About the cover
Macroscopic patterns in the animal world, such as zebra stripes and leopard spots, can be described by processes occurring at the scale of biological cells. In this issue, Michel C. Milinkovitch's team reveals a dynamical process that coordinates pattern formation on the skin of ocellated lizards. By recording the shifting patterns over four years, the team found that the colour changes occur at the level of individual skin scales and obey the rules of a hexagonal cellular automaton — a grid of elements whose states change according to the states of neighbouring elements. Numerical simulations and mathematical derivation show that such a discrete system can emerge at the mesoscopic scale from the continuous reaction-diffusion framework when variations in skin thickness are taken into account. The team's work suggests that cellular automata are not just abstract computational systems, but that they can directly correspond to processes generated by biological evolution. Cover image: Michel C. Milinkovitch.