Coiling of lugworm feces reveals universal mechanics for the shape of poo.

Abstract

The coiling of animal excreta has fascinated naturalists since Darwin's detailed observations of earthworm castings, yet the underlying physics remains poorly understood. We investigate lugworm (Arenicola marina) fecal castings as a natural system where soft biological material is extruded upward against gravity from subsurface burrows, spontaneously creating coiled morphologies. Through field observations at Roscoff, France, rheological measurements, controlled experiments, and theoretical modeling, we demonstrate that such "anti-gravitational" biological coiling follows the gravitational regime of elastic rope coiling theory, for which the coil radius is <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>R</mi><mo>~</mo><msup><mrow><mrow><mo>(</mo><mrow><msup><mrow><mi>d</mi></mrow><mrow><mn>2</mn></mrow></msup><mi>E</mi><mo>/</mo><mi>ρ</mi><mi>g</mi></mrow><mo>)</mo></mrow></mrow><mrow><mn>1</mn><mo>/</mo><mn>3</mn></mrow></msup></math>, where E is Young's modulus, d is the rope diameter, ρ is the rope density, and g is the gravitational acceleration. Using data from lugworm feces, a synthetic analog (pea dough), and spaghetti and rice noodle controls, we find <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>R</mi><mo>~</mo><msup><mrow><mrow><mo>(</mo><mrow><msup><mrow><mi>d</mi></mrow><mrow><mn>2</mn></mrow></msup><mi>E</mi><mo>/</mo><mi>ρ</mi><mi>g</mi></mrow><mo>)</mo></mrow></mrow><mrow><mn>0.31</mn><mo>±</mo><mn>0.03</mn></mrow></msup></math>, in quantitative agreement with mechanical theory. This contrasts with typical downward defecation in most animals, which transitions from the gravitational regime to an elastic (kinematical) regime as the fall height to the top of the pile decreases. Our findings provide a quantitative mechanical framework for understanding fecal morphology and reveal how directional differences in waste disposal create fundamentally different buckling regimes across the animal kingdom.