Why the Green Turns to Blue, a Detective Story

Chameleons, in general, tend to display more blue coloration in captivity compared to the wild, as evidenced by numerous examples, especially in animals  coming from several generations in human care. This blueing effect is notably present in Nosy Be panther chameleons (Furcifer pardalis), which naturally exhibit much less blue, with the so-called "true blue" rarely found in the wild; however, certain specimens might display a significant blue hue, particularly if they are old or ill. Similar occurrences have been observed in other species while in captivity, including Calumma parsonii, Trioceros quadricornis, Trioceros montium, Chamaeleo dilepis, and Chamaeleo calyptratus.

Interpreting color shifts in chameleons is more complex than in snakes due to the differing inheritance patterns of color pigments. While in snakes, the coloration is defined by the presence/absence of certain color pigments only, chameleon colors arise from the interplay of light refraction of two layers of cells containing guanine crystals. This output is combined with pigments such as melanin (brown, black) and carotenoids and pteridines (yellow, orange, red). This process of creating colors based on yellow, orange and red pigments in chameleons appears to be more passive than teh notoriously known work of melanine (concentrating and distributing in cells), as physical coloration is primarily influenced by the presence or absence of yellow pigments in the skin. For instance, the typical yellow mouth corners observed in certain populations of Furcifer pardalis (such as those from Nosy Be, Nosy Komba, Nosy Tanikely, Nosy Faly, Ambanja, etc.) or the orange mouth corners found in Furcifer major and bifidus are visible even on the exuviae of the mouth's corners. Furthermore, deceased chameleons tend to be predominantly yellow, a condition observed when there is no active color change; this is also the case when they are ill or during the night, when color change mechanisms are limited due to low light exposure (up to 1 Lux from moonlight). Yellow can also appear on shaded parts of the body, such as the belly exposed to moonlight from above or areas concealed by leaves or other objects.

The original natural color of chameleons, predominantly green, is a complex hue resulting from a combination of yellow and blue tones. Thus, a transition towards blue indicates a decreased level or absence of yellow. Blue coloration logically emerges from the former green shade when yellow pigments are missing. Therefore, a critical question arises: where do these yellow pigments come from, and why are they absent?

Currently, the most plausible explanation points to dietary factors. It suggests that some components in the wild diet of chameleons are not provided or are given in lower quantities in captivity. Carotenoids are suspected to play a significant role. Experience indicates that chameleons fed bee pollen, which typically contains up to 2% carotenoids by dry weight, exhibit slower or less likely blue coloration. 

This perspective also sheds light on the question of how chameleons utilize carotenoids, especially considering their inability to metabolize them into Vitamin A, contrary to many other vertebrates. The answer likely lies in their use of carotenoids as a means of physical coloration in their bodies. This is notably observable in the skin of various species and in their mucous membranes, particularly within their mouths. For example, the yellow inner space of the mouth can be seen in species such as Chamaeleo calyptratus, C. arabicus, C. dilepis, Furcifer pardalis, and F. oustaleti, as well as orange-colored inner spaces in Brookesia thieli, Rieppeleon kerstenii, Calumma globifer and C. oshaughnessyi.

The coloration can be influenced by chameleons' diet, as many of their food sources—such as Orthopterans, Dipterans, and Hymenopterans—average approximately 1 to 5 mg of carotenoids, particularly xanthophylls, per gram of body weight. Additionally, bee pollen can significantly augment this carotenoid intake if it becomes part of the gut content or if the chameleons ingest pollen collected by bees and bumblebees through specialized structures on their legs.

However, it's important to note that pteridines and their derivatives are more crucial for the yellow coloration in chameleons than carotenoids. While pteridines do not play a role in Vitamin A synthesis, they can be directly absorbed from the intestine into the bloodstream and transported to various parts of the body, akin to the behavior observed with carotenoids.

Another factor influencing coloration is the exposure to natural sunlight, especially the ultraviolet (UV) spectrum. Generally, chameleons that receive natural sunlight tend to display fewer blue hues and maintain more green tones in comparison to those that are kept under artificial light or artificial UV sources.

Nonetheless, all these assertions and connections remain speculative for now, as more extensive research is needed to fully understand the intricacies of chameleon coloration and its underlying mechanisms.