Crocodile Head Scales Are Not Developmental Units But Emerge from Physical Cracking

Michel C. Milinkovitch1 Liana Manukyan1 Adrien Debry1 Nicolas Di-Poï1 Samuel Martin2 Daljit Singh3 Dominique Lambert4 Matthias Zwicker3
1Laboratory of Artificial and Natural Evolution (LANE), Department of Genetics and Evolution, University of Geneva 2La Ferme aux Crocodiles, Pierrelatte, France 3Computer Graphics Group, University of Bern 4Department of Mathematics and Namur Center for Complex Systems, University of Namur

In Science 339, 2013

Spatial distribution of head scales. (A) Head scales in most snakes (here, a corn snake) are polygons (two upper panels) with stereotyped spatial distribution (two lower panels): left (yellow) and right (red) scale edges overlap when reflected across the sagittal plane (blue). (B) Polygonal head scales in crocodiles have a largely random spatial distribution without symmetrical correspondence between left and right. (C) Head scales from different individuals have different distributions of scales’ sizes and localizations (blue and red edges fromtop and bottom crocodiles, respectively).


Various lineages of amniotes display keratinized skin appendages (feathers, hairs, and scales) that differentiate in the embryo from genetically controlled developmental units whose spatial organization is patterned by reaction-diffusion mechanisms (RDMs). We show that, contrary to skin appendages in other amniotes (as well as body scales in crocodiles), face and jaws scales of crocodiles are random polygonal domains of highly keratinized skin, rather than genetically controlled elements, and emerge from a physical self-organizing stochastic process distinct from RDMs: cracking of the developing skin in a stress field. We suggest that the rapid growth of the crocodile embryonic facial and jaw skeleton, combined with the development of a very keratinized skin, generates the mechanical stress that causes cracking.

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