Surrey Amphibian and Reptile Group (SARG) 

Adder

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Nomenclature

Common name:  adder 
Taxonomy:  Vipera berus berus (Linnaeus, 1758)
Other Names:  Northern viper
Common viper
Gad
Gwiber (Welsh)

Introduction

The shy Adder is Britain's only venomous reptile.

The Adder is a native animal of the UK, and the only venomous reptile. It is a stocky snake of less than a metre long and is very shy of human presence.

Adders can be found at woodland edges, roadside verges, chalk grassland or dry heathland.

The Adder is present in almost every county of Great Britain, but very rare in London and the northern home counties, the Adder is believed to be in decline.

As the UK's only venomous reptile, there is a weath of British folklore associated with this elusive creature. The Druids believed that one of the strongest mystical charms was the Adder-stone or glain neidyr. This was a small glass-like stone with a hole, which was believed to be made by the snakes on Midsummer's eve. The charm is supposedly a cure for a wide range of ailments and could even cure the bite of an Adder.


Identification

Adder identification features

The most distinguishing feature of the Adder is the dark zig-zag pattern running along the animal’s back. No other native animal has this marking.

The adder has a red eye, and is the only native reptile with a vertically split pupil. Male Adders tend to be a cream or light brown colour with a black ventral zig-zag marking. Females are usually brown with a dark brown zig-zag.

The adder can demonstrate a tremendous diversity in colouration. Black adders are not unusual, and base colouration can vary from creams to browns or oranges.

The zig-zag markings of this female adder are unmistakable.
The zig-zag markings of this female adder are unmistakable.
The Adder has an entirely red iris with a vertically split pupil. This is a unique feature for native British reptiles.
The Adder has an entirely red iris with a vertically split pupil. This is a unique feature for native British reptiles.

Morphology

Comparison between usual morph and melanistic adder © Malene Thyssen

Adders can be seen in a bewildering variety of colours. Natural colours vary from the frequent melanistic (black) adders, through browns, brick reds, silvery and even albino specimens (very rare). Black adders are surprisingly frequent, and tend to be prevalent in certain populations yet completely absent in others. The colouring of an adder appears not to provide any significant evolutionary advantage or disadvantage.

A black adder may warm up faster, but it will also cool down faster. One disadvantage for a black adder is that the characteristic zig-zag marking may not be visible, and this has been shown to be a deterrent for some predators (aposematism). Melanistic (black) adders occur in about on fifth of adder populations and are not uncommon, so evolutionary pressures must only slightly favour the usual morphology.

Ginger morphs are usually associated with juvenile or sub-adult female adders although the colouration can be seen in some adults.
Ginger morphs are usually associated with juvenile or sub-adult female adders although the colouration can be seen in some adults.
The bright silver of a freshly sloughed male adder.
The bright silver of a freshly sloughed male adder.
Blue or grey tinged morphs are often associated with snakes that have just shed their skin.
Blue or grey tinged morphs are often associated with snakes that have just shed their skin.

Sexual Dimorphism

The male adder has black markings, sharply defined lateral spots and black flecks on the upper lip scales.

Accurate differentiation between male and female adders can be difficult. Generally, if the dorsal (along the back) zig-zag is black, then the animal is a male, if the zig-zag is brown, then the animal is likely to be female. This rule of thumb is about 90% accurate, unfortunately, at certain times of the year, when the snake has not sloughed its skin for a while, accurate differentiation can be difficult.

The lateral spots along the flanks are clear and distinct for males, with poorly defined, or blurred edges in the female.

Tail shape and length can be a key gender identification feature, with the male's tail appearing longer, with a short parallel portion behind the cloaca (which houses the hemipenes). The female tail tapers immediately after the cloaca.

A strong diagnostic feature is the colour of the non-white scales along the upper labial scales (upper lip). These colour flecks are amlost always brown for females and black for males. Unfortunately, this method is unreliable for melanistic (black) specimens.

The female Adder has a brown zig-zag pattern; and brown flecks on the labial scales.
The female Adder has a brown zig-zag pattern; and brown flecks on the labial scales.
 The male Adder has a black zig-zag pattern.
The male Adder has a black zig-zag pattern.
A female adder moving across country. Note the indistinct lateral spots.
A female adder moving across country. Note the indistinct lateral spots.

