Adder

Vipera berus, commonly known as the common European adder and the common European viper, is a species of venomous snake in the family Viperidae. The species is extremely widespread and can be found throughout much of Europe, and as far as East Asia. There are three recognised subspecies.
Known by a host of common names including common adder and common viper, the adder has been the subject of much folklore in Britain and other European countries. It is not regarded as especially dangerous; the snake is not aggressive and usually bites only when really provoked, stepped on, or picked up. Bites can be very painful, but are rarely fatal. The specific name, berus, is Neo-Latin and was at one time used to refer to a snake, possibly the grass snake, Natrix natrix.
The common adder is found in different terrains, habitat complexity being essential for different aspects of its behaviour. It feeds on small mammals, birds, lizards, and amphibians, and in some cases on spiders, worms, and insects. The common adder, like most other vipers, is ovoviviparous. Females breed once every two or three years, with litters usually being born in late summer to early autumn in the Northern Hemisphere. Litters range in size from three to 20 with young staying with their mothers for a few days. Adults grow to a total length of and a mass of. Three subspecies are recognised, including the nominate subspecies, Vipera berus berus, described here. The snake is not considered to be threatened, though it is protected in some countries.

Taxonomy

There are three subspecies of V. berus that are recognised as being valid including the nominotypical subspecies.

Subspecies	Taxon author	Common name	Geographic range
V. b. berus		Common European adder	Norway, Sweden, Bulgaria, Finland, Latvia, Estonia, Lithuania, France, Denmark, Germany, Austria, Switzerland, Northern Italy, Belgium, Netherlands, Great Britain, Poland, Croatia, Czech Republic, Slovakia, Slovenia, Hungary, Romania, Russia, Ukraine, Mongolia, Northwest China
V. b. bosniensis	Boettger, 1889	Balkan cross adder	Balkan Peninsula
V. b. sachalinensis	Zarevskij, 1917	Sakhalin adder	Russian Far East, North Korea, Northeast China

The subspecies V. b. bosniensis and V. b. sachalinensis have been regarded as full species in some recent publications.
The name 'adder' is derived from nædre, an Old English word that meant snake in the older forms of many Germanic languages. It was commonly used in the Old English version of the Christian Scriptures for the devil and the serpent in the Book of Genesis. In the 14th century, 'a nadder' in Middle English was rebracketed to 'an adder'.
In keeping with its wide distribution and familiarity through the ages, Vipera berus has a large number of common names in English, which include:
In Welsh, it is called gwiber, a name derived from Latin vīpera. In Denmark, Norway and Sweden, the snake is known as hugorm, hoggorm and huggorm, roughly translated as 'striking snake'. In Finland, it is known as kyykäärme or simply kyy, in Estonia it is known as rästik, while in Lithuania it is known as angis. In Poland the snake is called żmija zygzakowata, which translates as 'zigzag viper', due to the pattern on its back.

Description

Relatively thick-bodied, adults usually grow to in total length, with an average of. Maximum size varies by region. The largest, at over, are found in Scandinavia; specimens of have been observed there on two occasions. In France and Great Britain, the maximum size is. Mass ranges from to about.
The head is fairly large and distinct and its sides are almost flat and vertical. The edge of the snout is usually raised into a low ridge. Seen from above, the rostral scale is not visible, or only just. Immediately behind the rostral, there are two small scales.
Dorsally, there are usually five large plates: a squarish frontal, two parietals, and two long and narrow supraoculars. The latter are large and distinct, each separated from the frontal by one to four small scales. The nostril is situated in a shallow depression within a large nasal scale.
The eye is relatively large, equal in size to or slightly larger than the nasal scale, but often smaller in females. Below the supraoculars, there are six to 13 small circumorbital scales, usually eight to 10. The temporal scales are smooth. There are 10–12 sublabials and six to 10 supralabials. Of these, numbers 3 and 4 are the largest, while numbers 4 and 5 are separated from the eye by a single row of small scales.
There are 21 rows of dorsal scales midbody. They are strongly keeled scales, except for those bordering the ventral scales. The scales appear to be loosely attached to the skin, with the lower rows becoming increasingly wider. Those closest to the ventral scales are twice as long as those along the midline. The number of ventral scales is 132–150 in males and 132–158 in females. The anal plate is single. The subcaudals are paired and number 32–46 in males and 23–38 in females.
This species exhibits significant variation in colouration. Light-coloured specimens are characterised by small, incomplete, dark crossbars on their backs, while darker specimens display faint or clear, darker brown markings. Melanistic individuals are completely black and lack any apparent dorsal pattern. However, most specimens have a zigzag pattern on their backs that extends along their entire bodies and tails. A distinctive dark V or X marking is usually present on the dorsal surface of the head. A dark streak runs from the eye to the neck and continues as a series of longitudinal spots along the flanks.
Unlike most snakes, the sexes of this species can often be distinguished by colour. Females are typically brown with dark brown markings, whereas males are a uniform grey with black markings. The base colour of males is often slightly lighter than that of females, which makes the black zigzag pattern stand out more. Melanistic individuals are often female.

