How Far North Do Adders Live? The Snake of the Arctic Circle

How far north do adders go?

The common European adder (Vipera berus) has the largest range of any viper, and its northern edge runs through Fennoscandia, the region of Norway, Sweden, Finland and northwestern Russia. Here it crosses the Arctic Circle, at about 66.5°N, and the most northerly populations live in northern Norway, Swedish Lapland and Finnish Lapland.

Farther east, in Siberia, the adder’s range stays mostly south of the Arctic Circle, following the forest zone across Russia to the Pacific coast and Sakhalin Island. The far north of Europe is warmed by the Gulf Stream and the North Atlantic Drift, which helps explain why snakes can live farther north there than in Asia or North America.

No snakes live beyond the forest edge in the high Arctic. Iceland, Svalbard and Greenland have none at all. Ireland, much farther south, has none either, for historical rather than climatic reasons; see Where do adders live?

Which other reptiles live so far north?

Among snakes, no species comes close. The grass snake, Europe’s other widespread snake, reaches central Scandinavia but stops well south of the Arctic Circle. In North America, the most northerly snake is the common garter snake, which reaches roughly 60°N in Canada’s Northwest Territories.

The adder shares its northern home with one even hardier reptile: the viviparous or common lizard (Zootoca vivipara). This small lizard reaches about 70°N in northern Norway and is the most northerly reptile in the world. Like the adder, it gives birth to live young, which is no coincidence.

How do adders survive so far north?

Long hibernation

Snakes cannot make their own body heat, so they cannot be active when it is freezing. Adders solve the problem by hibernating underground below the frost line, in old burrows, root holes, rock crevices and banks. In northern Sweden, hibernation lasts about 8–9 months, from about September to May or June. Many adders share traditional dens and return to them year after year.

Even so, winter takes a heavy toll. Studies estimate that about 15% of adults and 30–40% of young adders die during hibernation. See Do snakes hibernate? for how snakes in general survive the cold.

Live birth

Most snakes lay eggs, but eggs buried in cold northern soil would develop too slowly or not at all. The adder keeps its eggs inside its body until they hatch, and the female gives birth to 3–20 live young, usually in late summer. A pregnant female spends much of her time basking, so the embryos develop at the warm temperature of her body rather than the cold temperature of the ground. Live birth has evolved many times in reptiles of cold climates and high mountains.

Dark coloring and basking

Adders bask in sunny, sheltered spots and flatten their bodies to catch as much sunlight as possible. Dark colors absorb heat faster, and fully black (melanistic) adders are more common in some cold, northern and mountain populations. Swedish studies have found that black adders warm up faster than patterned ones, although they are also easier for predators to spot.

On sunny days, adders can warm up even when the air is cold. In early spring they sometimes emerge to bask on snow-free patches and have been seen crossing snow.

A slow, patient life

In the far north, the active season is only about three to four months long. Females need to eat well to build up fat for their young, so many breed only every second or third year, and sometimes less often. Adders also grow slowly and live long, often 10–15 years in the wild, which gives them enough breeding seasons to replace themselves.

Northern adders are not small, either. Some of the largest adders on record, over 90 cm (35 in) long, come from Scandinavia.

Where can you see adders in the north?

Adders are common and familiar in much of Sweden, Norway and Finland, where they live in forests, bogs, rocky coasts and summer cottage areas. Sweden alone records around 1,300 adder bites a year, a sign of how often people and adders meet. In Finland the adder is the only venomous snake and is known as kyy.

If you walk in Nordic forests and coasts in summer, watch where you step and sit, keep dogs on a leash and never handle an adder. Bites are rarely fatal but always need emergency care: call 112 in Europe, 999 in the UK or 911 in the US. For more on the real risks, read Are adders dangerous?