft material is extruded, complex and elegant patterns and even structures with practical functions can be generated. In previous related work, researchers have used the interaction between moving substrates and coils to create a variety of microstructures that are both beautiful and useful. This study of the "dung pile shape" further enriches people's understanding of the behavior of soft materials in a gravity field.
"Beyond science," the research team also plans to bring these findings back to digital life. Bang revealed that he and his colleagues are designing a "second-generation poop emoji" and plan to formally make recommendations to the Unicode Alliance, which is responsible for unifying emoji standards. In the future, users are expected to see new "poop" emoticons on mobile phones that are different from existing images. Relevant research papers have been published in the academic journal Nature Communications and fact-checked by independent scientific authors.