THE BLUE-SPOTTED MORPH OF THE SLOW WORM, ANGUIS-FRAGILIS - COLOR POLYMORPHISM AND PREDATION RISKS

Blue-spotted individuals are found in many populations of the slow wor m (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, 750-160 0 m a.s.l.). Blue-spotted individuals were very rare (0.8% of the tota l sample; n = 744). However, the population inhabiting the town of Tar visio presented a significantly higher proportion of blue-spotted indi viduals than did the other sampled populations. All the blue-spotted i ndividuals 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 correlat ed with increase of longitude from Greenwich (Spearman's r = 0.73, n = 8, P = 0.038). A predation experiment with model slow worms demonstra ted that the blue-spotted individuals suffered higher risks of predati on than normal coloured ones. Taking into account the result of this e xperiment, and considering that (1) the frequency of occurrence of the blue-sported males within populations was inversely correlated with t he proportion of individuals with broken rails (Spearman's r = - 0.91, n = 8, P = 0.002), and that (2) higher frequencies of broken rails ma y mean higher risks of predation for natural lizard populations, we co nclude 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 augmen ting the individual success of male slow worms in sexual contexts.