Intraspecific variation and detectability of iridescence in the dorsal coloration of a wall lizard

Iridescence refers to the optical property of surfaces for which reflected wavelengths depend on viewing geometry. Although iridescence underlies some of the most striking animal colours, the sensory stimulation elicited by iridescent spectral shifts in relevant observers has seldom been explored. Wall lizards often show substantial intraspecific colour variation, which may influence the detectability of iridescence by varying the range of affected wavelengths. Here, we set out to study Podarcis liolepis dorsal coloration in two localities selected for their colour differences: the València Botanical Garden (EB), and La Murta Natural Park (LM). To determine the presence of angle-dependent reflectance, we measured dorsal coloration at three different configurations (0º, 60º, and 90º angles between incident light and observer location) in 87 lizards of either sex and used visual modelling to determine their detectability when viewed by conspecifics, raptors, and humans. Our results show that P. liolepis dorsal coloration varies chromatically with sex and locality, and also shows iridescence (i.e. reflectance peaks at shorter wavelengths with increasing viewing angle). Lizards from EB are brown dorsally, whereas most lizards from LM, especially males, show a green dorsal background coloration, reflectance peaking at shorter wavelengths in lizards from LM compared to lizards from EB. Angle-dependent shifts in peak location are similar across localities and sexes, yet increased overlap between the involved waveband and receiver cone sensitivities results in larger chromatic distance in lizards from LM (compared to EB) for every observer considered. In addition, P. liolepis dorsal iridescence may be more apparent to humans and raptors than to the lizards themselves. Our findings suggest that intraspecific colour variation may affect the detectability of iridescence consistently across observers, emphasizing the importance of using objective colour quantification and visual modelling methods when studying the ecological consequences of iridescence in nature.