Morphological variation and sexual dimorphism of the cephalic scales in Lacerta bilineata

The Western green lizard (Lacerta bilineata) is a lacertid distributed throughout Mediterranean and Central Europe. Little is known about the morphological variability and sexual shape differences in this species. In this paper, the variation of the cephalic scales in L. bilineata is analysed by means of a geometric morphometric approach. A main structural pattern is characterized by negative allometry of the frontal and interparietal areas, and positive allometry of the parietal and frontoparietal scales. Sexual differences are described both in size and shape. In males, the scales are generally larger, with relative shortening of the frontal area, frontoparietal enlargement, occipital lengthening and bulging of the parietal scales, which compresses and narrows the interparietal and occipital areas midsagittally. This pattern is based on a shared allometric trajectory, with males displaying a peramorphic morphotype. However, males show some shape differences in the occipital area that are not size-related, and cannot be interpreted in terms of general head enlargement. This structural trajectory can be related to the development of the skull, but the role of soft tissues (temporal, nuchal and masticatory muscles) must also be considered. The development of the jaw and nuchal muscles involved in intra- and intersexual behaviours could have played a pivotal role in the evolution of this pattern.