Colour variation in the polymorphic common wall lizard (Podarcis muralis): an analysis using the RGB colour system

Intra-specific colour variation may either reflect a discrete polymorphism, potentially related to life-history strategies; a continuous signal, which may be related to individual quality; or a combination of both. Understanding the true nature of this colour variation may thus help to identify the possible selective mechanism producing it. The common wall lizard (Podarcis muralis) exhibits polymorphic coloration, both sexes showing three pure morphs differing in throat and belly colouration (white, yellow and red) and three intermediate morphs (white-yellow, white-red, and yellow-red). In this study we used digital photography and image analysis (RGB system) to investigate colour variation among morphs, sexes and populations. We found that colouration of the six phenotypes results from the combination of three discrete colour tinges (white, red and yellow): pure morphs express a single colour, and no continuous variation exists among them. Similarly, ventral parts of the intermediate white-red and yellow-red morphs present a mixture of two discrete colours resulting from the concurrent expression in different scales of the same colours showed by the corresponding pure morphs. Finally, the white-yellow intermediates show the same colour tinge than pure yellow morph, but with a lower intensity. The finding that white-yellow lizards are smaller than all other ones suggests that they might be subadults not yet expressing the full yellow colouration. Morph colouration significantly varied among sexes, suggesting a possible role for hormone plasma levels in controlling colour expression. It is noteworthy that RGB values varied among sites, indicating that colour expression in this species holds some level of environmental plasticity. Therefore, our results confirmed the presence of discrete colour morphs, which may be genetically based in both sexes of common wall lizards. However, our findings also showed intra-individual variation in colour expression within a morph that is associated with habitat occupancy, which suggests phenotypic plasticity in morphs.