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

> About one in five land vertebrates has no legs, yet these animals get less than half a percent of locomotion research. This short introduction to the first ever symposium on limbless locomotion shows how snakes, legless lizards and even eels slither, sidewind and inch through sand, clutter, tunnels and trees, and why so much about their movement is still unknown.

Summary by Kersey Lawrence, Original Wisdom. Updated 9 November 2026. Page: https://www.originalwisdom.com/tracking-literature/2020-astley-long-limbless-locomotors-over/

## Citation and access

- **Authors:** Henry C. Astley
- **Year published:** 2020
- **Journal or source:** Integrative and Comparative Biology
- **Full citation:** Astley, H. C. 2020. Long limbless locomotors over land: the mechanics and biology of elongate, limbless vertebrate locomotion. Integrative and Comparative Biology 60(1): 134-139.
- **DOI:** 10.1093/icb/icaa034
- **Access status:** Hosted (freely available elsewhere)
- **PDF:** https://www.originalwisdom.com/wp-content/uploads/2026/11/2020_Astley_long-limbless-locomotors-over-land-the-mechanics-and-biology-of-elongate-limbless-vertebrate-locomotion.pdf

## Tags

- **Species:** Fish > American eel; Reptiles
- **Behaviour and ecology:** Behaviour; Movement
- **Methods and technology:** Literature review
- **Data type:** Review
- **Observer qualification:** Not needed
- **Discipline:** Behaviour and ecology
- **Year published:** 2020

## Detailed keywords

- **Species:** Studied (as a group, through the symposium papers): limbless and elongate vertebrates, mainly snakes, also limbless lizards, a basal snake and American eels (Anguilla rostrata). Named in the text: gliding tree snakes, armored cordylid lizards (Chamaesaura spp.), pygopodid geckos (Pygopodidae), eel-like fishes, limbless amphibians (three lineages of Lissamphibians) and early limbless tetrapods. Earlier studies cited include corn snakes (Elaphe guttata), brown tree snakes (Boiga irregularis), boa constrictors, the sandfish lizard and the fossil snake Najash. No field study area.
- **Location and habitat:** No study site; the paper is a symposium introduction. Habitats discussed: cluttered ground, tunnels and confined spaces, sand (on and beneath the surface), trees and branches, vertical steps, and land crossings by migrating eels where dams block rivers. The symposium was held at the Society for Integrative and Comparative Biology annual meeting, 3 to 7 January 2020, Austin, Texas, United States.
- **Topics:** limbless locomotion; snake locomotion; locomotor modes (lateral undulation or “slithering”, sidewinding, concertina, rectilinear); push points; movement through clutter, tunnels, sand and trees; gap bridging; climbing a vertical step; convergent evolution of the elongate, limbless body plan; vertebrae, ribs, muscles and tendons; neural control and visual perception; gliding; eels moving over land; snake robots (“snakebots”) and search and rescue; gaps in knowledge.
- **Statistics or models:** None
- **Data type:** Review: an introduction to a symposium collection of 11 papers, with no new data. Small inconsistencies in the paper: the eel paper is introduced as “Ward et al.” but cited and listed as Redmann et al. 2020; the rib paper is introduced as “Capano et al.” but cited as Capano 2020.

## Synopsis

### 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.

### 8. Implications for tracking (Original Wisdom's observations)

_These are Original Wisdom's observations, not the authors'._

- The paper does not discuss trails, but the four classic movement modes, and the finding that animals switch modes with the surface and slope (as the eels did), are useful background for anyone reading snake and other legless animal trails on sand, soil or slopes.

- The note that sidewinding may be more widespread than thought is a reminder not to identify a snake species from movement type alone.

## Cite this summary

Lawrence, K. 2026. Summary of: Long Limbless Locomotors Over Land: The Mechanics and Biology of Elongate, Limbless Vertebrate Locomotion (Astley, 2020). Original Wisdom, Tracking Literature. https://www.originalwisdom.com/tracking-literature/2020-astley-long-limbless-locomotors-over/