How Do Zebras Get Their Stripes?
Coat patterns such as a zebra’s stripes afford animals the ability to blend into their environment and among other individuals with the same patterns. Predators can’t see camouflaged animals or target specific individuals in a mass of spots or stripes. A regular pattern is fundamental to blending in, but how does a seemingly designed pattern appear on an animal’s coat?
Alan Turing, the man who cracked the Enigma code, discovered that regular patterns could be formed by a combination of long- and short-range processes. Imagine a room that has a series of walls perforated with different sized holes. Centered in the room is a long line of people attempting to go through the walls. Short people can easily go through because they will fit in any of the holes. But tall people make their way through the walls slowly, using only the larger holes. They also try to help other tall people—but short people try to prevent tall people from reaching the other side. Over time, tall people will aggregate near the first set of walls, their progress slowed by the size of holes and by short people, but their numbers are increased by their proclivity to assist other tall people. Short people will spread across the series of walls unimpeded.
If short and tall were wearing different colors—white and red, respectively—an overhead view of this process would reveal a regular pattern of red patches where the tall people have concentrated. This is the same process that governs the development of coat patterns. Chemical “activators” (in this scenario, the tall people) assist in the production of pigment in localized areas, while “inhibitors” (the short people), which move faster, prevent the spread of the pigment at long distances.
Regular pattern formation is not limited in nature to just animal coats. Long and short-range processes produce regular patterns in mussel beds, vegetation, and even microorganisms. Here plants and animals benefit from other individuals of the same species at short distances but are inhibited by competition among individuals at long distances.
Monica Granados is getting her PhD in Biology at McGill University.