Reptile Locomotion Notes

Reptile Locomotion

Recap of Amphibian Locomotion

• Amphibians exhibit fairly conserved locomotory modes.
• Anurans jump, walk, or swim.
• Salamanders walk with a primitive sprawling gait.
• Reptiles share the sprawling posture with lateral limb placement, which somewhat constrains their movement.

Reptilian Diversity and Adaptations

• Reptiles adapted to diverse habitats, resulting in diverse locomotory adaptations to navigate them.

Tetrapedal Sprawling Locomotion (Slow)

• Most lizard species walk similarly to salamanders at low speeds.
• This is especially evident in monitors (Varanidae) and skinks (Scincidae).
• As speed increases, they alter their stride and posture.
• Serpentine species may fold their legs and locomote via undulation (skinks).
• Step sequence: 1, 3, 2, 4 (Right front, left rear, left front, right rear).

Tetrapedal Locomotion (Faster)

• Lizards with roughly equal forelimb/hindlimb length switch to almost diagonal foot movement when increasing speed.
• The change involves reducing ground contact from three feet to two as stride length increases.
• The body is held much higher off the ground.

Forelimb Usage in Running

• Many lizards barely use their forelimbs when running at higher speeds.

Bipedal Locomotion (Fast)

• Common in lizard species with much longer hindlimbs (Iguanians) at high speeds.
• Short forelimbs can no longer support the body as stride length increases, so hindlimbs take over for balancing and propulsion.
• The body of these species is usually short and points upwards during bipedal running.
• Limbs remain laterally attached; they do not achieve the erect posture of mammals.
• Increasing stride length without dorsoventral spinal flexion has allowed consequential adaptations.

Bipedal Locomotion (Arboreal Species)

• Arboreal iguanians are predisposed to bipedal locomotion.
• Many forage on the ground.
• Bipedalism allows rapid transition to climbing upon reaching a trunk.
• Elevated body pitch may offer further adaptive advantages due to biomechanics.

Bipedal Locomotion (Riparian Water Runners)

• Sailfin dragons (Hydrosaurus, Agamidae) and Basilisks (Basiliscus, Iguanidae) inhabit riparian habitats and are semi-aquatic.
• Both exhibit remarkable convergent locomotion: running over water.

Adaptations for Water Running

• Lobate rear digits (more pronounced in heavier-bodied sailfins).
• Flight behavior involves diving into nearby water and bicycling the hindlimbs, similar to other bipedal iguanians.
• Large rear feet and increased surface area allow them to power across the surface for several meters.

Speedy Skinks

• A skink’s serpentine body and short limbs are adapted to a terrestrial, semi-fossorial ecology.
• Legs become obstructions for burrowing species.
• To increase speed or move through vegetation, skinks fold their legs laterally against the body and tail, using lateral undulations like a snake.

Climbing Adaptations: Claws

• Claws were a novel adaptation in early reptiles.
• Claw morphology varies, with large, robust, straight claws in burrowing species and slender, curved claws with sharp ends in arboreal species.
• Reference: D'Amore DC, Clulow S, Doody JS, Rhind D, McHenry CR. Claw morphometrics in monitor lizards: Variable substrate and habitat use correlate to shape diversity within a predator guild. Ecol Evol. 2018; 8: 6766–6778. https://doi.org/10.1002/ece3.4185

Climbing Adaptations: Toepads

• Geckos (and Anolis) have adhesive pads of B-keratin (same as their scales).
• Their lamellae contain hundreds of setae, with each seta having hundreds of spatulas.
• Reference: Cheng, Q. & Chen, Bing & Gao, Huajian & Zhang, Yong-Wei. (2011). Sliding-induced non-uniform pretension governs robust and reversible adhesion: A revisit of adhesion mechanisms of geckos. Journal of the Royal Society, Interface / the Royal Society. 9. 283-91. 10.1098/rsif.2011.0254.

Gecko Toepad Mechanics

• Setae are stretched and pretensioned by the gecko.
• This generates van der Waals forces, allowing adhesion to smooth surfaces.
• Very strong against shear force (side-to-side sliding).
• Toes are “peeled” off from the claw as geckos walk, deactivating van der Waals forces, and reapplied in the opposite sequence.
• Clinging strength is best at low temperatures and high humidity.
• References: Cheng et al. (2011) and Niewiarowski et al. (2008).

Capabilities of Gecko Toepads

• Attach/detach in milliseconds.
• Don’t stick to each other or anything the gecko doesn’t want.
• Work underwater or in a vacuum.
• Don’t degrade, foul, or get dirty.
• Stick to any surface except Teflon.
• Hold hundreds of times a gecko’s mass.
• Reference: Autumn, Kellar. (2006). American Scientist, 94(2).

