Wildlife Guide · United Kingdom & Scandinavia and northern Europe

Adder

Vipera berus

Britain's only venomous snake — a shy, cold-tolerant viper marked by an unmistakable zigzag.

Least Concern (IUCN, 2021); declining and legally protected in the UK

A coiled adder (Vipera berus) basking on heathland, its grey-brown body marked with the species' distinctive dark zigzag stripe running down the back and a V-shaped marking on the head.

Image: Source: Global Animal Guide (WARN) — verify license before production

In brief

An adder (Vipera berus) is a small venomous snake of Europe and Asia, identified by a dark zigzag stripe along its back. It is Britain's only venomous snake, hunts small mammals and lizards, and is shy, biting humans only in defence.

By the WARN Research & Conservation TeamChecked against IUCN Red List & CITES sourcesLast updated June 2026

The adder (Vipera berus), also called the European viper, is a small venomous snake famous for the bold zigzag stripe running down its back. It is the only venomous snake native to Britain and the only snake found inside the Arctic Circle, ranging from western Europe across Russia to the Pacific coast of Asia. Adders favour heathland, moorland, woodland edges and coastal dunes, where they bask to warm up and hunt small mammals, lizards and amphibians. Though its venom can give a painful, occasionally serious bite, the adder is shy and bites only defensively. Globally it is classed as Least Concern, but in the UK it is declining and legally protected.

~90 cm

Maximum length (most adders are shorter, around 60-70 cm)

3-20

Live young per litter, born in late summer

Britain's only venomous snake species

2021

Year of its most recent IUCN Least Concern assessment

Figures sourced from Wikipedia — Vipera berus (adder), IUCN Red List — Vipera berus, Encyclopaedia Britannica — adder and other references on this page.

Adder: Quick Facts

Vital statistics and key facts about the Adder
Common name	Adder (European viper)
Scientific name	Vipera berus
Typical length	60-70 cm (up to ~90 cm)
Venomous	Yes — mildly, rarely dangerous to healthy adults
Diet	Small mammals, lizards, amphibians, occasionally birds
Reproduction	Ovoviviparous; 3-20 live young in late summer
Habitat	Heath, moor, bog, woodland edge, dunes
Range	Europe and Asia; the world's most northerly snake
UK status	Declining; protected under the Wildlife and Countryside Act 1981
IUCN status	Least Concern (2021)

Scientific Classification

Scientific classification of the Adder
Kingdom	Animalia
Phylum	Chordata
Class	Reptilia
Order	Squamata
Family	Viperidae
Genus	Vipera
Species	Vipera berus

Conservation Status

IUCN Status: Globally Least Concern on the IUCN Red List, reflecting the adder's very large range across Europe and Asia. Despite this, the species is declining in parts of its range, and in the United Kingdom it is a conservation concern: populations are fragmented and shrinking, driven by habitat loss, fragmentation and disturbance. It is legally protected in the UK under the Wildlife and Countryside Act 1981, which prohibits killing, injuring or selling adders.
Population: No reliable global total; the population is large but unquantified. UK populations are localised and many are small and isolated.
Trend: Stable to decreasing globally; declining in the UK and several European countries.
Assessed: 2021
CITES: Not listed on CITES Appendices.

Local declines are largely driven by the loss and fragmentation of heath, moor and other open habitats, plus human disturbance — the same habitat pressures affecting many other reptiles.

Did you know?

The adder is the only snake on Earth found inside the Arctic Circle — its tolerance of cold, helped by giving birth to live young rather than laying eggs, lets it survive farther north than any other snake species.

Key Facts: Adder

The adder is Britain's only venomous snake and the only snake whose range crosses into the Arctic Circle.
Its trademark dark zigzag stripe down the back makes it one of the easiest European snakes to identify.
Adders give birth to live young rather than laying eggs, an adaptation to cool northern climates.
Bites are rarely dangerous to healthy adults but are painful and always need prompt medical attention.
Globally the adder is Least Concern, but UK populations are declining and are protected by law.
Adders are shy and will retreat if given the chance; almost all bites happen when a snake is trodden on or handled.

