Arguably Somerset’s most charismatic reptile species, the Adder, Vipera berus, is the only venomous snake in Britain, and currently the fastest declining reptile in the country.
Adders are sexually dimorphic; it being possible to distinguish males from females by external characteristics. The males are smaller than the females both in length and build. The average length of a series of 159 males all measured in Somerset was 48.1 cm while that of a series of 48 females, also all measured in Somerset, was 51.9 cm. Some individuals may exceed these averages by quite a long way. Of these males the longest was 65.5 cm while the longest female was 71.0 cm. The species is generally of a stocky build, but females are noticeably stouter than males. Among the same series of individuals the average weight for the males was 68.3g while that of the females was 96.3g. Sizes may vary between populations from different habitats; nevertheless, people encountering adders for the first time are often struck by how small they are.
The species is instantly recognisable by its markings, most obviously a characteristic dark mark on the top of the head usually appearing as an inverted V or X shape followed by a broad, dark zigzag line running the length of the back. There is also usually a row of dark spots running down the length of either flank. While these markings are shared by both sexes, the colouration between them differs. The ground colour of males is usually a steely grey or pale straw yellow while the overlying markings are jet black, providing a striking contrast between the pale background and the dark markings. In the females, the ground colour is darker than that seen in the males, usually some shade of brown, olive, fawn, or red, while the overlying markings are simply a slightly darker shade of brown, as opposed to the jet black seen in males. The overall impression is that the females tend to be coloured in muted pastel shades, while the males have sharp contrasting colours. Both sexes have red eyes, although those of the male tend to be a deep ruby red while those of the female are often a paler coppery colour. The pupil is elliptical as opposed to the round pupil of a grass snake.
The sexes are difficult to differentiate when adders are very young, but becomes clear by the time they reach maturity. A proportion of the young progress through a brick red colour phase during early growth, which may persist for several years and has in the past resulted in several authors erroneously proposing a new species: the small red viper.
Entirely black, or melanistic adders are not uncommon, yet while the ground colour of these individuals may be black, the characteristic markings of the species are often still discernible over the dark background.
Adders are generally associated with dry, open habitats where sunlight can reach the ground, offering plenty of opportunities for basking whilst providing patches of cover in which the snakes can seek refuge. They particularly favour warm, south-facing slopes and are not usually found in dense woodland where the canopy casts deep shade. In Somerset, the primary habitats for adders are heathlands, commons and moors, typically on light, acidic soils or peat, and often vegetated by any combination of heathers, gorse, broom, moor grasses, bracken and vaccinium. Together with these habitats, the species can also be found in open woodland, unimproved rough grassland, disused quarries and undisturbed embankments, such as those alongside railways. They tend to avoid areas with heavy clay soils.
During the winter months, adders hibernate underground either singly or in groups, sometimes sharing the hibernaculum with other species of reptiles. Features that are frequently used as hibernacula include crevices, either under tree roots or in rocky scree or shale, burrows excavated by small mammals, or sometimes man-made features, such as the footings of long-abandoned buildings.
The position and hydrology of the hibernaculum are important. Adders select areas of high ground with free draining soils to ensure that flooding will not occur, and the crevice or hole must be of sufficient depth to prevent the animals freezing during hibernation. South-facing slopes are preferred, to ensure maximum exposure to the winter sun enabling the adders to bask outside the hibernaculum during the last days of autumn and the first sunny days of spring. Thick shrubby vegetation across the surface can provide a thermal cushion for the hibernaculum, whilst gaps in the vegetation will allow the animals to bask in relative safety.
During hibernation, the metabolic rate is greatly reduced, but the animals are not completely comatose and a low level of activity is maintained. Therefore, during periods of mild, bright weather throughout the winter adders may emerge briefly to bask, but true emergence usually begins sometime in February or early March. The males emerge first, followed a few weeks later by the females and finally by the juveniles. Emergence is followed by several weeks known as the ‘lying out’ period. During this time the creatures take every opportunity to bask in the vicinity of the hibernaculum, retreating back underground when weather conditions are unsuitable. Such is the urge to bask at this time that even very weak or intermittent sunshine will draw them out despite temperatures that may only be a few degrees above freezing. Adders are adept at finding small suntraps and hollows, sheltered from the wind, where the microclimates can be several degrees above the ambient.