Climbing Adaptations: Chameleons

• Zygodactylous feet.
• Prehensile tail.
• Laterally compressed body.
• Diagonal walk sequence = Stable on perches narrower than its body (but not fast!)

Glissant Locomotion (Arboreal Species)

• Draco lizards are the best and most specialized extant reptile gliders (several fossil forms evolved patagia convergently).
• Glides of >50m are possible.
• Highly elongated, loose-ended ribs.
• Patagia = membrane between these (the ‘wing’).
• Secondary aerofoils: neck lappets.
• Evolutionary dead-end: their morphology (long body, short limbs) is entirely specialized around gliding from tall trees; they can barely run on the ground.
• Reference: Alex Siu Hong Lau et al. (2023). Physics of Fluids, 35(3).

Draco Gliding Mechanics

• Draco have ‘compound’ wings made of multiple parts.
• After the dive phase, they reach back and grab the anterior edge of the patagia.
• Glide is controlled by adjustments of the forearms.
• This allows a full range of movement in the forearms when not gliding.
• Reference: Dehling JM. (2017). PLoS One, 12(12):e0189573.

Glissant Locomotion Evolution

• Draco are not the first reptiles to glide.
• Draco gliding morphology (elongated ribs supporting a skin membrane aerofoil) isn’t even novel.
• Examples include: Coelurosauravus elivensis, Icarosaurus siefkeri, Sharovipteryx mirabilis

Gliding Geckos

• Several species of SE Asian gecko can also glide.
• They use fringes of skin along the tail, finger webbing, and laterally to the body.
• A glide is anything shallower than 45 degrees.

Limbless Locomotion in Snakes: Overview

• Snakes have a diverse range of four traditional modes of locomotion:
  • Concertina
  • Lateral undulation
  • Sidewinding
  • Rectilinear
• Recent expansions suggest up to 14 variations.
• Reference: Bruce C Jayne (2020). Integrative and Comparative Biology, 60(1), 156–170.

Variations of Snake Locomotion Modes

• Concertina
  • Flat-surface concertina
  • Tunnel concertina
  • Arboreal concertina with alternate bends
  • Arboreal concertina with helical wrapping
• Lateral Undulation
  • Forward aquatic lateral undulation
  • Backward aquatic lateral undulation
  • Terrestrial lateral undulation
  • Lateral undulation with a ventrolateral keel
  • Arboreal lateral undulation
  • Gliding lateral undulation
• Vertical undulation
• Sidewinding
• Rectilinear
• Lasso

Limbless Locomotion: Undulation

• Ancestral movement type in fishes and aquatic amphibians.
• Most common form of locomotion in snakes.
• Only works forwards (terrestrial).
• Generates movement by a wave that moves towards the tail.
• Also used by legless lizards and short-limbed skinks.
• Additional variations in snakes.

Limbless Locomotion: Swimming

• Lateral undulation is the primary method.
• Usually accompanied by adaptations to the tail to increase lateral surface for propulsion in aquatic species.
• Limbs (if present) are folded against the body.
• In snakes, works backward as well.

Gliding in Snakes

• Chrysopelea spp. use lateral undulations through the air and maximize body surface area, similar to geckos and Draco.

Snake Gliding Stability

• Lateral (and vertical) undulations make the glide highly stable and give control.
• Simulated dives without undulations were unstable.
• Reference: Yeaton et al. (2020). Nature Physics, 16, 974–982.

Limbless Locomotion: Climbing

• Double lines of ridges along the ventral scales allow some species (e.g., Chrysopelea, Gonysoma) to climb vertical trunks.
• Seems to be an adaptation in tropical species to very wide trees.
• Modified lateral undulation.

Vertical Undulation

• Snakes use vertical undulation to generate propulsion when moving over small obstacles repeatedly, at least in artificial conditions.
• The importance of this in the wild is unknown.
• Reference: Derek J. Jurestovsky et al. (2021). J Exp Biol, 224(13): jeb239020.

Snake Locomotion Specialization

• Snakes locomote using specialized scales and muscles.
• Enlarged ventral scales are present in almost all snakes (exceptions being blind snakes and some highly aquatic species).
• Loose skin around the ventral surface is attached to the ribs by costocutaneous muscles.
• This is absent from the tail (5-40% of snake body length).
• Reference: Bruce C Jayne (2020). Integrative and Comparative Biology, 60(1), 156–170.

