FOOD-HABITS, GROWTH-RATES, AND REPRODUCTIVE-BIOLOGY OF GRASS-SNAKES, NATRIX-NATRIX (COLUBRIDAE) IN THE ITALIAN ALPS

A five-year mark-recapture study at Sella Nevea, a montane (1100 m a.s .l.) site in the Carnic Alps, provided information on diets, growth ra tes, and reproductive output in an Italian population of the wide-rang ing grass snake, Natrix natrix. Our snakes resembled a previously stud ied population in lowland Sweden in terms of body size at sexual matur ation in females (70 cm) and mean adult female body length (82 cm). Ho wever, growth rates were lower in our population, and sexual maturatio n was delayed (6-8 years, versus 4-5 years in Sweden), perhaps because of the cool climate and relatively brief growing period each year. Fe males produced a single clutch of 4-24 eggs in late July each year. La rger females produced larger clutches, but clutch size relative to mat ernal size was lower than in Swedish grass snakes. Hatchling sizes and Relative Clutch Masses (RCMs) did not shift with increasing female si ze. RCMs may provide a useful index of 'costs of reproduction' in this population, because females with high RCMs were very emaciated after oviposition, and hence may experience a greater risk of mortality, as well as a high energy expenditure. Prolonged incubation gave rise to l onger, thinner hatchlings, but the low environmental temperatures at t he study site may favour early hatching (and hence, result in a shorte r fatter hatchling emerging from the egg, with more of its energy stor es unused). Compared to sympatric viviparous snakes (Coronella austria ca and Vipera berus), the oviparous grass snakes can achieve a much hi gher reproductive output owing to a larger clutch size and more freque nt reproduction (annual, rather than biennial or triennial). The abund ant prey resource used by grass snakes (amphibians) may also enable th em to recoup energy more rapidly after reproduction; dietary compositi on shifts ontogenetically in both sexes, with the largest prey (mice a nd adult toads) taken primarily by large female snakes.