# Why do tigers have vertical stripes? Check the math, say researchers.

Scientists have discovered that the stripes exhibited on animals like zebras and tigers can be defined by mathematical predictability.

|
Courtesy of Tom Hiscock
This image shows simulations of Turing stripes. On the left, stripes are evenly spaced, but their direction is variable. On the right, a signaling gradient has made the stripes align in the same direction.

Scientists have discovered that the stripes exhibited on animals like zebras and tigers can be defined by mathematical predictability.

The study, published today in the journal Cell Systems, offers a possible explanation for why different species’ stripes move the direction they do. It has to do with how much of one substance or another is produced while the baby animal is still in its mother’s womb.

The study builds upon early research in the field that was conducted by Alan Turing, the famed British mathematician and codebreaker who near the end of his life turned his skills to mathematical biology.

Dr. Turing’s work produced several insights, among them the theory that the unstable diffusion that occurs during a chemical reaction is what can cause different structures like zebra stripes to grow in a certain direction.

The Turing model gives a mathematical explanation for stripes and other patterns, but doesn’t explain why some species, like tigers, have vertical stripes and others, like okapis, have horizontal ones. This study seeks to answer that question.

"We wanted a very simple model in hopes that it would be big picture enough to include all of these different explanations," lead author Tom Hiscock, a PhD student at Harvard Medical School, said in a press release. "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."

In the study, Mr. Hiscock created a model that examined the physical changes that each contribute to stripe orientation: “production gradient,” the substance that amplifies stripe pattern density; "parameter gradient," or the substance that changes one of the parameters in stripe formation; and finally, the physical changes that contribute changes in direction to the molecular, cellular, and mechanical origin of stripes.

The model relies upon a simple equation that has also been used by other researchers when examining stripe formation. The equation rests on the logic of local pattern density and the concept that those local patterns will recur over a larger space like skin or fur.

“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," Hiscock said in the release.

The simple equation wasn’t reliable for more complex models or models that have multiple instabilities besides the Turing instability. But it was successfully able to predict more complex patterns, even if only partially.

Previous research has looked into the possibility that zebra stripes may be an evolutionary defense against flies. This study from Hiscock and the Harvard research team offers an explanation for why those stripes occur so regularly.

You've read  of  free articles. Subscribe to continue.
Real news can be honest, hopeful, credible, constructive.
What is the Monitor difference? Tackling the tough headlines – with humanity. Listening to sources – with respect. Seeing the story that others are missing by reporting what so often gets overlooked: the values that connect us. That’s Monitor reporting – news that changes how you see the world.

About a year ago, I happened upon this statement about the Monitor in the Harvard Business Review – under the charming heading of “do things that don’t interest you”:

“Many things that end up” being meaningful, writes social scientist Joseph Grenny, “have come from conference workshops, articles, or online videos that began as a chore and ended with an insight. My work in Kenya, for example, was heavily influenced by a Christian Science Monitor article I had forced myself to read 10 years earlier. Sometimes, we call things ‘boring’ simply because they lie outside the box we are currently in.”

If you were to come up with a punchline to a joke about the Monitor, that would probably be it. We’re seen as being global, fair, insightful, and perhaps a bit too earnest. We’re the bran muffin of journalism.

But you know what? We change lives. And I’m going to argue that we change lives precisely because we force open that too-small box that most human beings think they live in.

The Monitor is a peculiar little publication that’s hard for the world to figure out. We’re run by a church, but we’re not only for church members and we’re not about converting people. We’re known as being fair even as the world becomes as polarized as at any time since the newspaper’s founding in 1908.

We have a mission beyond circulation, we want to bridge divides. We’re about kicking down the door of thought everywhere and saying, “You are bigger and more capable than you realize. And we can prove it.”

If you’re looking for bran muffin journalism, you can subscribe to the Monitor for \$15. You’ll get the Monitor Weekly magazine, the Monitor Daily email, and unlimited access to CSMonitor.com.

https://www.csmonitor.com/Science/2015/1228/Why-do-tigers-have-vertical-stripes-Check-the-math-say-researchers.
https://www.csmonitor.com/subscribe

## Subscription expired

Your subscription to The Christian Science Monitor has expired. You can renew your subscription or continue to use the site without a subscription.

This message will appear once per week unless you renew or log out.

## Session expired

Your session to The Christian Science Monitor has expired. We logged you out.