The chameleon’s legendary ability to shift its skin color is a sophisticated biological and physical process involving specialized cells within its skin. This rapid color change is achieved through layers of structures that manipulate light and pigment in response to signals from the animal’s nervous system. The mechanism relies on a phenomenon called structural color, making it far more complex than just moving pigment.
Beyond Camouflage: Why Chameleons Change Color
The popular belief that chameleons change color primarily to blend into any background is largely a misconception. While camouflage plays a role, the most frequent and dramatic color shifts serve two other main functions: social signaling and thermoregulation. Chameleons are ectothermic, meaning they rely on external sources to manage their body temperature. They shift to darker colors to absorb heat when cold, or to lighter colors to reflect sunlight and cool down when warm.
Social signaling is the primary driver for rapid, vibrant changes, acting as non-verbal communication between individuals. Males display bright colors to assert dominance over rivals or to attract a mate during courtship. Conversely, a shift to duller or darker hues signals submission, stress, or fear.
Iridophores: The Physics of Structural Color
The most dynamic color changes are made possible by structural color, a physical process engineered by specialized cells called iridophores. These cells form at least two distinct superimposed layers beneath the outer skin. The upper layer contains a lattice of tiny, reflective structures made from guanine nanocrystals. The color we see is determined by the spacing between these nanocrystals, which acts like a photonic crystal to reflect specific wavelengths of light.
When the chameleon is calm, the nanocrystals in the upper layer are tightly packed, reflecting shorter wavelengths like blue and green light. When the animal becomes excited, the iridophore layer relaxes, increasing the distance between the nanocrystals by as much as 30%. This increased spacing shifts reflectivity to longer wavelengths, resulting in the skin reflecting yellow, orange, or red light instead of blue.
The lower layer of iridophores contains larger, more disorganized crystals that primarily reflect infrared light. This deeper layer plays a major role in reflecting solar radiation, assisting in thermoregulation.
The Role of Pigment Cells
While iridophores create dynamic structural colors, traditional pigment cells contribute fixed chemical colors and are essential for creating the final, visible hue. These pigment-containing cells, collectively known as chromatophores, are found in layers above and below the iridophores. Xanthophores contain yellow pigments, and erythrophores contain red pigments. The combination of structural color and chemical pigments produces the full color spectrum.
For instance, if the iridophores reflect blue structural light and this light passes through a layer of yellow xanthophores, the resulting color perceived is green. The deepest layer of cells consists of melanophores, which contain the dark pigment melanin. Melanophores are star-shaped cells with long projections that can cover the layers beneath them. By dispersing the melanin throughout these projections, the chameleon can darken its skin or create dark patterns, which influences the lightness of the reflected color.
How the Nervous System Controls the Change
The rapid, voluntary color shifts are fundamentally controlled by the chameleon’s autonomic nervous system. The process is triggered by external stimuli, such as a change in ambient temperature, the presence of a rival, or a potential mate. The brain quickly processes these cues and sends signals to the specialized skin cells. These nerve impulses and hormonal signals prompt the iridophores to either expand or contract, changing the spacing of the guanine nanocrystals. Similarly, the signals instruct the pigment cells to either disperse or aggregate their stored pigments.