The blue‐spotted morph of the slow worm,Anguis fragilis:Colour polymorphism and predation risks

Blue‐spotted individuals are found in many populations of the slow worm (Anguis fragilis) throughout Europe. The frequency of occurrence of this colour morph was studied in eight populations from a mountainous territory of northeastern Italy (Tarvisio Forest, Carnic Alps, 750–1600 m a.s.l.). Blue‐spotted individuals were very rare (0.8% of the total sample; n = 744). However, the population inhabiting the town of Tarvisio presented a significantly higher proportion of blue‐spotted individuals than did the other sampled populations. All the blue‐spotted individuals were adult males of large size (average SVL = 173.5 ± 6.8 mm), with heavy body (average body mass = 41 ± 6 g), while the sole blue‐spotted animal captured in Valais was subadult. The presence of blue‐spotted animals within a population was apparently not correlated with habitat type, altitude and latitude, but was positively correlated with increase of longitude from Greenwich (Spearman's r = 0.73, n = 8, P = 0.038). A predation experiment with model slow worms demonstrated that the blue‐spotted individuals suffered higher risks of predation than normal coloured ones. Taking into account the result of this experiment, and considering that (1) the frequency of occurrence of the blue‐spotted males within populations was inversely correlated with the proportion of individuals with broken tails (Spearman's r = ‐ 0.91, n = 8, P = 0.002), and that (2) higher frequencies of broken tails may mean higher risks of prédation for natural lizard populations, we conclude that blue‐spotted individuals should be rare in the areas where the density of potential predators is high. It is also suggested that the blue‐spotted colouration may be intrasexually selected for augmenting the individual success of male slow worms in sexual contexts.