A crocodile’s facial scales are in fact, not scales at all according to a study performed by Dr. Michel Milinkovitch of the University of Geneva, Switzerland. The study, published in Science, has revealed that the characteristic patterning on the face of crocodilians is actually the result of the animal’s skin physically cracking during embryonic development.
Asked what prompted the study, Dr. Milinkovitch commented that he “could not understand how the scales could be so disorganised (large variability in sizes and shapes) and so different… [both] between the left and right of the same animal and … among different individuals. So, I suspected there was something interesting happening there … I did not expect it would lead to the discovery of such a novel mechanism.”
Scales, like feathers and hair, are a type of keratinised skin appendage. They develop in the egg (or in utero) from, genetically governed, developmental units in the skin. The spatial organisation and patterning of these appendages is determined by reaction-diffusion mechanisms. The scales on a crocodile’s body are wonderfully compliant and develop along the lines just mentioned. The head scales however do not follow the same developmental path.
The observed polygonal, scale-like, pattern emerges during a period of rapid, embryonic, skeletal growth. The skin on a crocodiles head is extremely thick (2x thicker than elsewhere on the body), has a collagen rich dermis, and a heavily keratinised epidermis. As a result the skin is very rigid and inelastic. The rapid development of the underlying skull stresses the skin to a point that causes it to crack.
Video demonstrating some of the graphics tools utilised in mathematically analysing the head scale patterns.
Cracking in nature is, ordinarily, the result of an upper layer shrinking while the underlying substrate remains the same size. You have more than likely observed this pattern in mud that is drying under a hot sun. The upper layer of mud loses moisture more rapidly than the underlying layer, causing it to contract and crack. In the case of the crocodile, the same results are achieved by growth of the underlying layer while the outer layer remains the same size.
Despite being mechanically, rather than genetically, derived the “scalation” of the head will still progress with some degree of order. For example, the first cracks to appear do so across the width of the face. This is because the head grows more quickly in a longitudinal direction (i.e. from the back of the head towards the snout) than in any other direction. Dome pressure receptors, the tiny bumps which give crocodilians their famed hypersensitivity to touch, also help direct the development of these cracks. The bumps which give rise to these receptors appear at an earlier stage of development than the first cracks. The cracks tend to avoid these receptors and progress along lines where their local density is lowest.
So next time you find yourself parked in front of the mirror, vainly fretting over the inevitable onset of crow’s feet, spare a thought for the poor crocodile and remember that things could be a lot worse.
Photo: Courtesy of Raik Thorstad
Video: LANE webpage (see below)
For more information and supplementary materials on this study visit the Laboratory of Artificial & Natural Evolution’s Publications webpage.
Many thanks to Michel C. Milinkovitch for input offered during the composition of this post.