How the most northern lizard, Zootoca vivipara, overwinters in Siberia

The common lizard Zootoca vivipara has the largest range of all the terrestrial reptiles which includes the subarctic regions of the Palaearctic. The species provides a unique model for studying the strategies of adaptation of a reptile to extreme low winter temperatures. The aim of our research was to determine whether this species survives the severe winters of Siberia, including Yakutia, due to its exceptional cold hardiness or due to wintering in abnormally warm places. The cold hardiness limit of lizards from the southeast of Western Siberia was lower than in conspecific European populations (-4 A degrees C) and was the record low for all adult reptiles. In dry substrate (water content 13-14 %), 21 % of lizards survived at temperatures from -3 to -10 A degrees C, but in wet substrate (70-80 %) none of them survived even at slightly below-zero temperatures. The survivors remained in a supercooled state until the temperature dropped to about -3 A degrees C, and then they froze and could remain frozen for over 2 months. In most biotopes examined in the southeast of Western Siberia, soil temperatures at the depth of the lizard hibernacula (5-13 cm) were higher than -10 A degrees C. Despite very cold air, similar winter soil temperatures were recorded in the warmest lizard habitats in Yakutia, due to the soil-heating effect of unfrozen groundwater in talik zones. Thus, extensive distribution of the common lizard in Yakutia is determined not only by its exceptional cold hardiness but also by specific hydrogeological conditions maintaining winter soil temperatures above its tolerance limit.