More than meets the (human) eye: Cryptic chromatic diversity in a colour polymorphic lizard

Abstract

Understanding the evolution of polymorphisms, such as colour polymorphisms, is a key aim in evolutionary biology. Accurate, objective description of the different colour variants is crucial to correctly explore colour polymorphisms, a task that can be hindered by the existence of cryptic morphs. Colour polymorphisms are common in many lizards, including lacertids (fam. Lacertidae). The common wall lizard, Podarcis muralis, shows a complex colour polymorphism involving up to five discrete morphs that differ in their ventral coloration: white, yellow, and orange pure morphs, as well as white-orange and yellow-orange mosaic morphs. Here we report that colour polymorphism in this species is more complex than hitherto appreciated due to the presence of two different types of white coloration: one absorbing and another reflecting in the near ultraviolet (UV). Our results show that both colours are found in all populations although in different frequencies. Visual models indicate that both whites are likely to be perceived by the lizards as distinct colour categories. No equivalent cryptic colour variation was found among yellow or orange animals. However, the two whites are also present in white-orange mosaics. As both whites are present in different adult individuals from the same population, it seems more adequate to describe the ventral polymorphism of P. muralis as comprising not five, but seven morphs: UV-absorbing white (UV⁻white), UV-reflecting white (UV⁺white), yellow and orange pure morphs, as well as UV⁻white-orange, UV⁺white-orange, and yellow-orange mosaic morphs. Our results demonstrate that the two types of white are also found in other Podarcis species as well as in other lacertids, revealing that this phenomenon is not a derived character restricted to Podarcis. The existence of cryptic morphs highlights the impact of relying on anthropomorphic descriptions in studies of colour polymorphisms.