What Defines Different Modes of Snake Locomotion?

1. What they did

For decades, snake movement has been sorted into four main modes: rectilinear (moving the belly skin while the body stays straight), lateral undulation (side-to-side waves along the body), sidewinding, and concertina (bunching and stretching). These definitions are based on how the movement looks, not on how the muscles work.

In this review, the author asks whether four modes are enough. The review brings together recent studies, mainly in the laboratory, that recorded the electrical activity of snakes’ muscles (electromyography, EMG) while they moved, together with studies of body movement, friction and energy cost. It covers movement on flat ground, in water, in tunnels, on branches and cylinders, and across gaps between branches. The author adds some of their own unpublished observations, including video of three reticulated pythons doing concertina on a flat surface and muscle recordings from a boa constrictor climbing a sloping cylinder.

2. What they found

Rectilinear locomotion

• Uses different body parts from every other mode: the belly skin moves back and forth relative to the skeleton, and the ribs stay still. Muscles linking the skin to the ribs pull the skin forward and then pull the skeleton forward over it.
• These muscles are absent in the tail, so the part of the body that can use this mode varies widely between species (tails range from under 5% to over 40% of total length).

Lateral undulation

• Swimming and crawling on land share side-to-side bends that travel from head to tail, but they differ. In swimming snakes the waves grow larger toward the tail and each point on the body follows a different path. On land, all points follow roughly the same path, and the timing of muscle activity relative to bending stays the same along the body, unlike in swimming.
• Some sea snakes and the file snake can swim backward. The author has never seen a snake do lateral undulation backward on land.
• Some tree snakes move steadily with fewer than three points of support, and some can crawl on smooth surfaces. The gliding snake can sharpen a keel along the edge of the belly scales that seems to reduce slipping on branches.
• On narrow branches, unsupported parts of the body would sag unless the snake actively holds them up, which probably needs a different muscle pattern.

Sidewinding

• Parts of the body are lifted and others rest still on the ground, leaving separate, parallel tracks set at an angle to the direction of travel.
• It costs much less energy per distance than lateral undulation on land.
• The muscle pattern is like lateral undulation, except that one back muscle is active on both sides at once in some places, arching the back and lifting it off the ground.

Concertina

• On a flat surface, the pythons were never fully straight, their bends formed from front to back, and the rear of the body often slipped backward. Here only the snake’s weight holds the anchoring part in place.
• In tunnels, the snake pushes outward against the walls. This costs far more energy than lateral undulation or sidewinding.
• On branches, the snake bends downward to grip the branch, pressing inward, and uses a different muscle pattern from tunnel concertina. Some snakes instead wrap around the branch in a spiral like a coiled spring, the only known mode with no alternating left and right bends.

Bridging gaps

• Brown tree snakes crossing gaps use back muscles on both sides at once, for long periods, a pattern not seen in swimming or on the ground. The muscles change their roles depending on whether the snake is held at one or two points, turning, or going up or down. Going straight down needs almost no back muscle activity.

Traits for sorting gaits

The author lists 13 traits for classifying how limbless animals move, including the direction of bending, whether bends alternate left and right, whether the waves travel the whole body, the paths followed by points on the body, sliding versus still contact, how friction is generated, and how the muscles are used.

3. What they concluded

• Rectilinear locomotion and sidewinding are each a single, distinct mode.
• Lateral undulation and concertina are better used as groups of related gaits, the way “symmetric” and “asymmetric” gaits are groups for animals with legs.
• The author recognizes five types of lateral undulation (on land, forward swimming, backward swimming, with a belly keel, and on branches) and four types of concertina (flat surface, tunnel, on branches with alternating bends, and on branches with spiral wrapping). This gives at least 11 distinct modes where four were traditionally recognized, and more if gap crossing and climbing in different directions were counted.
• Open questions include whether sidewinding that leaves drag marks between the tracks on sand is still sidewinding (the author suspects it is), and whether backward slipping should define new modes (the author thinks not).
• Studying only the simplest cases can make animal movement seem more uniform and stereotyped than it is.

6. Other technologies, skills or innovations

• Electromyography: recording muscle activity with electrodes, combined with video or film of the moving snake.
• Measures of the energy cost of moving (net cost of transport) and friction of surfaces.
• Test surfaces in the laboratory: tunnels, pegs, treadmills, cylinders of set diameters and slopes, and gaps between supports.
• Supplementary videos are linked from the paper on YouTube.