# Wiggling Through the World: The mechanics of slithering locomotion depend on the surroundings > How does an animal with no legs move so well? Laboratory studies of snakes and the sandfish lizard show that snakes rely on belly scales that grip more sideways than forwards, and lift parts of their bodies as they slither, while the sandfish dives into sand in under a second and “swims” through it at nearly two body lengths a second. Summary by Kersey Lawrence, Original Wisdom. Updated 9 November 2026. Page: https://www.originalwisdom.com/tracking-literature/2010-goldman-hu-wiggling-through-world-mechanics/ ## Citation and access - **Authors:** Daniel I. Goldman and David L. Hu - **Year published:** 2010 - **Journal or source:** American Scientist - **Full citation:** Goldman, D. I. and Hu, D. L. 2010. Wiggling through the world: The mechanics of slithering locomotion depend on the surroundings. American Scientist 98(4): 314–323. - **Access status:** Hosted (freely available elsewhere) - **PDF:** https://www.originalwisdom.com/wp-content/uploads/2026/11/2010_Goldman_and_Hu_wiggling-through-the-world-the-mechanics-of-slithering-locomotion-depend-on-the-surroundings.pdf ## Tags - **Species:** Reptiles > Corn snake; Reptiles > Milk snake; Reptiles > Sandfish - **Region:** North America > United States - **Behaviour and ecology:** Behaviour; Movement - **Methods and technology:** Simulation - **Data type:** Modelling or theoretical; Review - **Observer qualification:** Not needed - **Discipline:** Behaviour and ecology - **Year published:** 2010 ## Detailed keywords - **Species:** Studied: milk snakes and corn snakes (juveniles; corn snake named as Elaphe guttata) and the sandfish lizard (Scincus scincus), a 10 cm desert lizard the paper describes as native to the deserts of north Africa. Mentioned for comparison: black mamba (a 2 m snake that can sprint at nearly 5 m per second), threadsnakes, anacondas, nematode worms, eels, sea snakes, moles and scorpions. - **Location and habitat:** Laboratory studies at the Georgia Institute of Technology, Atlanta, United States (with collaborators at New York University and Northwestern University). Snakes tested on cloth, inclined planes, mirrored surfaces, smooth plastic and photoelastic gelatin; the sandfish tested in a fluidized bed of 0.3 mm glass beads (and later 3 mm beads). Natural habitats mentioned: prairies, rocky slopes and desert sand. - **Topics:** limbless locomotion; lateral undulation (slithering); sidewinding and concertina gaits; push points; belly scales and frictional anisotropy (friction that depends on direction); body lifting and weight distribution while slithering; sand-swimming and burial; granular media (sand) behaving like a solid, fluid or gas; packing fraction (how tightly sand is packed); wave efficiency; bio-inspired robots. - **Statistics or models:** Resistive force theory (RFT), a technique first developed for small swimmers in fluids, applied to snakes on surfaces and to the sandfish in sand; Coulomb (dry) friction; empirical drag laws measured on a stainless-steel cylinder pulled through sand; molecular dynamics simulation of about 150,000 grains; a seven-segment robot sandfish. No statistical tests are reported. - **Data type:** A popular-science overview in American Scientist of the authors’ own laboratory experiments and models (published in more detail by Hu and colleagues in 2009 and Maladen and colleagues in 2009). No dates of data collection are given. Small wording difference: the text says the sandfish disappears beneath the sand “within half a second”, while the Figure 8 caption says it can bury itself “in less than a second”. ## Synopsis ### 1. What they did Many animals move without legs: snakes, worms, slugs and some lizards. The authors, a physicist (Goldman) and a mathematician and engineer (Hu), asked how such animals move so effectively in two very different settings: snakes slithering over the ground, and the sandfish lizard “swimming” beneath the surface of desert sand. - Snakes above ground. Working with juvenile milk and corn snakes, Hu and colleagues measured how the friction of the belly scales changes with direction, by briefly anaesthetizing snakes and laying them straight on tilted planes covered in cloth. They dressed snakes in a snug fabric “jacket” to stop the scales gripping, filmed snakes on mirrors and on light-transmitting gelatin to see which parts of the body touched the ground, and built a mathematical model to predict slithering speed. - The sandfish below ground. Goldman’s group filmed the sandfish