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 rates, and reproductive output in an Italian population of the wide-ranging grass snake, Natrix natrix. Our snakes resembled a previously-studied population in lowland Sweden in terms of body size at sexual maturation in females (70 cm) and mean adult female body length (82 cm). However, growth rates were lower in our population, and sexual maturation was delayed (6-8 years, versus 4-5 years in Sweden), perhaps because of the cool climate and relatively brief growing period each year. Females produced a single clutch of 4-24 eggs in late July each year. Larger females produced larger clutches, but clutch size relative to maternal size was lower than in Swedish grass snakes. Hatchling sizes and Relative Clutch Masses (RCMs) did not shift with increasing female size. 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 longer, thinner hatchlings, but the low environmental temperatures at the study site may favour early hatching (and hence, result in a shorter fatter hatchling emerging from the egg, with more of its energy stores unused.) Compared to sympatric viviparous snakes (Coronella austriaca and Vipera berus), the oviparous grass snakes can achieve a much higher reproductive output owing to a larger clutch size and more frequent reproduction (annual, rather than biennial or triennial). The abundant prey resource used by grass snakes (amphibians) may also enable them to recoup energy more rapidly after reproduction; dietary composition shifts ontogenetically in both sexes, with the largest prey (mice and adult toads) taken primarily by large female snakes.