Cracking the Code of Crocodile Scales


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  1. Science 339, 78 (2013)


  1. Liana Manukyan

  2. Nicolas Di-Poï

  3. Adrien Debry


Samuel Martin
La Ferme aux Crocodiles,

Matthias Zwicker
Computer Graphics Group,
University of Bern,

Dominique Lambert
Dept of Mathematics,
Namur Center of Complex Systems, Belgium.

Aurélien Roux
Dept Biochemistry,
University of Geneva,

Marcos Gonzalez-Gaitan
Dept Biochemistry,
University of Geneva,


Crocodylian ISOs

EvoDevo (2013)


EvoDevo (2015)

Colour Change in Chameleons

Nature Communications (2015)


  1. Crocs' Chaotic, Cracked Cranial Covering

  2. Using computer graphics and developmental biology techniques, we show that the scales on the face and jaws of crocodilians are not genetically controlled developmental units and that their spatial patterning is generated through physical cracking of the skin (Milinkovitch et al. Science 339, 78 (2013)).

Snakes and lizards have stereotyped, symmetrical, head scale patterns

Head scales in many snakes (below, a corn snake) and lizards are polygons with stereotyped spatial distribution: The left (yellow) and right (red) scale edges overlap when reflected across the sagittal symmetry plane (blue).

Crocodiles have a disorganized head scale pattern

On the other hand, polygonal scales on the face and jaws of Nile crocodiles have a largely random spatial distribution without symmetrical correspondence between left (yellow) and right (red).

These analyses are performed using Computer Graphics tools described in the video on the left below. On the right below, the same video with an interview of Michel Milinkovitch by Nadia Ramlagan (The American Association for the Advancement of Science; AAAS).


Similarly, scales from different crocodile individuals have different distributions of sizes and localizations (blue and red edges from top and bottom crocodiles, respectively).

All body and head scales in snakes and lizards are developmental units

In snakes and lizards, each body scale differentiates from a primordium (in situ hybridization with Shh gene probe), and head scales also develop from primordia, with positional cues determining scale identity.

Similarly, postcranial scales (right = zoom on trunk) in crocodiles also develop from primordia (Ctnnb1 probe) that differentiate into symmetrical then oriented asymmetrical overlapping scales.

Face and jaws scales in crocodiles are not developmental units

Crocodile head scales never form scale primordia but instead develop a pattern of sensory organs that detect surface pressure waves [inset detail; dermal pressure receptors (DPRs)] before any scale appears (probe: Ctnnb1).

Starting about 55 days after oviposition (E55), grooves progressively appear, propagate and interconnect (while avoiding DPRs) to form a continuous network across the developing skin.

Skin sections (left) indicate that cracks correspond to epidermal bulges that reach the stiff underlying tissues. Immunohistochemnistry (right) indicates increased cell proliferation (green) within the skin grooves corresponding to cracks. Abbreviations: primary (pc) and secondary (sc) cracks (ep, epidermis, de, dermis, bo, bone tissue).

Crocodile head scales are generated by cracking

This process is entirely analogous to true physical cracking of a shrinking material layer adherent to a nonshrinking substrate, as in drying or cooling pavement. However, in crocodiles, it is not the cracking layer that shrinks but the underlying substrate layer (the embryonic skull) that grows, hence generating the mechanical stress that causes the skin to crack.

Other cases of physical cracking are drying mud (left), cooling ceramics (top right), and drying adhesive plastic sheets (bottom right).

Please, consult the full publications below for references & much additional information.

  1. Milinkovitch M.C., Manukyan L., Debry A., Di-Poï N., Martin S., Dhillon D.S., Lambert D., Zwicker M.
    Crocodile Head Scales Are Not Developmental Units But Emerge from Physical Cracking
    Science 339, 78 (2013) -- DOI: 10.1126/science.1226265
  2. BulletDownload a FREE reprint (PDF) of the article

  3. BulletDownload the Supplementary Materials file

  4. BulletWatch the Supplementary Movie (also available HERE and in YouTube)

  5. BulletCheck the slideshow from Science

  6. BulletCheck Sarah C. P. Williams’ article in ScienceNOW

  7. Coverage

  8. BulletCheck the slideshow from BBC Nature

  9. BulletCheck the Movie illustrating the Computer Graphics Tools (version 1)

  10. Check more coverage (TV programs, Websites, and News Papers) HERE

Other related publications

  1. Di-Poï N. & M.C. Milinkovitch
    Crocodylians Evolved Scattered Multi-Sensory Micro-Organs
    EvoDevo 2013, 4:19
  2. BulletOpen Access

  3. Coverage

  4. Check Q&A in Biome: online highlights from BioMed Central journals

  5. Check coverage (TV programs, Websites, and News Papers) HERE

  6. Martins A., Bessant M., Manukyan L. & M.C. Milinkovitch.
    R2OBBIE-3D, a Fast Robotic High-Resolution System for Quantitative Phenotyping of Surface Geometry and Colour-Texture
    PLOS ONE (2015)

  7. BulletOpen Access

  8. BulletSupporting File 1 (Supp. Figs A-I, Legends of Supp. Movies S1-S5, Links to Supp. 3D-Model Files).

  9. BulletSupporting Movies on the PLOS ONE web site

  10. BulletSupporting movies S1-S5

  11. Coverage

  12. Bulletcheck here

  13. Tzika A.C., Helaers R., Schramm G. & M. C. Milinkovitch
    Reptilian-transcriptome v1.0, a glimpse in the brain transcriptome
    of five divergent Sauropsida lineages and the phylogenetic position of turtles
    EvoDevo 2011, 2: 19
  14. BulletOpen Access

  15. Access to the Reptilian Transcriptome website

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