What does an adder look like and how do you identify it?

The adder is a stocky, modest-sized snake, usually 60-70 cm long, with the largest individuals — most often females in the north of the range — exceeding 90 cm. Its most reliable feature is the continuous dark zigzag stripe running the length of the back, set against a paler ground colour. Males tend to be grey or whitish with a black zigzag, while females are typically brown or reddish with a darker brown pattern. The head carries a distinctive V- or X-shaped marking, and the eyes have vertical, cat-like pupils with a red or coppery iris. Some adders are entirely black (melanistic), which can make the zigzag hard to see — these are more common in cooler, upland areas where dark skin helps absorb heat. In Britain the adder can be confused with the harmless grass snake and the legless slow worm, but neither has the bold zigzag, and the grass snake instead shows a yellow-and-black collar behind the head. The adder's short, slightly upturned snout and relatively chunky build also help separate it from Britain's slimmer non-venomous reptiles.

Where do adders live and what do they eat?

Vipera berus has one of the largest ranges of any terrestrial snake, stretching from Britain and western Europe across Scandinavia, Russia and northern Asia to the Pacific. Remarkably, it lives farther north than any other snake and is the only species found inside the Arctic Circle, thanks to its tolerance of cold. Adders favour habitats with a mix of open basking spots and dense cover: heathland, moorland, bogs, woodland rides, hedgerows, rough grassland and coastal dunes. They are cold-blooded and spend much of the day regulating their temperature, basking in the morning sun before becoming active hunters. The diet is mostly small mammals such as voles, mice and shrews, along with lizards (including common lizards and slow worms), amphibians like frogs and newts, and occasionally nestling birds. Prey is killed with a quick venomous bite, then released and tracked by scent until the venom takes effect. In the cooler parts of their range adders hibernate through winter, often communally in shared underground refuges called hibernacula, emerging in early spring to bask and breed.

Are adder bites dangerous to humans?

Adder venom is medically significant but, compared with many of the world's vipers, relatively mild. The snake is not aggressive: it relies on camouflage and will almost always slip away if given the chance, biting only when trodden on, cornered or handled. A bite is painful and typically causes swelling, bruising and sometimes nausea, dizziness or a drop in blood pressure; serious systemic reactions are uncommon but possible, especially in children, older people or those with allergies. Fatalities are very rare — in Britain only a small number of deaths have been recorded over more than a century. Even so, every adder bite should be treated as a medical emergency: keep the affected limb still and below heart level, remove rings or tight clothing, and seek hospital care promptly, as antivenom may be given in severe cases. Dogs are bitten more often than people, usually on the face or paws while investigating a basking snake, and also need veterinary attention. The sensible response to meeting an adder is simply to step back and leave it alone — it poses no threat to anyone who keeps their distance.

How do adders breed and survive cold climates?

The adder's reproduction is closely shaped by its cold northern home. Rather than laying eggs, females are ovoviviparous: the eggs develop inside the body and the young are born live, typically 3 to 20 at a time in late summer or early autumn. This strategy lets the mother carry her developing young to warmer basking sites, giving the embryos enough heat to develop in climates where eggs left in the ground might never hatch. Breeding is demanding, so females in cooler regions often reproduce only once every two or three years, recovering condition in between. In spring, males emerge from hibernation first and perform a 'dance of the adders', a ritualised wrestling contest in which rival males rear up and try to push each other down to win access to females. Adders are long-lived for a small snake, and hibernation is a dangerous period — a significant proportion of juveniles and some adults do not survive the winter. These slow life-history traits, combined with the loss and fragmentation of heath and moorland, help explain why UK adder populations are sensitive to disturbance and slow to recover.

