A unity underlying the different zebra striping patterns

To elucidate the relationship between the complex striping patterns of the different species of zebras, a simple conceptual experiment has been performed. Using data from horse embryos, the normal growth of the zebra from early foetus to adult has been reversed to see what happens both to the spacing and to the orientation of the stripes. It turns out that for each species, there is a point in time when all the body stripes would have been perpendicular to the dorsal line and equally spaced. Moreover the spacing is roughly the same (0·4 mm) for the three main species of zebra at this time. This point is during the third week of development for E. burchelli, fourth week for E. zebra and fifth week for E. grevyi. As striping only appears at about the eighth month of foetal development, it seems that the pattern is determined a long time before the cells actually lay down pigment. Further analysis of the pattern so laid down on a rapidly-growing foetus shows how shadow and gridiron stripes can arise. The reason why leg stripes are orthogonal to body stripes cannot however be derived from this phenomenological approach. These results suggest that a single mechanism generating equi-spaced stripes of separation 0·4 mm could lay down the body stripes of zebras and that species differences arise from pattern formation occurring at different times in embryogenesis.