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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)