Rectilinear Creeping

• Used by heavy-bodied species.
• Relatively slow.
• Contracostal muscles tighten the ventral skin in patches (grip) then haul the skeleton forwards.
• Species reliant on rectilinear creeping usually have very short tails.
• Snakes can move forwards while completely straight

Sidewinding

• Used by species moving on loose or smooth substrates.
• Travel perpendicular to the direction of the body.
• Use a wave travelling along the body for grip, the same as undulation.
• The body is only in contact with the ground in two places when going fast.
• Can reach high speeds

Limbless Locomotion: Concertina

• Used for climbing thin perches that the snake can wrap around to some degree.
• Always minimum two gripping areas.
• Similar to humans climbing a rope.
• Also used for traveling through tunnels (but the grip is on the outside of the coils).

Limbless Locomotion: Lasso

• Allows them to climb much wider trees than concertina.
• Only one area of grip
• Demanding and slow.
• Reference: Julie A. Savidge et al. (2021). Current Biology, 31(1), R7-R8.

Internal Concertina

• Amphisbaenians (worm lizards) use internal concertina with some lateral undulation.
• Concertina works similarly to rectilinear in snakes
• Skeleton is pulled ahead of the skin, allowing semi-independent movement.
• Reference: Hohl et al. (2014). J Zool, 294: 234-240.

Slowcomotion: Tortoises

• Characterized by:
  • No lateral bending of the spine.
  • Limited options for modifying gait for increased speed.
  • To speed up, they just accelerate their standard movement.
  • Extended periods where at least three feet are in contact with the ground (stability).
• Reference: Manuela Schmidt et al. (2016). J Exp Biol, 219(17): 2693–2703.

Challenges in Studying Tortoise Locomotion

• Difficult to measure the muscle and skeletal involvement during walking because:
  • The shell/ osteoderms on the legs.
  • Tortoises are stubborn.

Tortoise Locomotion Mechanics

• 35% of the stride length is generated from rotation in the shoulder girdle (inside the shell).
• The rest is from the antebrachium (forearm) and manus (foot) in combination flexing backwards and upwards.

Crocodilian Locomotion: Slow

• At low speeds, crocs crawl on their bellies.
• Traditional sprawling tetrapod gait like tortoise/lizard/salamander.
• Reference: Hutchinson, J.R., et al. (2019). Sci Rep 9, 19302.

Crocodilian Locomotion: Faster ('High Walk')

• Crocs speed up with a ‘high walk’.
• Characteristics:
  • Legs held erect.
  • Body high from the ground.
  • Tail base off the ground (end drags).
  • Diagonal sequence of steps.
• All croc species show these first two gaits.
• Reference: Hutchinson, J.R., et al. (2019). Sci Rep 9, 19302.

Crocodilian Locomotion: Gallop (FASTER!)

• The vertebral column flexes dorso-ventrally now (up and down).
• Characteristics:
  • Tail and body off the ground.
  • Fore and hindlimbs move almost as pairs.
• Reference: Hutchinson, J.R., et al. (2019). Sci Rep 9, 19302.

Crocodilian Locomotion: Symmetrical Bound (FASTEST!!)

• The vertebral column flexes dorso-ventrally (up and down).
• Increases stride length.
• Characteristics:
  • Tail and body off the ground.
  • Fore and hindlimbs move as pairs.
• Reference: Hutchinson, J.R., et al. (2019). Sci Rep 9, 19302.

Galloping Crocodilians

• Currently, only members of Crocodyloidea have been proven to use these derived gaits
• Alligatoroidea (alligators and caimans) do not (quite) but can achieve similar speeds
• Unknown for gharials and Tomistoma, some anecdotes that young gharial will gallop
• Example: Crocodylus rhombifer (Cuban crocodile) can gallop
• Reference: Hutchinson, J.R., et al. (2019). Sci Rep 9, 19302.

Reptile Locomotion Summary

• Lizards
  • Ancestral tetrapod sprawling gait (slow)
  • Bipedal (hindlimbs>forelimbs)
  • Lateral undulation (skinks)
  • Specialized: Gliding, water running, internal concertina
• Snakes
  • Lateral undulation (most common and versatile)
  • Concertina (climbing, tunnels)
  • Sidewinding (loose surfaces, speed)
  • Rectilinear creeping (heavy-bodied)
  • Lasso (climbing wide trunks)
  • Vertical undulation (lab only, multiple obstructions close together)
• Tortoises
  • Ancestral tetrapod sprawling gait (modified for lack of flexion)
  • Can only speed this up to move faster
• Crocodilians
  • Sprawling belly crawl (slow)
  • High walk (faster)
  • Gallop (faster, only crocs)
  • Bound (fastest, only crocs)