Could be mistaken for

Sub-adult Aesculapian snake © John Newton

Any snake can be mistaken for an adder, most commonly grass snakes are wrongly identified as adders, although a glance at the identification pages will show a host of differences between the two species. If see a snake around a garden pond, it is almost certainly a harmless grass snake.

Our other native snake, the smooth snake is far more adder-like than the grass snake; however both these snakes have a round pupil and no zig-zag along their back. The Smooth snake is extremely rare, and unless you are on dry heathland in the Southern counties you will not see one (you probably won't see one even if you are in the right spot!).

Even the slow worm which is a legless lizard can occasionally be mistaken for an adder. Slow worms have a metallic colouration, blunt tongues and eyelids (snakes have no eyelids and cannot close their eyes).

Adders are rarely seen in gardens, unless your garden backs onto heathland, woodland or large scrub areas. If you have a garden pond stocked with fish, or frogs are abundant, and you have a snake, it's probably a grass snake!

The Aesculapian snake is an alien species with an extremely restricted range in the UK, there are only two colonies, one in north Wales and the other in central London. More commonly encountered species are escaped exotic pets such as corn snakes and king snakes.

The grass snake is the most commonly encountered snake; often misidentified as an adder.
The grass snake is the most commonly encountered snake; often misidentified as an adder.
The smooth snake has no zig-zag pattern along the back; but is rarely encountered.
The smooth snake has no zig-zag pattern along the back; but is rarely encountered.
 The slow worm has a metallic appearance and is a legless lizard.
The slow worm has a metallic appearance and is a legless lizard.

Distribution

Global distribution for adder © http://seh-herpetology.org

The Adder is the most northerly distributed reptile, and is well adapted to cool conditions. They can even be found within the edge of the arctic circle.

The UK subspecies; Vipera berus berus can be found across Europe as far east as the Ural mountains in Russia, and as far south as the Mediterranean. Its northerly extent includes Scandinavia and Russia as far north as the arctic circle. The UK represents the westerly extreme of the adder’s range.

The Adder can be found across England, Scotland and Wales, but is absent from Northern Ireland and Eire. Strongholds for the adder comprise the southern counties, Wales, Cunbria and the North East.

Surrey has a very strong population of adders, although this may be declining. Surrey is certainly one of the stronghold counties within England, with more records reported than any other county except Dorset.

UK distribution of adder © NBN
UK distribution of adder © NBN
Surrey distribution for adder © SARG
Surrey distribution for adder © SARG

Habitat

Adders; basking typically adjacent to vegetative cover, in this case, close to a hibernaculum.

Adders require a combination of thick vegetation for cover from predators, open sunlight spots in which to bask, a food source comprising other reptiles, frogs or small mammals and readily draining soil for flood-proof hibernacula.

The need for well drained soils means that adders are frequently found on sandy heathland and chalk downland. They tend to avoid heavy clay soils.

Hedgerows can provide an abundance of prey. Although associated with chalk grassland and heathland, the adder is essentially a woodland margin species, and can be readily found at woodland edges or along forest rides.

Hibernation areas are usually on drier and higher ground, but it is usual for (particularly male) animals to migrate over 1km to wetter hunting areas during the summer.

Heathland provides good habitat for adder; although woodland margins are preferred.
Heathland provides good habitat for adder; although woodland margins are preferred.
Forest rides provide high concentrations of adder sightings due to the lack of suitable basking locations.
Forest rides provide high concentrations of adder sightings due to the lack of suitable basking locations.
Chalk grassland provides excellent drainage for adders.
Chalk grassland provides excellent drainage for adders.

Activity

Male adders will slough their hibernation skins around the first week of April.

The adder is a diurnal predator, however on particularly warm summer nights it is known that adders also hunt during the hours of darkness.

Best survey time is from mid-March until mid-July for both visual and refugia survey.

The mating period varies, depending upon annual weather conditions, but usually the last two weeks of April or the first week of May.

After the mating period, male Adders (and juveniles) migrate to wetter communal hunting areas shared by adders from various hibernacula. This can make summer months difficult for survey, although most females stay close to hibernacula sites, those in reproduction basking openly to aid development of their unborn young.

All animals return to the proximity of the hibernacula site towards the end of September. Juveniles may follow scent trails back to a different hibernacula area to their orign, thus aiding genetic diversity.


