Cryptic variation within and among islands in the ultraviolet reflectance of lizard throat coloration

Cryptic polymorphism can bias tests of intraspecific phenotypic variation across space and time. Because many colours exist outside the range of human perception, colour variation is not always easily detected even when it plays an important role in organismal biology. In colour polymorphic lizards, recent work has demonstrated the presence of cryptic variation in ultraviolet (UV) reflectance, but the factors influencing this variation across space and time are less well known. To test among factors influencing UV reflectance variation across the landscape, we captured 771 wall lizards (Podarcis erhardii), a colour polymorphic species, across 35 islands in the Greek Cyclades and examined how UV traits vary across habitats, predator communities, and lizard densities. We discovered evidence for a cryptic, low-UV-morph phenotype nested within the known white morph phenotype, whose presence varies from absent to being fixed within several islands. Island size and geographic proximity were significant predictors of UV reflectance, but we did not find a significant correlation between either predator presence or any other island-wide environmental variables and UV-morph phenotype. We did find that on islands with both low- and high-UV-morph phenotypes, higher UV individuals are more likely to occupy greener, moister areas. We also found that lizards on islands with majority low-UV phenotype individuals are significantly larger than in mixed- and high-UV populations. Our results suggest that the presence of a cryptic, low-UV morph may have novel ecological implications for our understanding of wall lizard social dynamics, species interactions, and evolution.