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Geographic variation in diet composition of the grass snake (Natrix natrix) along the mainland and an island of italy: the effects of habitat type and interference with potential competitors

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The diet of grass snakes (Natrix natrix) on the mainland and an island of Italy was compared by pooling literature data and original data. A total of 535 prey items were recorded (444 prey items from specimens >40 cm SVL), but the number of items was very variable between sites. Body lengths (both sexes) varied between geographical areas, and females were larger than males in all study areas. Specimens from the island (central Sardinia) and from one mainland mountainous locality (Duchessa Mountains) were significantly smaller than those from all the other localities. Amphibians were the main prey for both sexes, but females ate more toads and fewer frogs or tadpoles than males; females also consumed more rodents than males. There was a strong effect of locality on diet composition i.e. newts/salamanders were found only in two montane areas; hylids were found only in the single island area; and rodents were commonly preyed upon only at a single mainland locality. Two lizard corpses (Podarcis muralis) were scavenged by grass snakes at a mainland locality. The presence of the piscivorous snake Natrix tessellata, a potential competitor for food, did not have any apparent effect on the food types eaten by grass snakes because grass snakes consumed fish when sympatric with N. tessellata, but not at other sites. The dietary variation exhibited by grass snakes suggests that, by shifting their diets to other prey, they might be able to persist in areas where their usual natural prey has declined drastically, but this remains to be demonstrated.
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Keywords: BODY SIZES; COLUBRIDAE; COMPETITION; FOOD HABITS

Document Type: Research Article

Publication date: October 1, 2005

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