Many of us still harbour a primal apprehension of anything that scuttles or slithers. Instantly recognisable by their striking zigzag patterning (sometimes called “diamonds”), adders are an enthralling part of Britain’s countryside for many. Unfortunately, as Britain’s only venomous reptile, they also have a fearsome reputation that, despite the hyperbole, poses conservation challenges.
The unanimous outcome of the UK’s first national adder symposium, held in Kent during November 2011, was that: “The adder is in more urgent need of new conservation efforts than any other reptile or amphibian species in Britain”. Indeed, increasingly, reptile surveys in England are reporting fewer adders, to the point where some researchers are anticipating imminent localised extinctions. The adder is protected from deliberate harm under the Wildlife & Countryside Act (1981) and has been made a conservation priority more recently, but these have failed to halt the decline.
That which follows is a summary of adder natural history. The detailed article for this species will follow in due course.
European adder at a glance
Three subspecies or three species, depending on preferred scheme. Type specimen (Vipera berus berus) found in UK and throughout northern and Central Europe. Sometimes called “viper” (Northern or Common), although this is much less common. Old English name Naedre, meaning “creeping thing” or “serpent”. Britain’s only venomous reptile.
Size: Typically, adults up to 65cm (26 in.); males (ave. 55-60cm / 22-24 in.) are smaller than females. Largest in UK at 87cm (34 in.), some specimens from Sweden recorded at 90cm (35cm) & two records of 104cm (41 in.) from Scandinavia. Weigh 50-180g (1.8-6.3 oz.).
Colour/Appearance: Reasonably thick-bodied snake, with a flat snout and red iris with vertical slit black pupil. Considerable colour variation by age, sex and moult (slough) proximity. Males often grey with dark black zigzag pattern running length of body; females brown with dark zigzag; juveniles often brick red. Colours brighter immediately post-slough. Pre-slough individuals often dull coloured with cloudy (‘milky’) blue eye. Completely black (melanistic) individuals well known in some populations, possibly more common in moorland or high-altitude habitats where may expedite warming. Oddities appear periodically, including individuals without dorsal markings (esp. in continental Europe). Must slough skin to grow; males sloughing soon after emergence and females a few weeks later. Juveniles slough more frequently than adults.
Distribution: Globally the most northerly distributed European snake species, with a range into the Arctic Circle. Widespread throughout Britain, although more abundant in southern England, (particularly East Anglia, Kent, Sussex, Hampshire, Dorset and Cornwall) west Wales, northern England and Scotland. In north, Pembrokeshire Coast, Isle of Skye and Dumfries and Galloway support good populations.
Habitat: A habitat generalist inhabiting open deciduous woodland, mixed farmland (absent from intensive agriculture), chalk downland, moorland, heathland, scrubland, mashes/bogs and several “urban” areas, such as quarries and brownfield sites. More recently, perhaps partly in response to reduced habitat suitability elsewhere (see Threats), recorded on railway embankments, disused quarries, mineral work spoil heaps and brownfield sites.
Longevity: Ten years appears average in wild, although occasionally 15 yrs. Some reports suggesting upto 30 years.
Sexing: Males often silvery-grey with black zigzag, while females darker with brown zigzag showing less contrast in markings. Colour isn’t, however, a reliable method of sexing. Tail and rostral (nose) scales most reliable method without handling. Males have (proportionally) longer tail than females. Some indication eye colour may vary with sex (dark red/purple in males vs. bright red in females) or that jawline scales in males are black while females have brown between them.
Activity: Hibernate from late autumn to early spring (temperature dependent), males entering and leaving earlier than females. First weeks post-hibernation spent basking to trigger sperm production in anticipation for female emergence. Ideal basking spots in direct sunlight, clear of vegetation but near cover for retreat. Body temperature of 25-30C (77-86F) required for full activity and achieved by basking; as spring moves to summer and air temperature rises, basking is less necessary and may not be required at all during hot periods in July and August. Pregnant females bask for most of day during late summer to assist development of young. Typically active by day, but may continue so through warm/hot summer nights. Climb and swim well.
Territory/Home Range: Non-territorial, but males will fight over access to females. May travel 200m (660 ft.) per day looking for mates, including underground, and move upto 2km (1.2 mi.) between breeding and hibernation sites.
Diet & Feeding: Wide prey base; mainly small mammals (e.g. mice and voles), but also small birds, frogs and lizards. Juveniles eat mostly froglets and small mammal nestlings. Prey swallowed whole. Jaw sections aren’t fused, allowing expansion to swallow objects larger than skull. As body temperature is managed environmentally and not via metabolism (per mammals), snakes eat remarkably infrequently (maybe 9-10 meals per year), particularly when taking nutritious prey. Fast for 2 or 3 wks before entering hibernation.
Highly efficient venom delivery apparatus consisting of two hollow mobile hinged fangs in upper jaw that can be rotated forward upon bite, folding against the roof of the mouth when not in use. Venom gland at base of each fang transfer venom to tip, allowing injection deep into prey tissue. Adder follows prey scent trail to feed once venom has taken effect.
Food identified primarily by highly sensitive senses of smell/taste and vibration; vision probably short sighted and heavily movement-based.
Reproduction: Males probably locate females by scent and can detect female pheromones from considerable distance, following them to potential matings. Hypothesised they may also follow pheromones of other males in hope they’ll lead to females. Males fight rivals by hissing and lunging until one backs down. If neither retreat they’ll rise up and shove one another (“dancing”), each trying to pin the other to the ground. Winner usually the larger of the combatants. Female sexuality maturity related to body size; appears to be a critical threshold (~51cm / 20 in.) below which she will not breed. Most mating during April.
Courtship involves male flicking female with tongue and tapping her rapidly with his chin; female waves tail in response. Sexes press bodies together and mate with tails raised. Copulation may last upto 2hrs followed by period of mate-guarding by male, during which they may mate several times. Despite mate-guarding, multiple paternity sometimes occurs. Female gives birth to clutch of 3-18 (ave. 10) pencil-sized live young late July to early September (peak in August). Reach maturity in 3-4 years depending on habitat quality, but many won’t breed until eighth season.
Behaviour and Sociality: Appear predominantly solitarily, excluding initial emergence and mating. Upon emergence from hibernation in spring will bask in groups during which males spend time writhing over one another “scent checking”. Juveniles may remain in area of birth for a while after birth, but don’t appear to interact with siblings or mother.
Predators: Gulls, corvids, buzzards, herons, chickens and, in particular, pheasants. Hedgehogs and badgers may take them opportunistically as would small mustelids (stoats & weasels).
Threats: Decline in heathland (including moorland burning) and dune systems as plantation forestry, urbanisation and agriculture has spread. Intensive agriculture, mowing and grazing has also reduced suitability of many downland and woodland habitats. Climate change; milder, wetter winters may increase metabolism (reduce hibernation) and flood hibernacula (per New Forest), although warmer summers may increase range.
Britain’s Reptiles & Amphibians - by Howard Inns
WildGuides Ltd. -- 2011 -- ISBN: 978-1903657256
Collins Field Guide: Reptiles & Amphibians of Britain & Europe - by Nicholas Arnold & Denys Ovenden
HarperCollins -- 2004 -- ISBN: 978-0002199643
RSPB Spotlight Snakes - by Jules Howard
Bloomsbury Wildlife -- 2020 -- ISBN: 978-1472971692
The Adder - by Peter Stafford
Shire Publications -- 1987 -- ISBN: 978-0852638798
The Private Life of Adders - by Rodger McPhail
Merlin Unwin Books -- 2011 -- ISBN: 978-1906122294