structural:scholar_edition
Reproduction

In Hungary, mating takes place in the last week of April, whilst in the north it happens later (in the second week of May). Mating has also been observed in June and even early October, but it is not known if this autumn mating results in any offspring. Females often breed once every two years, or even once every three years if the seasons are short and the climate is not conducive.

Males find females by following their scent trails, sometimes tracking them for hundreds of metres a day. If a female is found and then flees, the male follows. Courtship involves side-by-side parallel 'flowing' behaviour, tongue flicking along the back and excited lashing of the tail. Pairs stay together for one or two days after mating. Males chase away their rivals and engage in combat. Often, this also starts with the aforementioned flowing behaviour before culminating in the dramatic 'adder dance'. In this act, the males confront each other, raise up the front part of the body vertically, make swaying movements and attempt to push each other to the ground. This is repeated until one of the two becomes exhausted and crawls off to find another mate. Appleby (1971) notes that he has never seen an intruder win one of these contests, as if the frustrated defender is so aroused by courtship that he refuses to lose his chance to mate. There is no record of any biting taking place during these bouts.

Females usually give birth in August or September, but sometimes as early as July, or as late as early October. Litters range in size from 3 to 20. The young are usually born encased in a transparent sac from which they must free themselves. Sometimes, they succeed in freeing themselves from this membrane while still inside the female.

Neonates measure 14 to 23 cm (5.5 to 9.1 in) in total length (including tail), with an average total length of 17 cm (6.7 in). They are born with a fully functional venom apparatus and a reserve supply of yolk within their bodies. They shed their skins for the first time within a day or two. Females do not appear to take much interest in their offspring, but the young have been observed to remain near their mothers for several days after birth.