Timon lepidus, known familiarly as the ocellated lizard, wears the signs of aging on its scaly back. As the lizard matures, the spots that adorn its youthful skin break up and rearrange into a labyrinthine design that marks adulthood. Michel Milinkovitch of the University of Geneva and his colleagues don’t know why the lizard transforms that way—perhaps to camouflage itself, perhaps to signal potential mates—but they now know how.
By monitoring three lizards over several years, the researchers deduced that the patterns on the animals’ backs were updating according to a well-defined algorithm: Over a period of a month or so, a given scale will change color—from green to black or black to green—with a probability p that depends on the colors of the scales around it. For, say, a green scale surrounded by green neighbors, p is around 50%. For a green scale with two or fewer green neighbors, p drops effectively to zero. (The animated graphic is a computerized representation of one lizard’s skin changing over time.) The researchers confirmed the algorithm by modeling the reaction–diffusion equations that govern the evolving distribution of the lizard’s various color-generating cells.
In essence, the reptile is the embodiment of a cellular automaton, a type of discretized model made popular by John Conway’s Game of Life and used to simulate the spread of wildfires, the firing of neurons, and other phenomena. Although some cellular automata evolve indefinitely, the rules governing the ocellated lizard eventually steer it to a static pattern. Around the time the lizard turns four, its pixelated look becomes permanent. (L. Manukyan et al., Nature 544, 173, 2017.)