Cryptic ontogenetic changes in the ventral coloration of a colour polymorphic wall lizard (Podarcis muralis)

During growth, many animals undergo irreversible colour changes. Despite their ecological and ethological relevance, ontogenetic colour changes (OCCs) are often overlooked. The problem is compounded when OCCs involve wavelengths invisible to humans. Wall lizards can perceive ultraviolet (UV) light, and their conspicuous ventral and ventrolateral coloration —including UV-reflecting patches— likely serves social communication. To human vision, adults of the European common wall lizard (Podarcis muralis) may show throats and bellies with uniform orange, white, and/or yellow coloration, or two mosaic colorations. In addition, adult males (and some females) present a row of ventrolateral UV-blue patches in their outer ventral scales (OVS). Here we set out to describe OCCs in the ventral (throat and belly) and ventrolateral (OVS) coloration of juvenile Podarcis muralis lizards as perceived by conspecifics. We measured reflectance in newborn and yearling lizards raised under semi-natural conditions and used visual modelling to estimate chromatic distances within individuals and across life stages (i.e. newborns, yearlings, and adults). Newborns typically exhibit UV-enhanced white (UV+ white) on their ventral surfaces (throat, belly, and OVS), a colour that is likely perceived by conspecifics as distinct from the most frequent adult colours (i.e. orange, yellow, and UV-reduced white; UV- white). This distinction, invisible to humans, highlights two types of white coloration differing in UV reflectance. The prevalence of UV+ white decreases with age, with the decline being less pronounced in female bellies. OCCs related to the UV-blue patches seem to be delayed with respect to changes in the throat and belly. Throat colours in yearling lizards are indistinguishable to conspecifics from the equivalent colours in adult throats. In contrast, chromatic distances between the UV-blue patches of adults and yearlings are large enough for discrimination by conspecifics. We discuss a possible function of P. muralis OCCs in delaying the onset of adult colours involved in social signalling.