with high-speed x-ray video as it dived into and swam through a bed of tiny glass beads. Air blown up through the bed let them set the sand to loosely or tightly packed. They measured drag on small rods pulled through the sand, then modelled the lizard’s swimming with a resistive force model, a computer simulation of individual grains, and a robot sandfish. ### 2. What they found Snakes - A snake’s belly scales overlap like roof shingles. On cloth, milk snakes slid most easily forwards (friction coefficient 0.10), less easily tailwards (0.14) and least easily sideways (0.20). This difference with direction is what lets a snake move forward on flat ground with no obvious push points. - Snakes in the fabric jacket, or on very smooth surfaces, slithered in place without moving forward. - A model assuming the snake’s weight is spread evenly along its body predicted only half the speed seen in the laboratory (8 cm per second, or 0.2 body lengths per second). - Mirror and gelatin experiments showed that snakes lift parts of their body while slithering. In the model, lifting the curves of the body and putting weight elsewhere increased speed by 35% and efficiency by 50%. The sandfish - The 10 cm lizard runs on the surface using its legs, but when startled dives into the sand within about half a second, using its legs and body together. - Once under the sand, it holds its legs against its sides and swims by sending a wave down its body from head to tail, at nearly two body lengths per second, faster than milk snakes move on flat ground. It went to a depth of about 4 cm and then stopped. - Drag in tightly packed sand was nearly double that in loose sand, yet the lizard moved the same way in both and was, on average, faster in tightly packed sand, by raising its wave frequency (up to four undulations per second). - Its wave efficiency (forward speed divided by the speed of the body wave) was about 0.5, nearly twice that of nematode worms in fluids (about 0.2) and more than snakes on surfaces (about 0.3), and did not change with how tightly the sand was packed. - The resistive force model predicted a wave efficiency of 0.4 to 0.7 and a best body-wave height of about 0.2 of the wavelength. The lizards’ movements clustered at this best value. - The grain simulation and the robot (seven motor segments in a fabric skin, swimming in 6 mm plastic beads) agreed with each other within 5%, and both swam fastest using the sandfish’s own movement pattern. ### 3. What they concluded - Above ground, the direction-dependent friction of snake belly scales produces thrust; below ground, the sandfish uses the fluid-like but frictional behaviour of sand. Although the forces differ, resistive force models first made for swimmers in water work for animals on and within dry land. - Snakes lifting their bodies suggests that movement in three dimensions improves performance, and the sandfish dives at a shallow angle rather than swimming flat, so 3D effects need further study. - Questions remain, such as whether speed or efficiency leads limbless animals to switch from slithering to sidewinding, and how animals move through wet material. - These models could help design limbless robots that move through complex terrain faster and more efficiently than animals. ### 6. Other technologies, skills or innovations - High-speed x-ray video to watch an animal moving inside sand. - A fluidized bed: air blown up through the sand sets it to repeatable loose or tight packing. - Photoelastic gelatin between crossed polarizing filters, which lights up where a snake presses hardest, and mirrored surfaces to show when the belly lifts. - Molecular dynamics simulation, which follows the motion of every grain, and a seven-segment robot sandfish built from hobby servo motors. ### 8. Implications for tracking (Original Wisdom's observations) _These are Original Wisdom's observations, not the authors'._ - The study did not involve tracking. It does describe how sidewinding snakes leave a trail in sand of separate “footsteps” rather than a continuous winding path, and shows that slithering snakes also lift parts of their bodies, so their weight on the ground is uneven along the body. This may help trackers think about why the pressure along a snake trail can vary. ## Cite this summary Lawrence, K. 2026. Summary of: Wiggling Through the World: The mechanics of slithering locomotion depend on the surroundings (Goldman and Hu, 2010). Original Wisdom, Tracking Literature. https://www.originalwisdom.com/tracking-literature/2010-goldman-hu-wiggling-through-world-mechanics/