Chameleons are cloaked in color-changing crystals
Chameleons rapidly change color using tiny crystals in their skins, researchers report this week in Nature Communications. These crystals may also protect the lizards from hostile environment.
A chameleon is a queer creature. Eyes that rolls independent of one another. Cartoon-like, zygodactylous feet. A long tongue that spurts out from its mouth. The most striking feature of all, at least in some species, is its skin that changes color for camouflage and social cues, such as courtship. The color change is due to pigments in the skin, or so it has been thought. But researchers from University of Geneva, Switzerland, report an additional mechanism in panther chameleons: guanine crystals.
Panther chameleons (Furcifer pardalis) are known for their ability, regardless of gender and age, to rapidly change color by pigments in their skins. Mature males, however, have a larger color variation that serves to attract females and to intimidate rivals. The researchers observed that this “spectacular display” – shifting from blue to green to red – could not be the work of pigments alone.
The researchers used high-resolution microscopy to study the chameleons’ skin in detail. They discovered that panther chameleons have two layers of specialized cells, or iridophores, that contain tiny, neatly arranged guanine crystals. (Guanine is a common compound that also makes up a part of DNA.)
By stretching and shrinking iridophore cells, panther chameleons are able to change the distances between neighboring crystals. Shifting the crystal alignment causes the crystals to reflect certain wavelengths of light that we perceive as certain colors, such as blue (short wavelengths) or red (long wavelengths). The mature male chameleons use crystals, together with the pigments, to achieve their dynamic color change, the researchers explain. For example, the background appears green when the chameleons are relaxed. This is due to the combination of yellow pigments and crystals reflecting blue light. When excited, the iridophores shift from reflecting blue to red. Thus, the skin changes from green to orange (a combination of yellow and red). For the portion of skin that is red from the beginning due to red pigments, the researchers note that the color does not change but the brightness increases when excited. (Check out the video at the end of the post that shows these color changes…and have a laugh at quirky, rolling eyes.)
The upper layer of iridophore cells is only fully developed in adult males. The second layer, found deeper into the skin, is present in all panther chameleons, regardless of age and sex, and contains bigger and less ordered crystals. The deeper layer is not involved in color change (because the crystals do not reflect visible light) but instead reflects wavelengths at near-infrared range, including thermal radiation from the sun. In fact, the researchers calculate that about 45% of the sunlight is blocked by this deeper crystal layer. The deeper layer could function as a thermal protection that could be the key to survival in a dry and hot environment, as found in native habitats of panther chameleons in parts of Madagascar.
The rapid change in color achieved by rearranging crystals is not unique to panther chameleons. The mechanism is similar to those previously observed in some species of squid, fish, and other reptiles, such as Siamese fighting fish (Betta splendens) and day geckos (Phelsuma lizards). However, the deeper layer is possibly unique, found only in a handful of chameleon species, the researchers say:
This combination of two functionally different layers of iridophores constitutes an evolutionary novelty that allows some species of chameleons to combine efficient camouflage and dramatic display, while potentially moderating the thermal consequences of intense solar radiations. — Teyssier et al. (2015) Nature Communications
This research report is open access, which means that the entire article (the text, figures, and movies) is freely accessible to the public.