Food

Adult female adder predating a rat - prey is habitually swallowed head first. © Terry Longley (www.seeing.org.uk)

Like all native snakes, the adder will eat any animal that it can overpower and swallow, however; the adder tends to specialise in small mammals, particularly voles.

Research has shown that adders take a significant period of time to digest their prey. It is not infrequent for an adder to survive from eating only one small mammal per month.

There are reports that some adder populations primarily predate amphibians, such as the common frog. Presumably this is due to a lack of small mammals, but demonstrates this snake's dietary flexibility.

Nesting birds are also taken when the opportunity is presented. Other reptiles may also be taken, with many reports of common lizard and slow worm making up a part of the adder's diet. It is believed that juvenile adders are perticularly dependent on common lizard prey, and it is unusual for find adders at locations where common lizards are not present.

Although it is generally believed that the adder is an ambush predator, they can often be seen actively hunting for prey. It is believed that the adder strikes its victim, then immediately releases (once a lethal dose of venom has been injected). By doing this the adder avoids any possible injury from struggling prey. The adder will then track its victim, scenting the trail by use of its flickering tongue 'tasting' the air. The adder can track the individual prey item, regardless of crossing tracks left by other animals of the same species.

Recent studies have shown that adders spend a significant amount of time underground, in vole runs and burrows. Presumably such proximity to a prey source is not coincidental.

Small mammals such as the Field vole are the staple diet of Adders.
Small mammals such as the Field vole are the staple diet of Adders.
 At some sites - Adders are known to favour common frogs as prey.
At some sites - Adders are known to favour common frogs as prey.
Reptiles also make up a significant part of the Adder
Reptiles also make up a significant part of the Adder's diet - particularly for juvenile snakes.

Predators

The Adder will only assume this defensive posture; head raised, body coiled to strike, hissing loudly, when flight is not an option.

Most of the adder's predators are birds. Birds of prey will take an adder, but so will members of the crow family, some sea gulls and even herons. Perhaps the main predator of young and small adders is the pheasant!

Smooth snakes, although rare will eat adders of up to 30cm in length. Mammal predators may include badgers, foxes and even hedgehogs, but this is opportunistic and does not form part of their usual diet. In certain parts of the country, pole cats and their relations the stoat and pine marten may predate on adders.

Research has shown that the zig-zag markings along the adder's back are recognised by predators as a warning that the adder can defend itself with venom. This defence mechanism is known as aposetism. The adder's first defence against all predator threats is to move quickly and silently into deep cover, such as a gorse thicket.

Only as a last resort will the Adder bite, and even then will posture and hiss to warn a predator that it can defend itself.

There are around 100 adder bites to humans each year in the UK. Victims are usually male, who have picked up the Adder with their hands! There have been only 12 reported deaths from Adder bite, and none since the mid 1970s. Some of these fatalities are believed to have been due to the administration of early antivenins, when the risk of untreated allergic reaction was higher than today.

Healthy humans usually recover fully within three weeks (children often within two weeks), although some nagging aches can persist for longer periods. Antivenin is only used for severe bites, although anybody bitten by an adder should seek immediate medical attention.

Up to 5% of the buzzards diet comprises reptiles and amphibians; including adders.
Up to 5% of the buzzards diet comprises reptiles and amphibians; including adders.
Smooth snake is capable of predating adders of up to 30cm in length.
Smooth snake is capable of predating adders of up to 30cm in length.
Mustalids; as well as foxes and badgers may be capable of the occasional predation of adders.
Mustalids; as well as foxes and badgers may be capable of the occasional predation of adders.

Shelter

Adders use dense vegetation for cover, but are often found close to wooden objects such as old tree stumps.

Adder hibernacula (hibernation places) are of vital importance for the long-term survival of the species. Some adders may use the same hibernacula for life, and often generations of the animal have used the same hibernacula; however there is strong evidence to support the possibility that adders can chage hibernacula.

Hibernation sites must be resistent to flood, frost and predators. Typical hibernacula include living root systems, unused burrow networks, and especially the root systems of trees that have been blown over by gales and have overgrown.

In Surrey, the majority of hibernaqcula are situated on slopes (to aid drainage?) associated with bracken, birch or gorse.