Long periods of exposure during basking, together with the concentration of adders around the hibernation slopes, make the lying out period the easiest time of year to observe the species, particularly the males who are highly active, both behaviourally and metabolically.
Basking stimulates the maturation of sex cells and production of a new skin beneath the old one. In the case of the males, the shedding of the winter skin, usually around mid-April, brings them into breeding condition. The females, however, may not shed their first skins of the season until several weeks later, after mating has occurred. Up until this initial slough the male adders have appeared rather drab and been somewhat unresponsive and approachable. The shedding of the skin however, transforms them. Suddenly their colours and patterns are at their most striking and vibrant, activity intensifies and they become increasingly alert and pugnacious. The males now track down females, and on discovering one the male adder will coil up with her and guard her from others, sometimes for several days, before she becomes responsive to his advances, and copulation occurs. At this time the females are rarely found unaccompanied by a male, but competition is fierce for females and a mate-guarding male will be challenged by other passing males. The combat that ensues has been described as ‘the dance of the adders’ but is perhaps better described as a wrestling match! These bouts consist of both males in a state of high excitement, chasing each other around to position themselves alongside one another. They rear the foremost half of their bodies off the ground, intertwining them around one another in an attempt to force their opponent back down to the ground. Biting does not occur and once one individual, usually the smaller, has been forced back down to the ground he will disengage and leave the victor to return to the female, or engage the next challenger!
Mating is a lengthy affair. When the female becomes receptive the male becomes highly excited moving around and over the female, flicking the tongue repeatedly, his head and body adopting distinctive twitching movements, while his tail curls rapidly underneath that of the female attempting to align their cloacae for intromission.
Once one or other of the hemipenes has successfully penetrated the female, the snakes will lie quietly during the mating process which may take several hours.
Production of young is energetically expensive. At such northerly latitudes as here in Britain where summers are short and relatively cool, females will not breed annually, some may breed biannually, but equally there may be a gap of three or four years between clutches.
By the end of April or the middle of May, mating is over and the adders disperse from the hibernation areas to their summer feeding grounds. They now become very difficult to find as they are not only more widely dispersed around the landscape, but the sun is powerful, reducing the time required to bask, and the vegetation will have become long and dense. Much of their time during high summer is spent under cover, in the thatch created beneath unmanaged grass or bracken, or in the bryophyte layer, where available.
Females are considerably more sedentary than males, often choosing not to join the dispersal to the summer feeding grounds, instead spending the summer close to the hibernaculum. Those that are gravid seem particularly reluctant to disperse, although some of the non-breeding individuals may go.
Where the adders go during summer depends on the extent of habitat connectivity from the hibernation area and the distribution of resources, primarily food and water. The summer grounds are often on lower ground and are generally damper than the hibernation area, providing cooler conditions during high summer while still offering plenty of opportunities for basking, and cover for retreat. If these conditions can be found close to the hibernaculum then adders may not disperse very far from the hibernation area, but individuals have been found, where necessary, to have travelled up to two kilometres to a suitable summer ground.
Activity during this summer period consists primarily of feeding, with each feed followed by periods of reduced activity, spent hidden within the thatch or litter layer, while digestion proceeds.
A wide range of prey species are taken including small mammals, lizards, nestling birds and amphibians, often incapacitated by a rapid venomous bite. The adder will not hold on to its prey, but will recoil immediately after the strike, allowing the envenomated victim to escape, before subsequently following the scent trail to find and consume the now deceased prey item.
By late summer adders begin to return to the hibernation areas, re-joining the breeding females who have spent the summer around the hibernaculum. These females are now heavily gravid and their gestation is reaching full term. As gestation progresses so the gravid females spend more of their time basking, until sometime during September or October the young are born. Adders are ovoviviparous, retaining unshelled, membranous eggs within the body until the embryos have developed to full-term, at which point the eggs are laid and the young immediately break out through the membranes. Clutch sizes vary; females breeding for the first time may only produce three or four young while older females can produce up to fifteen.
