Crocodile heads teach us a cracking lesson

I recently came across a study by Milinkovitch and colleagues on the development of crocodile head scales. I think it highlights how nature sometimes chooses unusual ways to approach development, so I thought I would share a short summary here!

Scales, feathers and hairs are evolutionary adaptations to terrestrial life, fulfilling functions such as preventing water loss or protecting from UV radiation. It is unclear whether they are homologous structures or the result of convergent evolution, but all three structures are generated in a similar way. During embryogenesis, individual developmental units are genetically specified, each of them later forming, for example, an individual scale. The final surface organisation of these developmental units is thought to be established following a model first proposed by Turing over 60 years ago. According to this model, the local concentration of several chemical components, and the ability of cells to differentiate at unique thresholds of these components, generates different patterns.

A recent paper by Milinkovitch and colleagues, however, suggests that surface patterns may be determined using a different mechanism. By making 3D models of crocodile heads and mapping scale edges and nodes, the authors noticed how these scale patterns, unlike other reptiles, were not symmetrically organised in individual heads and were generally randomly distributed. Using these models, they analysed several features, such as edge angles and area distribution, concluding that the scales of the crocodile head are generated simply by physical cracking of the skin during development. This was evident during crocodile embryonic development: initially only major cracks are present, and these then branch and interconnect over time to form the final scale complexity.

Crocodile heads choose an alternative to the more common theme of genetic developmental determination of skin patterns, and it will be interesting to see whether a similar mechanism is used by other organisms (or body parts!). This work also shows how physical processes can be major players in certain developmental processes, an important reminder that genes do not always explain everything.

Milinkovitch MC., Manukyan L., Debry A., Di-Poi N., Martin S., Singh D., Lambert D., Zwicker M. (2013). Crocodile head scales are not developmental units but emerge from physical cracking, Science, 339 78-81

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