Distribution and habitat

Vipera berus has a wide range. It can be found across the Eurasian land-mass; from northwestern Europe across southern Europe and eastern Europe to north of the Arctic Circle, and Russia to the Pacific Ocean, Sakhalin Island, North Korea, northern Mongolia and northern China. It is found farther north than any other snake species. The type locality was originally listed as 'Europa'. Mertens and Müller proposed restricting the type locality to Uppsala, Sweden and it was eventually restricted to Berthåga, Uppsala by designation of a neotype by Krecsák & Wahlgren.
In several European countries, it is notable for being the only native venomous snake. It is also one of only three native British snake species. The other two, the barred grass snake and the smooth snake, are non-venomous.
Sufficient habitat complexity is crucial for this species to be present, in order to support its various behaviours—basking, foraging, and hibernation—as well as to offer some protection from predators and human disturbance. It is found in a variety of habitats, including: chalky downs, rocky hillsides, moors, sandy heaths, meadows, rough commons, woodland edges, sunny glades and clearings, scrubby slopes and hedgerows, rubbish tips, coastal dunes, and stone quarries. If dry ground is available nearby, it will venture into wetlands and may therefore be found on the banks of streams, lakes, and ponds.
In much of southern Europe, such as southern France and northern Italy, it is found in either low-lying wetlands or at high altitudes. In the Swiss Alps, it may ascend to about. In Hungary and Russia, it avoids open steppeland; a habitat in which V. ursinii is more likely to occur. In Russia, however, it does occur in the forest steppe zone.

Conservation status

In Great Britain, the killing, injuring, harming or selling of adders is illegal under the provisions of the Wildlife and Countryside Act 1981. A similar situation exists in Norway under the and in Denmark. In Finland killing an adder is legal if it is not possible to capture and transfer it to another location and the same provision also applies in Sweden. The common viper is categorised as 'endangered' in Switzerland, and is also protected in some other countries in its range. It is also found in many protected areas.
This species is listed as protected under the Berne Convention.
The International Union for Conservation of Nature Red List of Threatened Species describes the conservation status as of 'least concern' in view of its wide distribution, presumed large population, broad range of habitats, and likely slow rate of decline though it acknowledges the population to be decreasing. Reduction in habitat for a variety of reasons, fragmentation of populations in Europe due to intense agriculture practices, and collection for the pet trade or for venom extraction have been recorded as major contributing factors for its decline. A citizen science based survey in the UK found evidence of extensive population declines in the UK, especially affecting smaller populations. A combination of public pressure and disturbance, habitat fragmentation and poor habitat management were considered the most likely causes of the decline. The release of 47 million non-native pheasants and 10 million partridges each year by countryside estates has also been suggested to have a significant impact on adder populations across the UK, with the possibility the reptile could be extinct by 2032.

Behaviour

This species is mainly diurnal, especially in the north of its range. Further south, it is said to be active in the evening, and it may even be active at night during the summer months. It is predominantly a terrestrial species, though it is known to climb up banks and into low bushes in order to bask or search for prey.
Adders are not usually aggressive and tend to be rather timid, only biting only when they are cornered or alarmed. They will generally only bite if are stepped on or picked up. They will usually disappear in the undergrowth at the slightest hint of danger, but will return once all is quiet, often to the same spot. Occasionally, individual snakes will reveal their presence with loud, sustained hiss, presumably to warn off potential aggressors. These are often pregnant females. When threatened, the front part the adder's body is drawn into an S-shape in preparation for a strike.
This cold-adapted species hibernates in winter. In Great Britain, males hibernate for around 150 days and females for around 180 days. In northern Sweden, hibernation lasts 8–9 months. On mild winter days, they may emerge to bask in areas where the snow has melted, often travelling across snow in the process. Approximately 15% of adults and 30–40% of juveniles die during hibernation.