Strict ontogenetic control rather than environmental plasticity drives colour morph stability in the common wall lizard

Understanding whether colour polymorphisms reflect fixed developmental programs or plastic phenotypic transitions is central to explaining how these systems are maintained through time. Here, we used a four-year capture-recapture dataset to test if adult common wall lizards ( Podarcis muralis ) exhibit plastic colour transitions or instead follow a fixed developmental program. Using standardized digital photography and Residual Randomization Permutation Procedures , we evaluated the hue and saturation variation across 167 adult individuals (216 transition events). Our results reveal an exceptional degree of categorical stability, with 97.7% of individuals retaining their discrete colour morph among captures; the rare transitions observed were exclusively unidirectional, involving initially white individuals acquiring pigmentation. Although individual identity accounts for a substantial portion of spectral variation, within-individual differences in hue and saturation are modest and do not allow reliable discrimination among individuals within morphs. However, measurable within-individual variation in colour properties is still present across captures. Phenotypic trajectory analysis demonstrated that as individuals grow, morphs significantly diverge in both colour intensity and direction. Notably, the yellow morph follows a unique, morph-specific maturation path, suggesting a canalized developmental program that enhances signal clarity with age. Unlike the socially induced and hormonally mediated colour transitions documented in U. stansburiana , the P. muralis system appears driven by a fixed ontogenetic commitment. These findings suggest that this polymorphism is maintained by a robust genetic architecture where adult maturation might serve to minimize cross-talk among socio-sexual signals. Highlighting the analysis of adult ontogenetic trajectories as a vital practical tool for deciphering the evolution of complex biological polymorphisms.