Cold hardiness in the boreal adder, Vipera berus.

In three freezing experiments we examined the freeze tolerance in newborn adders, Vipera berus, from East-central Sweden. After a two to three hours exposure to freezing, ten out of eleven fast frozen snakes survived and recovered completely after being exposed to -3.1 degrees C on average. In the other two experiments with fast and slow freezing followed by long exposures lasting for 22-30 hours, none of the snakes survived average exposures at -4.8 degrees C or -3.8 degrees C, respectively. The glucose content of blood from frozen snakes was significantly higher than in unfrozen ones. The increase was small and its contribution to freeze tolerance doubtful. Compared to other freeze tolerant reptiles, the adder was categorised as virtually non freeze tolerant capable of surviving only a short exposure not colder than approximately -4 degrees C. Supercooling could play a role in winter survival but their precise choice of hibernation site is probably the most important.