Many animals cannot remain active in cold northern countries when night temperatures are close to freezing and daytime temperatures are still low. They cope with this by seeking a sheltered place in which to spend the winter in a state of torpor. Their body temperatures fall, they breathe slowly and they stop moving. This is not sleep but a different state which allows their bodies to ‘shut down’ and not waste energy until it becomes warm enough to resume active life.
Reptiles and amphibians sense hibernation is close when night temperatures start to fall, days get shorter and the sun starts to lose its warmth. They may stay in hibernation for up to six months in an English winter, less if it the autumn and spring are mild. Some hibernate together in a group: adult Slow-worms may gather at the bottom of an old mammal burrow with the young at the top, so that if there are heavy frosts which reach their shelter, the young may die of cold. However, the adults should survive to breed again and replace them. Workmen have dug up football-sized groups under old walls. Many of the Slow-worms will have returned to the same hibernation spot year after year.
Adders also hibernate communally – often with Common Lizards, which seem to sense that they are safe from being eaten because the Adder cannot feed before hibernation. Food would rot in its stomach if it did so and it would not survive the winter.