Slithering Locomotion

1. What they did

Animals without legs, such as worms, snails and snakes, move over land by sliding their bellies along the ground. Swimming, flying and walking have been studied much more than sliding. In this book chapter, the authors review and expand their own earlier work on how snakes slither across flat surfaces, using dry friction between belly and ground.

• Background. They compiled published speed and size data for 140 snakes, described the four main snake “gaits” (lateral undulation or slithering, sidewinding, concertina and rectilinear movement), and summarised earlier studies of the energy cost of each gait.
• Friction measurements. Using juvenile corn snakes and milk snakes (about 30 cm long), they measured how easily the snakes slide forwards, backwards and sideways on cloth and on smooth fiberboard.
• A computer model. They built a model of a slithering snake that uses these friction values and the snake’s wave-like body shape to predict speed and efficiency.
• Body lifting. They filmed snakes from the side and added to the model the way snakes lift parts of their body off the ground while slithering.

2. What they found

Snake gaits, size and speed

• Over 2,900 snake species range from 10 cm threadsnakes to 10 m anacondas, and their body proportions stay about the same at every size.
• Speed in the undulating gaits rises in step with body length, unlike birds and fish, where speed rises more slowly with size.
• Snakes lighter than about 1 kg mostly use undulating gaits; heavier snakes mostly use concertina or rectilinear movement. The authors explain this by strength: a large snake cannot pull its whole body against friction at once, so it moves one part at a time.
• From earlier treadmill studies: slithering costs about as much energy per kilogram per metre (23 joules) as running does for a mammal or bird of similar size. Sidewinding is the cheapest gait (8 joules) and concertina the most costly (170 joules).
• Because snakes shed their skin rather than healing it, the authors estimate that the distance a snake can travel before wearing through its belly skin gets shorter as the snake gets longer, so avoiding wear matters most for large snakes.

Friction and the belly scales

• The overlapping belly scales catch on tiny bumps in the ground. On cloth, the snakes slid most easily forwards (friction 0.11), less easily backwards (0.14) and least easily sideways (0.19).
• The key to speed is how much harder it is to slide sideways than forwards. On cloth this was about twice as hard for milk snakes; on smooth fiberboard it was about the same, and a milk snake “slithered in place”, hardly moving forward. The model matched this.
• Inertia hardly matters: friction is far stronger, so a snake stops almost as soon as it stops slithering.
• The snakes slithered as fast on cloth as on boards with pegs, because the tiny bumps in the cloth act as “microscopic push points”.
• On a cloth incline, a milk snake slithered in place at 7 degrees and slid backwards on steeper slopes (supplementary video).

Body lifting

• Slithering snakes lifted the peaks and troughs of their body wave off the ground, pressing down harder with the parts in between.
• In the model, moderate lifting made the snake 35% faster and raised its efficiency from 0.3 to 0.55, not counting the effort of lifting.

3. What they concluded

• Snakes use two main mechanisms to slither on flat surfaces: belly scales that give more grip sideways than forwards, and lifting parts of the body to reduce drag and press harder where the body pushes forward.
• How well a snake slithers depends heavily on the surface. On surfaces too smooth for the scales to grip, snakes switch to other gaits or lift their bodies.
• Microscopic push points are a new idea that should be considered in models of snakes on all surfaces.
• The model is crude: it does not include the snake’s ability to change the angle of its scales or the energy spent lifting the body, which may be large for big snakes. Pressure sensors might help measure how snakes shift their weight.
• Body lifting shows that limbless movement has things in common with walking, where animals lift rather than drag their limbs. This matters for designing efficient snake-like robots.

6. Other technologies, skills or innovations

• Friction measured on unconscious snakes laid out straight, on cloth and fiberboard.
• Time-lapse and side-view video of slithering snakes, with four supplementary videos on YouTube.
• Photoelastic gelatin, which glows where force is applied, was tried but was too sticky for snakes to move naturally.