Module 1
Scales, Skin & Ecdysis
Reptile skin does three jobs no mammalian hide does: it waterproofs via a thick β-keratin corneum, it patterns in a reaction-diffusion lattice that Alan Turing predicted in 1952, and (in geckos) it sticks to glass by van der Waals force alone. This module dissects α- vs. β-keratin biochemistry, the physics of Turing scale patterning, the mechanics of ecdysis, and the nanomechanics of gecko setae.
1. α- vs β-Keratin
Mammalian hair, nail, and hoof are built of α- keratin: coiled-coil dimers (∼46 nm rods) assembled into 10 nm intermediate filaments. Reptiles retain α-keratin in the inner epidermal layers but also synthesise β-keratin— a distinct family of small (10–18 kDa) cysteine-rich proteins that fold as twisted β-sheets and assemble into 3 nm filaments in an amorphous cementing matrix.
β-keratin is tougher and more hydrophobic than α-keratin (Young’s modulus ~2.5 GPa for claws and beaks; Filshie 1962). The same gene family produces feathers (Aves), turtle shells, snake scales, and gecko setae. Crystal-structure work by Fraser & Parry (2011) mapped the twisted-sheet filaments; Dalla Valle 2010 showed the cluster of β-keratin genes sits on chromosome 2 in Anolis carolinensis with 34 paralogs (Alfoldi 2011).
2. Scale Morphogenesis — Turing in Skin
Alan Turing’s 1952 reaction-diffusion model predicted that two interacting morphogens — a short-range autocatalytic activator and a long-range inhibitor — can spontaneously pattern stripes, spots, or hexagonal lattices. Reptile scales are a textbook realisation. Milinkovitch 2013 showed crocodile head scales are NOT developmental units but emerge from physical cracking of the growing skin; ocellated-lizard (Timon lepidus) scales form discrete hexagonal units that flip colour as a probabilistic cellular automaton (Manukyan 2017, Nature).
\[ \frac{\partial u}{\partial t} = D_u \nabla^2 u - u v^2 + F(1-u),\qquad \frac{\partial v}{\partial t} = D_v \nabla^2 v + u v^2 - (F+k)v \]
Gray-Scott (above) with Du < Dv and tuned F, k produces labyrinthine or hexagonal spots whose wavelength is set by λ ∼ 2π√(D/(F+k)).
Simulation 1: Turing Pattern (Gray-Scott)
200×200 Gray-Scott simulation to steady state, producing the labyrinthine / spot pattern that underlies reptile scale morphogenesis.
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3. Ecdysis — the Molt Cycle
Squamates shed epidermis periodically. A thyroid-controlled proliferative phase produces a new under-layer; enzymatic degradation at the boundary between the old and new corneum generates a fluid-filled cleavage zone; the lizard or snake then physically works free. Snakes shed in one piece starting from the rostral scale; lizards shed in flakes. Maderson 1965 described four stages — germinal proliferation, new keratin synthesis, cleavage-zone lymphatic infiltration (“opaque phase” visible as cloudy spectacle scales), and peeling.
Molecular drivers include Shh, BMP2/4 and Wnt signalling, mirroring the feather / hair cycle in other amniotes. Dysecdysis (stuck shed) is the commonest clinical skin problem in captive reptiles and typically reflects low humidity or thyroid dysfunction.
4. Gecko Setae — van der Waals Adhesion
Tokay gecko (Gekko gecko) toes carry ~14 000 setae per square millimetre. Each seta (50–130 µm long) branches into ~1000 spatulae whose tips are ~200 nm across. Individual spatula adhesion is purely van der Waals; Autumn 2000 measured Fadhesion ≈ 200 µN per seta on atomic-force cantilevers.
\[ F_{vdW} \;=\; \frac{A R}{6\,d^3}\quad\text{(sphere-plane, A = Hamaker)} \]
Cumulative force across ~109 spatulae yields safety factors of ~100–200× a 50 g gecko’s weight. Release is controlled by angle: at a critical pull-off angle (~30°) the spatula detaches passively. Geim 2003 and Sitti 2013 extended the principle to synthetic dry-adhesive tapes.
Simulation 2: Gecko Setal Adhesion
Per-spatula van der Waals force vs. separation, scaled to the full gecko toe pad to reproduce the Autumn 2000 ~200× weight-safety factor.
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Code will be executed with Python 3 on the server
5. Specialised Integument
- Osteoderms in crocodiles and anguid lizards — dermal bone tiles reinforce the scales.
- Scutes on turtles: keratinised plates over a bony carapace (fused vertebrae + ribs).
- Chromatophores — melanophores + xanthophores + iridophores produce the dynamic colour change of chameleons and anoles (Teyssier 2015 iridophore lattice).
- Snake belly scales (ventrals) have pronounced friction anisotropy — key to locomotion (M3).
Key References
• Turing, A. M. (1952). “The chemical basis of morphogenesis.” Phil. Trans. R. Soc. B, 237, 37–72.
• Milinkovitch, M. C. et al. (2013). “Crocodile head scales are not developmental units but emerge from physical cracking.” Science, 339, 78–81.
• Manukyan, L. et al. (2017). “A living mesoscopic cellular automaton made of skin scales.” Nature, 544, 173–179.
• Autumn, K. et al. (2000). “Adhesive force of a single gecko foot-hair.” Nature, 405, 681–685.
• Autumn, K. et al. (2002). “Evidence for van der Waals adhesion in gecko setae.” Proc. Natl. Acad. Sci., 99, 12252–12256.
• Maderson, P. F. A. (1965). “The structure and development of the squamate epidermis.” Proc. Zool. Soc. Lond., 146, 98–112.
• Fraser, R. D. B. & Parry, D. A. D. (2011). “The structural basis of the filament-matrix texture in the avian/reptilian group of characteristic β-keratins.” J. Struct. Biol., 173, 391–405.
• Teyssier, J. et al. (2015). “Photonic crystals cause active colour change in chameleons.” Nat. Commun., 6, 6368.