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Chameleons Say It With Color

John Roach
for National Geographic News
September 26, 2005
 
Chameleons are famous for their ability to change their skin color to
blend in with their surroundings. But experts say camouflage is only
half the story of the tropical lizard's remarkable trait.

"Communication is also partly the function of coloration," Christopher Raxworthy, associate curator of herpetology at the American Museum of Natural History in New York, wrote in an e-mail interview.

One of the world's foremost chameleon experts, Raxworthy has discovered several new species and is actively engaged in protecting chameleon habitat in Madagascar.

Part of his research involves studying what the lizards communicate with each other via changes in their color. He's found that the color shifts often express territorial dominance or unwillingness to mate.

"Males become more brightly marked to advertise their dominance," Raxworthy said. "Females become dark or flash red spots to advertise their hostile response to males or their non-receptive status. Aggressive chameleons may become very dark."

Whatever the color signals mean, the tropical reptiles' unusual ability has earned them a fan base among humans.

Undergraduate college student Chris Anderson edits several chameleon-related Web sites, including the Chameleons! Online E-Zine, while studying biology at Cornell University in Ithaca, New York.

He said chameleons are rightfully considered masters of camouflage, but that people often mistake color change as an effort to blend in when in fact the lizards could be showing signs of stress.

Color Change

According to Anderson, the ability of chameleons to change color stems from special cells called chromatophores found in the upper layers of their skin. These cells are filled with different kinds of pigment.

The lizards have three layers of chromatophores. The deepest layer contains melanophores, which have black pigment. Cellular branches extend from these cells and allow the pigment to flow up to and interact with the pigment in upper layers.

The middle layer of cells, called guanophores, regulate blues shades, and cells in the uppermost layer, called xanthrophores, contain yellow and red pigments.

"Basically there's a neurological control mechanism that stimulates the pigments" to move around and cause the chameleon's skin color to change, Anderson explained.

Whether chameleons are actively aware of their color changes is an open question, Anderson said. He suspects that the ability to do so is a trait borne out through the process of natural selection.

Camouflage and Communication

Raxworthy, the American Museum of Natural History herpetologist, says understanding why and when chameleons change color is an ongoing scientific pursuit that began with field observations in the 1960s.

He says the lizards' rapid changes in color, which can occur in about 20 seconds, are most dramatic when chameleons are interacting with one another. He adds, though, the changes also play an important role in allowing the lizards to hide or blend in with their environment.

"Most of the time, chameleons are behaving as highly cryptic animals trying to avoid detection from predators," he said.

In a broadcast of the Pulse of the Planet radio program airing today, Raxworthy says the best time to find chameleons is at night, because they turn pale then and are easily illuminated with a flashlight.

(The National Science Foundation funds the radio program and this related National Geographic News series.)

When asked precisely why the lizards turn pale at night, Raxworthy says, "We are not sure. But it seems to be related to the closing of the [chameleon's] eyes."

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