Answer: Chromatophores

Of all the phenomena in the animal kingdom, very little has delighted humankind so consistently through the ages as the color changing and camouflaging abilities of creatures around the world—as far back as 400 BC, ancient scholars, like Aristotle, were already noting the ability of cephalopods, for example, to change color.

This ability stands in stark contrast to our rather boring human coloration. We, like other mammals and birds, have a coloration determined by the presence of cells known as melanocytes. However light-skinned or dark-skinned we appear, no matter the coloration of our canine and feline pets, or the coloration of animals we see outside our homes like squirrels and birds, it is all determined (and often permanently fixed) by melanocytes.

In the case of other species like amphibians, fish, reptiles, crustaceans, and cephalopods, however, their coloration is governed by a totally different set of cells called chromatophores. In the vast majority of cases, these chromatophores are inert and a fish that is blue today will be blue tomorrow, just like a cat that is black today will be black tomorrow. But in some special cases, like that of the chameleon and some cephalopods, the organism has complex mechanisms for controlling the display of chromatophores, thus allowing it to change coloration and patterns.

Interestingly enough, the mechanism of control is completely different in invertebrates (like the octopuses and squids) and vertebrates (like chameleons). Octopuses have complex multicellular organs that are manipulated via muscular contraction to change the translucency, reflectivity, or opacity of the pigment contained inside. This action is akin to opening or closing the shutters on colored stage lights to change the color on the stage.

It was long believed that chameleons used a similar mechanism and dispersed pigment-containing organelles within their skin to change color. We now know, however, that they change color by altering the density of a lattice of guanine nanocrystals in a specialized layer of skin in such a way that it affects which wavelengths of light are reflected and which are absorbed. This trick is less like the muscle “shutter” method octopuses use and more akin (in some ways) to the technology found in color-shifting prismatic paint sometimes used for high-end automotive paint jobs.