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HOW COME? Why a tiger never changes his stripes

Why do tigers have orange and black stripes? asks students in Howard Cohen's class at Francis Lewis High School, in Fresh Meadows, Queens. Tigers are the biggest of the big cats; some can weigh more than 800 pounds and measure 11 feet long. Scientists estimate that only about 3,000 to 3,900 tigers are left on Earth outside of zoos.

But why the stripes? Many animals have evolved "protective coloration," a kind of camouflage that helps them blend into their surroundings and avoid becoming someone else's dinner. But tigers are among nature's biggest predators; their only real threat are human beings. Scientists think that a tiger's stripes -- usually black on gold or brown -- help the animals blend in, avoiding detection by their prey.

A tiger's tawny base color matches colors found in forests and the reedy banks of rivers and streams. Their stripes evoke shadowed stems and trees. Meanwhile, tigers, like many animals, have lighter undersides and darker tops. When sunlight falls on a solid-colored object, the top appears lighter than the bottom, and the object's shape stands out clearly. But when sunlight shines on a counter-shaded animal, it's a bit harder to see. Counter-shading plus stripes help a tiger lurk in the shadows.

How does a tiger's body create its stripes? One answer may have come from math.

During World War II British mathematician Alan Turing devised methods to break the secret codes of Hitler's military. In the early 1950s Turing developed mathematical equations he thought could help explain repeating patterns in nature.

Turing suggested that patterns found in animal bodies -- from the spacing of alligator teeth to leopard spots and tiger stripes -- might be triggered by the interaction of two chemicals. An "activator" might cause a stripe or spot or tooth to develop in one place; an "inhibitor" might intervene here and there to stop the process. The result: a regular pattern.

Sixty years after Turing wrote his paper, two such chemicals were identified. In 2012 researchers from Kings College in London discovered that the ridges in the roof of a mouse's mouth were a kind of Turing pattern. The scientists found chemicals in mouse embryos that acted as activator and inhibitor, resulting in regularly spaced ridges.

Tiger stripes, which extend into the skin, may form in a complex but similar process. And scientists say that Turing-type repeating patterns can be seen throughout the universe, in weather systems, the spacing of plants in dry landscapes, and the distribution of stars in galaxies, including our own Milky Way.

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