Cold Resistance and Northern Range Limit of the Grass Snake (Natrix natrix, Squamata, Reptilia)

The cold resistance of the Grass snake (Natrix natrix), the northernmost oviparous snake species in Eurasia, was instrumentally studied. Its records at the northern limit of the range were compared with soil temperatures in potential overwintering horizons. The Grass snake was shown to be a cold-intolerant species. At temperatures below 3°C, it loses mobility, but, contrary to existing beliefs about its intolerance to small positive temperatures, it can survive for more than 4 months at a range of 0–1°C. Natrix natrix is capable of surviving in a supercooled state at –1°C for 5 days, but with further cooling, it freezes and dies. In terms of cold resistance, the Grass snake is close to the North American Garter snake (Thamnophis sirtalis, T. radix) and the North American Grass snake (Nerodia sipedon), but it is less resistant than the Common adder (Vipera berus) which can withstand supercooling to –2.2°C for up to 4 months. The position of the northern range limit of the Grass snake (from 64° N in Fennoscandia to 56° N in the Baikal region) is related to neither the geographical distribution of soil temperatures nor the depth of soil freezing as recorded at weather stations. The Grass snake is found in areas that, based on its cold resistance, are unsuitable for habitation. A probable explanation is traditional: its overwintering in particular habitats with positive temperatures in the soil (in river valleys, swamps and rocky outcrops, etc.). The northern limit of the Grass snake range seems to be determined not by winter soil temperatures, but more likely by summer factors, one of which may be the availability of places with the summer temperatures necessary for egg incubation.