Color Change in Chameleons
Kimmie McGowen and Madeline Robin
How Chameleons Change ColorThe mechanism behind chameleon color change isn’t very well understood yet. It is believed that in chameleons, the autonomic nervous system is responsible controlling color change (Berger and Burnstock, 1979). The autonomic nervous system does this by its neurons triggering a response in chromatophores. Chromatophores are cells within the dermis of chameleons that contain pigments and reflect light, which are responsible for creating coloration.
In chameleons, there are four types of chromatophores: xanthophores, erythrophores, iridiophores, and melanophores (Cooper and Greenberg, 1992).
Xanthophores and erythrophores are the closest to the surface of the skin and contain the pigments carotenoid and pteridine, which make xanthophores appear yellow-orange and erythrophores appear red.
Iridiophores are beneath the xanthophores and erythrophores in a chameleon’s skin. These organelles contain colorless crystals of guanine pigment. These crystals “stack” as platelets and the stacking and spacing arrangements affect how light is scattered. If they stack in a way that preferentially scatters shorter wavelengths of light and transmit longer wavelengths, a phenomena called “Tyndall scattering”, the iridiophores will appear blue.
The deepest layer of chromatophores in the chameleon dermis is made up of melanophores. Melanophores contain the pigment melanin, which is brown or black in color. They are large, star-shaped, and have long dendrites, or “arms”, that extend between and overlay the other types of chromatophores.
The coloration of chameleons is dependent on the density and arrangement of these chromatophores. Different chameleon species are able to change color to varying degrees and produce different colors and patterns, which is due to the types and arrangements of chromatophores that is characteristic of a particular species.
Color change happens when the melanin pigment moves. Chameleons darken when melanin disperses or concentrates in the dendrites of the melanophore, and the skin becomes pale when they collect at the center of the melanophore, which is nestled deeper within their skin. When the dendrites accumulate melanin, the melanin blocks the iridiophores, which typically reflect light, but not the xanthophores and erythrophores, so the chameleon appears dark orange or red (Bagnara and Hadley, 1973). Whether or not the movement of the other types of chromatophores in chameleons influences color change is not known.
Color Change TriggersThere are four categories of triggers that can cause chameleon color change, all of which vary by species:
Whether or not temperature will trigger a change in coloration in chameleons depends on the native habitat of . Two species native to low-latitude, mid-elevation evergreen forests, Trioceros jacksonii and T. ellioti, do not change reflectance of light due to temperature. However, Chamaeleo dilepis, which is native to areas of subtropical savannah environments with seasonally variable weather exhibit significant changes in reflectance of light (Walton and Bennett, 1993).
Chameleons also change pigmentation in response to light, particularly in areas of higher latitudes or altitudes. C. dilepis have been seen to perch on the top of a bush in the morning, compress their body, and take on a position that orients one side toward the sun. Once this position is achieved, the chameleons change to a black-brown color, but only on the side that faces the sun (Burrage, 1973). They may also take on a pale coloration during midday, which has been seen on roads and in open areas (Walton and Bennett, 1993).
Color change in chameleons is thought to be controlled by a combination of their nervous systems and hormones. In 1928, color change change as a response to direct stimulation to spinal nerves was shown in Bradypodion pumilum (Hogben and Mirvish, 1928), but it wasn’t until 1973 that hormones, specifically adrenocorticotropic hormone (ACTH), were shown to activate color change in Trioceros jacksonii (Bagnara and Hadley, 1973).