European Adder

The European adder kills with venom, then follows the trail. After the bite, it follows the scent of the dying animal and swallows it head first. Adults eat small mammals such as mice, voles, and shrews, plus lizards, slow worms, frogs, newts, and birds, especially nestlings and eggs. Juveniles eat nestling mammals, small lizards, frogs, worms, and spiders, and switch to an adult-like diet at about 30 cm (12 in). In some places, alpine salamanders can make up almost half of a juvenile’s diet. Adders hunt by ambush or by actively searching for prey, and sources disagree on whether they hunt mostly at dusk or by day. Venom yield is low, 10 to 18 mg in snakes 48 to 62 cm (19 to 24 in) long, and works mainly by breaking down tissue and blood.

The European adder lives farther north than any other snake, reaching north of the Arctic Circle, and its range stretches east across Eurasia to Sakhalin Island and northern China. In the Swiss Alps it may reach about 3,000 m (9,800 ft). It relies on basking, even climbing banks and bushes to catch the sun. Hibernation usually begins in September or October, in dry, sheltered spots, and is long: about 150 days for males and 180 days for females in Great Britain, and 8 to 9 months in northern Sweden. Adders sometimes emerge on mild winter days to bask, even crossing snow. Winter is risky: about 15 percent of adults and 30 to 40 percent of juveniles die during hibernation. In mild climates, adders may stay active all year.

European adders give birth to live young. Females usually breed every two years, or every three when seasons are short or the climate is poor, and mating takes place in spring after emergence from hibernation. Gestation lasts about 3 to 4 months, so births usually come in August or September, sometimes July or early October. Litters hold 3 to 20 young. Each newborn measures 14 to 23 cm (5.5 to 9 in), averaging 17 cm (6.7 in), and arrives in a transparent sac with a working venom apparatus and yolk reserves. Newborns shed their skin within a day or two and may stay near their mother for several days. Young adders reach sexual maturity at about 3 to 4 years old.

Male European adders settle rivalries with a famous ritual, the dance of the adders. Two males raise the front part of their bodies, intertwine, and try to push each other to the ground. There is no record of biting during these bouts. Appleby reported in 1971 that he never saw an intruding male win. More than two males may join the same contest. Males follow scent trails that can extend hundreds of metres each day while searching for mates. The contests happen in spring, when males emerge from hibernation and the mating season begins in late April or May across much of the range.

Courtship in European adders depends on chemical signals. Males follow scent trails left by females, and during courtship they use pheromones, tongue-flicking, tail vibrations, and body quivering to signal interest. Adders have no outer ear and detect sound through vibrations carried through the skin, jaw, and bones. Mating takes place soon after adders emerge from hibernation: in the last week of April in Hungary and in the second week of May farther north, though mating has also been seen in June and early October. After mating, a pair may stay together for a day or two, and a male may guard his mate for several hours.

European adders are timid and bite mainly when cornered, stepped on, or picked up. They usually flee at the first sign of danger. When threatened, an adder pulls its front body into an S-shape before striking, and pregnant females sometimes hiss loudly. Bites are rarely fatal: Britain recorded 14 known deaths from 1876 to 2005, the last in 1975, while Sweden sees about 1,300 bites a year. In cold regions, adders hibernate communally in burrows, with up to 100 individuals in one den. Their predators include foxes, badgers, large birds of prey, owls, and larger snakes. The species is declining in many places because of habitat loss, fragmentation, and persecution, and in parts of Britain it survives only in small, isolated patches.