Is the blue-spotted phenotype more widespread in the eastern slow worm Anguis colchica (Nordmann, 1840) than the western slow worm Anguis fragilis Linnaeus, 1758 and does it correlate with the male body size? A case study from Central Europe

The blue-spotted phenotype in a slow worm can be considered as an alternative colour morph or a secondary sexual characteristic. This phenotype is known to entail an elevated predation risk; thus, its continuous presence in a population must be balanced by additional and positive fitness consequences. In this study, we show that blue-spotted males are characterised by a greater snout-vent length (SVL) than typical specimens. Importantly, the SVL of blue-spotted males reaches the magnitude of the average female size. This indicates that the presence of blue spots may involve a correlated positive effect on growth and body size. The greater body size of the blue-spotted males could enhance their survival and mating success, and thus facilitate the continued presence of a high fraction of this morph within the population. In addition, we found that the blue-spotted phenotype is more common in the eastern than the western slow worm, and the proposed fitness consequences of the blue-spotted phenotype might enhance its tendency to spread in the eastern Anguis lineage.