This Is Why Tigers Have Parallel Stripes

Math holds the answer.

Why do tigers have stripes rather than being one solid color?  These stripes are amazing because they are not only evenly spaced, but also perpendicular to the spine.  Scientists have been trying to figure this mystery out since the 1950s, and now they have developed a model to identify what controls stripe formation in living creatures.

It turns out stripes are quite simple to model mathematically, and a lot of work has already been done on the subject.  Alan Turing was the first to suggest that stripe patterns occur due to a pair of morphogens that work together as an ‘activator’ and ‘inhibitor’.  However, what Turing’s model didn’t explain is how stripes orient themselves in a particular direction.

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“We wanted a very simple model in hopes that it would be big picture enough to include all of these different explanations,” said lead author Tom Hiscock, a PhD student at Harvard Medical School.  “We now get to ask what is common among molecular, cellular, and mechanical hypotheses for how living things orient the directions of stripes, which can then tell you what kinds of experiments will (or won't) distinguish between them.”

Hiscock’s investigation looked at why, for example, tiger stripes are perpendicular to its body while zebrafish stripes are horizontal.  What surprised him is that in the model it only takes a small change to switch between horizontal and vertical stripes.  However, Hiscock is not yet sure how the model translates to living things.

“We can describe what happens in stripe formation using this simple mathematical equation, but I don't think we know the nitty-gritty details of exactly what molecules or cells are mapping the formation of stripes,” said Hiscock.  

Hiscock’s model predicts three main things that affect how stripes orient themselves.  The first is a change in “production gradient,” which is a substance that amplifies stripe pattern density.  The second is a change in “parameter gradient,” — a substance that changes one of the parameters that forms the stripe — and the third is a physical change in the direction of the molecular, cellular, or mechanical origin of the stripe.

Although this idea is currently theory, Hiscock believes it won’t be long before they have the tools to determine whether any of these are true in living things.