The young, measuring about 15cm, go into hibernation within weeks of their birth, depending on their remaining yolk sac for nutrition until the following spring when they take their first prey. Those young that survive to adulthood will reach maturity and begin to breed at about four to five years of age, and may live in excess of thirty years.
By late October, when breeding is completed and autumn is well established, all the adders will once again be hibernating and will not usually re-emerge until the following spring.
As Britain’s only venomous snake the bite from an adder has historically been greatly feared. However, the danger from adder bite has always been exaggerated, and while they are venomous they could not realistically be described as dangerous to humans.
Many venomous species of snakes have the fangs fixed in place in the upper jaw, limiting the length to which they can grow while still enabling the mouth to close around them. However, the adder’s fangs are hinged on the jaw and can be laid flat along the jawbone when the mouth is closed, rapidly swinging down into an erect position when they are to be deployed. This enables them to grow particularly long and bites can be deep and painful. However, the venom of an adder is relatively mild in comparison to some of the other viper species found in Europe and human deaths from adder bite are exceptionally rare, much rarer than those attributed to bee stings or dog bites! In Britain the last recorded death from adder bite occurred in 1975 and in total there have been only fourteen documented deaths in the UK. The number of documented bites is many times higher.
Most people recover quickly and completely from adder bite. However, there are circumstances in which the bite can be life threatening, and there is one local fatality on record. Fatalities usually occur where the victim is either very young or very old, or where a pre-existing condition can aggravate the effects of the bite, particularly an allergy to the proteins and enzymes that constitute the venom. An allergic reaction to the venom can result in the patient lapsing into anaphylactic shock which, according to an inquest held in Flax Bourton, was what happened on May 23rd 1961 when Hilary Brown aged 12 from Bristol died from an adder bite inflicted while having a picnic at Priddy Mineries in the Mendip hills.
Apart from the pre-existing condition of a bite victim, there are several other factors that can determine the severity of an adder bite, not least the quantity of venom employed. Venom has evolved primarily to enable the capture of prey and is only secondarily used for defence. Once deployed, venom needs to be replenished and a snake with a depleted source of venom is a less efficient predator and is itself increasingly vulnerable to predation. Therefore it is in the snake’s interests to use venom sparingly and a snake can employ venom in quantities proportional to the level of perceived threat. Many bites from adders are ‘dry’ bites, where no venom is deployed at all, or bites where only small quantities of venom are dispensed although, clearly, in all instances medical treatment should be sought.
Adders are widely distributed across Europe and Central Asia, ranging from within the Arctic Circle in Scandinavia, south to Greece and eastwards across Russia to the Pacific Coast. However, they do not extend into far western Europe, being absent from Western France and the Iberian Peninsula.
In Britain adders are found throughout England, Scotland and Wales but are absent from Ireland. They are more common in southern England than in the north, and are particularly rare in the Midlands. Here in Somerset they are close to the western edge of their range.
Adders are generally uncommon and localised in Somerset, although in some of those areas of the county where they do exist, populations are still relatively healthy.
There are three strongholds in Somerset where adders can be considered reasonably abundant: the Mendips, the Quantocks, and Exmoor.
On the Mendips adders are still common in the west, particularly at several sites within the AONB, while east of the A37, where the habitat is strikingly different, they become exceptionally rare. In West Mendip, patches of heathland and gorse scrub provide the core habitat for adders although these are often interspersed with unimproved grassland, open scrub and limestone scree, all of which suit the species well. The network of dry stone walls that radiates across the Mendip landscape provides not only good cover, but also dispersal opportunities for adders to move between patches.
Adders are less abundant on the Quantocks than they are on the Mendips, yet reasonable numbers can be found, predominantly on the large continuous tracts of heathland on the high plateau and the mosaic of unimproved grassland and gorse scrub covering the western slopes. Additionally they can be found on several of the low hills and commons at the south of the range around Cothelstone, Broomfield and Aisholt.
