Hormonal Color Change
Boa constrictors are one of few snakes that undergo a physiological color change, which is the movement of pigment between dermal and epidermal melanophores, a type of chromatophore (pigmentation cells). This results from hormonal and light cycles and is initiated by melanophore stimulating hormone (MSH), which is excreted from the pituitary gland. The physiological color change allows for advanced sexual signaling and thermoregulation.
Boids can darken their appearance by moving the pigment melanin from dermal melanophores to epidermal melanophores. Darkening typically occurs during the day, in response to low temperatures, and during dry seasons. Darkening allows for rapid heat retention which is particularly beneficial in cooler environments, it also provides protection from ultraviolet radiation. Similarly, they can lighten their appearance by moving melanin from epidermal melanophores to dermal melanophores. Lightening typically occurs during the night, in response to high temperatures, and during wet seasons.
This change in melanin location between dermal and epidermal melanophores also results in chromatic changes. Besides melanophores, other types of chromatophores found in snakes include xanthophores (yellow pigments), erythrophores (red pigments), and leucophores (white pigments). These chromatophores are located beneath melanophores. When melanin is concentrated in epidermal melanophores, such as when the animal is darkened, long-wavelength light is absorbed before reaching xanthophores, erythrophores, and leucophores. This reflects short-wavelength light (320–500 nm). When melanin is concentrated in dermal melanophores, such as when the animal is lightened, long-wavelength light reaches and is reflected by xanthophores, erythrophores, and leucophores. This reflects long-wavelength light (500-700 nm). Humans can observe electromagnetic radiation between wavelengths of 400-700 nanometers, giving us the perception of color. Thus, when we compare a boa constrictor reflecting a wavelength of 400 nm, to a boa constrictor reflecting a wavelength of 700 nm. The former would appear to have increased reds.
Snake scales mostly consist of mostly transparent hard beta keratins forming a layer that coats the epidermis, which consists of the stratum corneum, stratum granulosum and stratum basal. Below the epidermis lies the dermis which contains chromatophores which give a snake it’s coloration. These chromatophores are arranged into stacks, variations in snakes include iridophores, xanthophores (yellow), erythrophores (red), leucophores (white), and melanophores (black/brown). Iridophores lay on above the other pigmentation cells and unlike the others, they are crystalline in structure.
When light enters these crystalline structures and while reflecting off the bottom of the surface the wavelengths interfere with each other. This can come in the form of constructive interference which shifts the light to a longer wavelength, producing reds and yellows. Alternatively, destructive interference can shift the light to a shorter wavelength producing violets and blues.