Snakes are limited in cold climates by incubation. An egg laid in northern Scandinavia would rarely accumulate enough warmth in a short summer to develop. The adder is viviparous: the female retains the developing young internally and can raise their temperature by basking, moving her whole reproductive investment to whatever warm patch of ground exists.
That solution has a cost. Gestation is demanding enough that northern females typically reproduce only every second or third year, and reproductive output across a lifetime is low. Adders also hibernate for long periods, often communally in frost-free crevices and old burrows, emerging early in spring to bask.
The species is the only venomous snake native to Britain. Bites are uncommon and rarely fatal, with very few deaths recorded in Britain over the past century, though envenomation can be serious and warrants medical attention. Adder populations have declined in parts of their range, and the causes are debated between habitat fragmentation, disturbance and management change.