Curiously, adders live in much lower density on the Quantocks than they do on the Mendips, despite the many square kilometres of apparently suitable habitat. Arthur Ballied in correspondence with Gerald Leighton in 1901 recalls a labourer’s claim to have killed as many as fourteen in one day on the Quantocks. It is improbable that such numbers would be encountered anywhere on the Quantocks today. Additionally, the Quantock animals appear to be smaller than those found on the Mendips. Morphometric data collected from adders on both the Quantocks and the Mendips suggest that while there is little difference between the average length of snakes from either of the two ranges, those on the Quantocks are of a significantly lighter build. As yet there is little indication why this should be the case, but it fits with observations from other parts of the country where researchers have found that adders living on extensive, uniform tracts of heathland, such as that found on the Quantocks, tend to be smaller than those from more heterogenous habitats (N. Hand pers comm). A possible explanation may be the effect of habitat on prey availability. The abundance of lizards on the Quantock heaths might suggest they comprise a greater proportion of the diet than on the Mendips, where other prey may predominate. Alternatively, the land management techniques differ between the Quantocks and the Mendips, with heather burning, or swaling, widely practised on the Quantocks. This may potentially disrupt the adders’ food source or, more seriously, the complete removal of cover caused by burning may leave the adders themselves vulnerable to increased predation, skewing the population toward younger and smaller animals.
Whatever the explanation for this interesting observation, it could potentially help to explain the lower density of adders on the Quantocks as body condition may have various effects on their population ecology, such as fecundity and survivorship during hibernation.
On Exmoor adders are abundant and can be found wherever suitable habitat exists, although there is a general feeling among the National Park staff that the majority of adders in the region are found on coastal sites; a view supported by the existing records. However, adders have never been as extensively surveyed on Exmoor as they have on either the Mendips or the Quantocks, and only further surveys will determine whether this perception is correct or simply reflects increased sightings in those areas more densely populated by people.
As well as these three strongholds, there are smaller populations of adders scattered across various locations in the county. Several sites on the Blackdown Hills have significant numbers of adders and the relative scarcity of records from the area is certainly unrepresentative of their true abundance. This is probably simply down to a lack of recording effort in this part of the county together with the position of the county boundary which places most of the favoured south facing slopes of the Blackdowns in Devon, leaving predominantly north facing slopes within Somerset. But there are several well-populated sites adjacent to or straddling the boundary.
From the locations mentioned, one could get the impression that adders are confined to hill ranges, and it is certainly the case that most of the good adder sites within Somerset are associated with high ground. However, this association probably has more to do with the distribution of suitable habitat for the species, than with any specific preference for high ground. The farmland landscape of improved grassland pastures and arable fields enclosed by hedgerows, so familiar across the lowlands of Somerset, does not suit adders. Thus, most of them tend to be on the hills where poor soils and steep slopes mean that farming has historically been less intensive, and the ‘wilder’ habitats preferred by adders persist. However, lowland populations do exist within the county where suitable habitat can still be found.
They can still be found on parts of the Somerset levels, although numbers are not high. They are absent from the levels south of the Polden ridge, where the clay soils do not suit them, but north of the Poldens they inhabit a number of sites on the Avalon Marshes. Before peat extraction was industrialised in the 1960s and 70s, much of the Brue Valley was covered by raised peat bogs providing suitable habitat for adders. Now only small remnants of these raised peat bogs persist and the adder population of the region is highly fragmented, most being found on or around these remnants such as those at Westhay, Catcott and Street heaths. Adders are also familiar to regular visitors to Shapwick NNR, where a hibernaculum is located immediately within the entrance to the reserve, attracting a lot of attention.
Occasional records occur from other small isolated populations scattered throughout the county where remote tracts of suitable habitat still exist, both on high ground such as at Ham Hill and on low lying ground such as Langton Heathfield reserve, but sightings are irregular from such sites.
In North Somerset adders are primarily associated with the Mendips, including to the west of the M5 motorway, although they are also found on the Lulsgate Plateau south of Backwell, and on the Tickenham ridge. They are scarce in BANES, although occasional records are collected in the vicinity of Pensford.