Scientists long believed that chameleons change their coloring similarly to the way octopuses switch shades, but biologists and physicists somewhat recently realized that something else entirely is going on. "The color change you are seeing is based on structural changes of crystal-like cells called iridophores underneath the skin that refract light," Flynn says.
When anything agitates or exhilarates the chameleon, things start to get interesting. "When they get excited, feel threatened, or want to display positively, towards a mate, or negatively, to a rival, the crystals separate or compress and give off the appearance of different colors," Flynn says.
In a 2015 study published in the journal Nature Communications, researchers with the University of Geneva in Switzerland revealed that iridophores act like tiny mirrors that selectively reflect and absorb different colors. While a lot of animals have color patterns — birds and fish for example — the cells that make their vibrant displays possible typically absorb or reflect certain regions of the visible light spectrum. Chameleon cells are different — iridophores can actually absorb or reflect any and all colors of the spectrum.
"Some species change more colors than others," Flynn says. "For example, some will change shades lighter to darker, while others will change to more vibrant, bright colors. There are quite a few species, so there is a lot of variance. Among all of them, they can change all different types of colors — even colors that aren't visible to the human eye!"
The pigments inside the iridophores are typically contained inside tiny sacs, but when a chameleon's mood changes, its nervous system activates the cells to contract or expand. When the animal is relaxed, the cells stay close together and reflect short wavelengths, like blue. A rush of excitement pushes those cells farther apart, enabling each iridophore to reflect longer wavelengths, like red, orange and yellow.
But if the naturally relaxed cells reflect blue, why are chilled out chameleons typically green like their surroundings? "Basically, when a chameleon is relaxed, they are naturally visible as green to help them camouflage with the green leaves and trees," Flynn says. "This is due to the natural yellow pigment combined with the relaxed state of the crystal cells which reflect blue light. Blue plus yellow equals green." It may not be easy being green, but it's necessary for the chameleon, which has no real defenses; blending in is its best bet against predators.
So, if the color changes are due to special cells under the skin, why are the effects external? The outermost layer of the chameleon's skin is actually transparent. Beneath that outer layer lies several more layers containing a variety of specialized skills, including the blue and white light-reflecting iridophores. What's really wild is if the chameleon needs a dramatic quick-change (like when it's trying to show off for a potential mate), it can quickly shed its outer layers of skin to expose the iridophores to direct sunlight in a hurry.
Learn more about chameleons in "Chameleon, Chameleon" by Joy Cowley. HowStuffWorks picks related titles based on books we think you'll like. Should you choose to buy one, we'll receive a portion of the sale.
Originally Published: Jun 21, 2019