Javier Abalos , Alicia Bartolomé, Guillem Pérez i de Lanuza, Fabien Aubret, Enrique Font

Many animals undergo irreversible ontogenetic colour changes (OCCs), yet these changes are often overlooked despite their potential ethological relevance. 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. Here, we describe OCCs in the ventral (throat and belly) and ventrolateral (outer ventral scales, OVS) coloration of juvenile common wall lizards (Podarcis muralis) 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 discriminable to conspecifics from the most frequent adult colours in the throat (i.e. orange, yellow, and UV-reduced white; UV^-white) and OVS (i.e. UV-blue). The prevalence of UV⁺white decreases with age, with the decline being less pronounced in female bellies. OCCs to UV-blue in the OVS are more apparent in males than in females and appear delayed relative to changes in the throat and belly. While throat colours in yearlings are indistinguishable to conspecifics from adult throat colours, yearling UV-blue patches remain chromatically distinct from those of adults. This delay may reflect variations in the mechanisms of colour production or distinct selective pressures acting on these patches. Overall, our results show that OCCs in P. muralis fulfil a key requirement for social signals by being perceptible to conspecifics. This supports the hypothesis that OCCs may play a role mediating interactions between juveniles and adults, as well as delaying the onset of colours involved in social communication.