Colour polymorphisms, the coexistence of two or more colour morphs of a species within a population, have long fascinated evolutionary biologists interested in the mechanisms generating and maintaining phenotypic variation in nature. The functional significance of colour polymorphisms (i.e. their adaptive value) is often linked to the selective mechanisms responsible for their maintenance over time. In lizards, the hypothesis that colour morphs may reflect alternative reproductive strategies involving differential sociosexual behaviour and/or alternative reproductive strategies has come to dominate the field. Wall lizards (family Lacertidae, genus Podarcis), with several geographically distant species that exhibit two or more alternative ventral colour morphs, have often been identified as a group in which alternative reproductive strategies and frequency-dependent selection likely underpin colour polymorphisms. However, several key aspects regarding the link between behaviour, fitness, and lacertid colour polymorphism remain overlooked or inadequately studied. In this thesis we tried to fill these gaps by experimentally addressing some of the most commonly assumed ideas about the functional significance of colour polymorphism in the European common wall lizard (Podarcis muralis, Laurenti 1768). In some populations of this species (e.g. eastern Pyrenees), adult lizards of both sexes may show up to three “pure” alternative colour morph (orange, white, yellow), and two mixed morphs consisting of a mosaic of differently-coloured scales (orange-white, yellow-orange). Although colour morphs are frequently thought to function as social signals mediating intraspecific interactions, morph categorization has been never assessed from the viewpoint of the intended receivers (i.e. conspecifics). Here, using a discrimination experiment and visual modelling, we found that lizards likely perceive the alternative colour morphs as chromatically distinct and show evidence of discriminating among them based on hue, rather than luminance. To evaluate the role of male coloration (including alternative colour morphs) in intra-sexual competition), we conducted lab-staged dyadic contests among size-matched males. We found lower fighting ability in lizards showing a smaller fraction of their outer ventral scales covered with dark melanin-based spots, and in lizards with orange ventral colour (which could result from the existence of a subordinate non-territorial strategy in this morph). However, our results in later studies (with a free-ranging population and ten experimental mesocosm enclosures), strongly refute the hypothesis that alternative socio-sexual behaviour or space use strategies characterize colour morphs when lizards interact under more natural conditions. In the free-ranging populations, colour morphs did not differ in inter-annual site-fidelity, home-range size, or male-female overlap. In the mesocosm enclosures, spatial dominance was the prime determinant of male fitness across colour morphs. In a later experiment, we conducted controlled matings among pure colour morphs and found no overall effect of female morph on clutch size or juvenile mass, and no effect of morph combination on offspring viability or prospective fitness. These results refute the existence of alternative reproductive strategies in female morphs and are also in disagreement with predictions from both correlational selection and heterosis. Lastly, by keeping the juveniles of known crosses in outdoor enclosures for a year, we studied the inheritance and ontogeny of P. muralis colour polymorphism. Specifically, our results confirmed that orange and yellow colour expression depends on two recessive alleles located at two separate autosomal loci and revealed that the whitish coloration exhibited by newborn lizards is likely perceived by conspecifics as a chromatically distinct colour different from any of the morph colours expressed by adult lizards. Overall, in this thesis we have hopefully presented compelling arguments to revise our perspective on the functional significance of lacertid colour polymorphisms, suggested promising lines of research for future work, and generally contributed to our understanding of the processes maintaining intra-specific variation in natural populations at large.Colour polymorphisms, the coexistence of two or more colour morphs of a species within a population, have long fascinated evolutionary biologists interested in the mechanisms generating and maintaining phenotypic variation in nature. The functional significance of colour polymorphisms (i.e. their adaptive value) is often linked to the selective mechanisms responsible for their maintenance over time. In lizards, the hypothesis that colour morphs may reflect alternative reproductive strategies involving differential sociosexual behaviour and/or alternative reproductive strategies has come to dominate the field. Wall lizards (family Lacertidae, genus Podarcis), with several geographically distant species that exhibit two or more alternative ventral colour morphs, have often been identified as a group in which alternative reproductive strategies and frequency-dependent selection likely underpin colour polymorphisms. However, several key aspects regarding the link between behaviour, fitness, and lacertid colour polymorphism remain overlooked or inadequately studied. In this thesis we tried to fill these gaps by experimentally addressing some of the most commonly assumed ideas about the functional significance of colour polymorphism in the European common wall lizard (Podarcis muralis, Laurenti 1768). In some populations of this species (e.g. eastern Pyrenees), adult lizards of both sexes may show up to three “pure” alternative colour morph (orange, white, yellow), and two mixed morphs consisting of a mosaic of differently-coloured scales (orange-white, yellow-orange). Although colour morphs are frequently thought to function as social signals mediating intraspecific interactions, morph categorization has been never assessed from the viewpoint of the intended receivers (i.e. conspecifics). Here, using a discrimination experiment and visual modelling, we found that lizards likely perceive the alternative colour morphs as chromatically distinct and show evidence of discriminating among them based on hue, rather than luminance. To evaluate the role of male coloration (including alternative colour morphs) in intra-sexual competition), we conducted lab-staged dyadic contests among size-matched males. We found lower fighting ability in lizards showing a smaller fraction of their outer ventral scales covered with dark melanin-based spots, and in lizards with orange ventral colour (which could result from the existence of a subordinate non-territorial strategy in this morph). However, our results in later studies (with a free-ranging population and ten experimental mesocosm enclosures), strongly refute the hypothesis that alternative socio-sexual behaviour or space use strategies characterize colour morphs when lizards interact under more natural conditions. In the free-ranging populations, colour morphs did not differ in inter-annual site-fidelity, home-range size, or male-female overlap. In the mesocosm enclosures, spatial dominance was the prime determinant of male fitness across colour morphs. In a later experiment, we conducted controlled matings among pure colour morphs and found no overall effect of female morph on clutch size or juvenile mass, and no effect of morph combination on offspring viability or prospective fitness. These results refute the existence of alternative reproductive strategies in female morphs and are also in disagreement with predictions from both correlational selection and heterosis. Lastly, by keeping the juveniles of known crosses in outdoor enclosures for a year, we studied the inheritance and ontogeny of P. muralis colour polymorphism. Specifically, our results confirmed that orange and yellow colour expression depends on two recessive alleles located at two separate autosomal loci and revealed that the whitish coloration exhibited by newborn lizards is likely perceived by conspecifics as a chromatically distinct colour different from any of the morph colours expressed by adult lizards. Overall, in this thesis we have hopefully presented compelling arguments to revise our perspective on the functional significance of lacertid colour polymorphisms, suggested promising lines of research for future work, and generally contributed to our understanding of the processes maintaining intra-specific variation in natural populations at large.