進化：ワニはどのようにして皮膚を得たのか

The distinctive pattern of scales on the face and jaws of crocodiles is formed by a precise mechanical process of skin folding, a study in this week’s Nature reveals. The findings validate previous suggestions that the scales emerge from a mechanical process rather than a genetic mechanism, while offering new insights into the details of the underlying processes.

Animal appendages, such as feathers, hair, and scales, usually develop as genetically controlled units during embryo development. There are some exceptions to this rule, such as the head scales on crocodiles, which have been found to be produced by purely mechanical processes. However, the precise mechanism driving the patterning of head scales has been hard to determine, in part owing to the technical difficulties of experimenting with crocodile embryos.

By combining experiments in Nile crocodile embryos and computer simulations, Michel Milinkovitch and colleagues generate a three-dimensional mechanical growth model that recapitulates the patterning of crocodile head scales. They find that the scales self-organize through simple mechanical processes, such as compressive folding originating from both the skin growing faster than the underlying bone and the contrasting stiffness of these different tissues. This process produces irregular geometric patterns in the head scales as the crocodile grows in a distinctly different way to how the body scales develop as genetically controlled units, the authors conclude.