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Insular populations offer excellent opportunities to study the factors that influence phenotypes. We observed island dwarfism in a widespread snake, the nose-horned viper (Vipera ammodytes). Island vipers were ~20% smaller than mainland individuals. They also produced fewer and smaller offspring. In snakes, food availability has a positive influence on body size, fecundity and offspring size. Consequently, low energy intake is a plausible explanation for insular dwarfism. The diet of island vipers was principally represented by lizards and centipedes, whereas the most profitable prey items (e.g. rodents) were regularly found in the stomach of mainland vipers. Furthermore, the proportion of individuals captured with a full stomach and good body condition were lower on the island compared with the mainland. Thus, island vipers were likely to be experiencing permanent energy restriction, with cascading effects on adult body size and reproductive output. Large prey promotes high relative jaw length in snakes. Island vipers displayed smaller relative jaw length compared with mainland populations, suggesting that plasticity played a role in insular dwarfism. But the difference in relative tail length between island and mainland populations, a trait not subjected to food-induced plasticity, indicates local adaptation. Both plasticity and adaptation might influence the phenotype of island vipers.

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records (79.8%), followed by V. berus with 145 records (18.8%), and V. ursinii with 11 records (1.4%). Results of the present study showed that in our country there are three areas crucially important for the diversity of vipers, with all three species present: Koritnik Mt. and Žljeb Mt. in Metohija, as well as Mokra Gora Mt. in south-western Serbia. We also found 25 10×10 km UTM squares with sympatry of two species of vipers. The designation of areas of special importance for vipers (e.g. Mokra Gora Mt., Dukat Mt., Besna kobila Mt., Kamena Gora Mt.) should be one of the priority actions for improvement of their conservation statuses in our country.