This fallen-tree stump has overgrown yet will be full of hollows; ideal for an adder hibernacula
This fallen-tree stump has overgrown yet will be full of hollows; ideal for an adder hibernacula
 This tarpaulin-covered pile of logs is an active adder hibernacula on a working dairy farm.
This tarpaulin-covered pile of logs is an active adder hibernacula on a working dairy farm.

Reproduction

Gravid adders are often conspicuous as they need to bask for long periods during the summer, to aid development of their young.

In April, the male adder actively seeks out a potential mate. This ‘mate searching’ activity can be frantic, with male adders patrolling at speed, searching for pheromone trails. At this time it is possible to approach adders without any apparent shyness from the animal.

When a male finds a potential mate, he will slither along her side tongue-flicking the female. If successfully received, the two will mate. After mating, the male will then ‘mate guard’; coiling next to the female in an attempt to prevent other males from mating with her.

Should a rival male approach, the two males will slide along each other, almost as if they are comparing their lengths. If the challenge progresses, both males will wind around each other, trying to force their opponent’s head to the ground. This produces an awe inspiring spectacle known as the ‘dance of the adders’ and should you be lucky enough to witness this graceful spectacle, it is an event you will never forget. This ‘fight’ lasts for only a few minutes before one of the males will concede defeat and leave the site at high speed, chased for several metres by the successful male. Should the original male win, he will resume his mate guarding adjacent to the female. Should he lose, the new victor will attempt to woo the female, and the cycle starts anew.

The adult female adder will bear live young every two or three years. The energy used to produce the young cannot be replaced in a single year, so adders are (generally) not capable of giving birth every year. Between three and twenty young are born towards the end of September (in the UK). The young snakes are left to fend for themselves, but can often be seen basking in twos or threes for the first month of life.

The male Adder (top right) is mate-guarding the female (top left). The male at the bottom is an intruder.
The male Adder (top right) is mate-guarding the female (top left). The male at the bottom is an intruder.
When detected - the mate-guarding Adder challenged the intruder
When detected - the mate-guarding Adder challenged the intruder
 The challenge escalates into the
The challenge escalates into the 'Dance of the Adders'.

Juveniles

Never pick up an Adder ! - Juvenile held by an expert to demonstrate size © Paul Smith

Juveniles are live-born in late summer. The parents play no part in their development, however the young adders are often observed in the same area as the mother for a few days after birth. In the 19th century, naturalists believed that the mother adder would protect its young by swallowing them, regurgitating them when the danger had passed. Although some credible witnesses attested to this occurrence, it is not now believed to be true.

The markings of juvenile adders are identical to that of adults. The patternation of each animal is as unique as a fingerprint, and can be used to identify an individual animal throughout its life. Juvenile colouration tends to be more orange than the adults, with ginger specimens commonly observed.

Juvenile Adders specialise in the predation of juvenile, or even adult common lizards and Slow worms, although any living prey of a similar size is likely to be taken. As with adults, the juveniles may regurgitate recently swallowed prey items when threatened in order to better make an escape.

This juvenile Adder shows the characteristic ginger colouration.
This juvenile Adder shows the characteristic ginger colouration.
The adult common lizard (foreground) was regurgitated - having been caught and eaten by this small adder! © Liam Russell
The adult common lizard (foreground) was regurgitated - having been caught and eaten by this small adder! © Liam Russell

Conservation

Adders are a protected species across the United Kingdom They are protected from being taken from the wild, being killed and from being sold. They are not legally protected from disturbance, as are some of the rarer species.

Unfortunately, Adders are still persecuted by humans, with many being killed each year from fear or ignorance. The main threat to the adder is loss of habitat, and the increased use of commercial pesticides that disrupt the adder's food chain. Being shy by nature, the adder is also susceptible to human disturbance. Of critical importance to the adders well-being, is the identification and protection of traditional hibernacula.

There are an estimated 65,000 breeding pairs of Adder within the United Kingdom, although this figure is believed to be declining. Key factors affecting this decline include:

  • Loss of habitat due to development.
  • Loss of suitable basking sites due to unsympathetic management practices.
  • Loss of suitable basking sites due to intentional afforestation.
  • Direct persecution/pressure by humans.


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Information

Principal Author(s):Main Illustrator:Edited by:Updated:
Steve LanghamSteve LanghamSteve Langham19 Sep 2018