Adder vs grass snake: telling Britain's snakes apart

Feature	Adder (Vipera berus)	Grass snake (Natrix helvetica)
Venomous	Yes (mild)	No
Back pattern	Bold dark zigzag stripe	No zigzag; plain with dark bars on flanks
Head marking	V or X on head; yellow/orange collar absent	Bright yellow-and-black collar behind head
Pupils	Vertical, cat-like	Round
Typical length	60-70 cm (up to ~90 cm)	Often 90-120 cm, longer than the adder
Behaviour if threatened	Retreats; bites only in defence	Often flees to water; may play dead or release foul musk

What WARN Does

The World Animal Rescue Network does not run field projects specifically for the adder — its native range lies well outside the countries where WARN's partners work partner countries — so this guide is part of WARN's free educational work to help people understand and respect wildlife. The pressures that threaten adders, above all the loss and fragmentation of habitat, are the same forces that endanger the animals WARN does protect, which is why sharing accurate, calm information about misunderstood species like venomous snakes matters.

If clear, fear-free wildlife guides like this one are useful to you, a small gift helps WARN keep producing them and supports the animals in our care.

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Adder: Frequently Asked Questions

Is the adder the only venomous snake in the UK?

Yes. The adder (Vipera berus) is the only venomous snake native to the United Kingdom. Britain's two other native snakes — the grass snake and the smooth snake — are non-venomous, and the legless slow worm is a lizard, not a snake. Adders are shy and bites are rare, but the snake is legally protected and should never be handled or harmed.

How dangerous is an adder bite?

Adder bites are painful but rarely life-threatening to healthy adults. A bite usually causes swelling, bruising and sometimes nausea or dizziness, and serious reactions are uncommon. Deaths are very rare, with only a handful recorded in Britain in over a century. Even so, every bite needs prompt medical care: keep still, remove tight items and go to hospital, as antivenom may be needed in severe cases.

What does an adder look like?

An adder is a small, stocky snake, usually 60-70 cm long, with a continuous dark zigzag stripe running down its back and a V- or X-shaped mark on its head. Males are typically grey with a black pattern; females are brown or reddish. The vertical, cat-like pupils and reddish eyes are distinctive. Some adders are entirely black (melanistic), which can hide the zigzag.

Where do adders live?

Adders have a vast range across Europe and Asia, from Britain through Scandinavia and Russia to the Pacific coast — farther north than any other snake, even reaching inside the Arctic Circle. They favour heathland, moorland, bogs, woodland edges, dunes and rough grassland, where they can both bask in the sun and find dense cover to hide in. In the UK they are now patchily distributed and declining in many areas.

Do adders lay eggs?

No. Unlike many snakes, adders do not lay eggs. They are ovoviviparous: the eggs develop inside the female and she gives birth to live young, typically 3 to 20 at a time in late summer or early autumn. This adaptation lets the mother keep the developing embryos warm by basking, which is essential for breeding successfully in the cold northern climates the adder inhabits.

Are adders endangered?

Across their huge global range, adders are classed as Least Concern by the IUCN, most recently assessed in 2021. However, the picture is very different locally: in the UK adder populations are declining and fragmented, largely due to habitat loss and disturbance, and the species is protected by the Wildlife and Countryside Act 1981, which makes it illegal to kill, injure or sell them.

What does an adder eat?

Adders hunt small mammals such as voles, mice and shrews, along with lizards, amphibians like frogs and newts, and occasionally nestling birds. Prey is killed with a quick venomous bite, then released and tracked by scent until the venom takes effect. They bask on heathland, moorland and woodland edges before becoming active hunters.

How long do adders live?

Adders are long-lived for a small snake, though this guide does not cite a specific maximum age. Females in cooler regions often reproduce only once every two or three years, and a significant proportion of juveniles fail to survive their first winter hibernation — traits that make UK populations slow to recover from habitat loss.

How big is an adder?

Most adders are 60–70 cm long, with the largest individuals — usually females in the north of the range — exceeding 90 cm. The species is Britain's only venomous snake, identified by the continuous dark zigzag stripe running down its back and a V- or X-shaped mark on the head.

Sources & References

Population figures, conservation status and biological facts on this page are drawn from authoritative scientific and intergovernmental sources.

Terms used on this page

Plain-English definitions, from our wildlife and conservation glossary.

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