Long Limbless Locomotors Over Land: The Mechanics and Biology of Elongate, Limbless Vertebrate Locomotion

1. What they did

Snakes and other long, legless animals move well through places where legged animals struggle: thick clutter, narrow tunnels, loose sand and branches. Yet very little research has looked at how they do it. Henry Astley organised the first ever symposium on limbless locomotion, held in Austin, Texas, in January 2020, bringing together biologists, physicists and roboticists.

This paper introduces the collection of papers from that symposium. It explains why limbless movement matters, sets out how little is known, and summarises each contribution under four headings: locomotor behaviour, control and perception, functional morphology (how body structure works), and limbless animals other than snakes.

2. What they found

Why it matters

• Being long and legless has evolved again and again: about two dozen separate times in lizards and snakes alone, and also in fishes, amphibians and early four-legged animals. This may make it the most frequently converged-upon body plan in animals.
• About 4300 vertebrate species are functionally limbless, 19% of all land vertebrates, yet studies of limbless movement make up under 0.5% of the research on animal locomotion.
• In clutter, slithering snakes push against obstacles and actually move faster as obstacles get denser (until they become so dense they hem the snake in), while legged animals slow down.

Locomotor behaviour

• Snake movement is traditionally divided into four modes: lateral undulation (slithering), sidewinding, concertina and rectilinear. A review of recent data shows a wide range of movements, some of which are hard to fit into these categories.
• Sidewinding was thought to be limited to a few vipers and two water snakes. New evidence suggests sidewinding and related movements are more common than once thought.

Control and perception

• A sand specialist snake used a very regular wave shape that worked well on uniform sand but often failed to push off an obstacle. A generalist snake used irregular movements that were slower on sand but very good at pushing off obstacles.
• Studies looked at how gliding tree snakes use vision, how snakes and snake robots climb a vertical step (body posture and a flexible body both help), and a robot controller that produces both slithering and concertina movement purely in response to its surroundings.

Body structure

• The long tendons of tree-climbing snakes give a clear advantage when they reach out across a gap.
• Ribs have to be both flexible against the ground and firm enough to pass on forces.

Beyond snakes

• Limbless lizards and a basal snake showed widespread convergence in their vertebrae and movement.
• American eels crossing land (for example around dams on their migrations) used lateral undulation on level ground but switched to concertina on slopes, a movement not described before in fish.

3. What they concluded

• These papers are significant new progress in a “sadly neglected” field, and the author hopes they will spur more interest.
• Much remains almost or completely unknown: sensory feedback during movement, how the muscles contract, the effects of body size, and how often each mode is used during natural behaviour.
• These gaps are great opportunities for discovery. In the author’s words, few blank spaces on the map present more opportunity than the one labelled “Here Be Serpents.”

6. Other technologies, skills or innovations

• Snake robots (“snakebots”), which may help solve real-world problems such as search and rescue, and whose control has been improved by data from real snakes.
• A decentralised controller that uses local feedback from the surroundings to copy snake movement and performance across several environments.
• Mathematical and engineering tools used to analyse body curvature, reaction forces and the snake muscle and tendon system.