The smaller of our two snakes, and the only venomous one, female Adders Vipera berus average 55 cm and the smaller males 50 cm in length. Their colour is very distinctive, with a dark zigzag line along the back, two rows of dark spots on each flank and a dark head-marking of variable shape, which can look like a V, U, X or H. The background colour varies between individuals, populations and the sexes. Males generally have lighter, brighter colours: grey, off-white, cream, yellowish, or occasionally bluish or greenish grey. Females are mostly brown or reddish-brown. Juveniles are reddish brown. Rare adults of either sex can be plain black (Beebee and Griffiths, 2000). These variations in colour scheme enable researchers to identify individual Adders, so that movements, territories and longevity can be assessed.
Adders hibernate from November to March, though sunny days in February can bring some snakes out to bask. They use underground sites such as scree or rabbit burrows, in dry ground with good cover. Many hibernation sites are communal.
Males shed their skin in April, at the start of the mating season. Females shed in May, and then both sexes shed again before the autumn. Females bear young only every second year in our region.
In summer, Adders can move around into different habitats, feeding mostly on small mammals with occasional amphibians, reptiles and nestling birds. The prey is bitten, released, and, if still mobile, followed until it dies.
The onset of cooler weather in the autumn starts the return to the hibernacula. Females shed their skin, and give birth to an average of eight or nine live young close to the hibernaculum. The young Adders use their yolk and fat reserves to last them through their first winter. Mortality is very high in the first year, after which most adults survive for another five or six summers (Beebee and Griffiths, 2000).
Although there are up to 100 cases of adder bites to humans in the UK each year, most have minor effects and there have been no fatalities for over 30 years. Often no venom is injected. Most cases result from basking snakes being accidentally handled or trodden on, with some resulting from snakes being picked up. Dogs are more often bitten but rarely with fatal results.
Adder distribution and habitat in our region is quite similar to that described for Common Lizard Zootoca vivipara, with Adders being more associated with heather moors, especially where the heather is not burnt. Like Common Lizard and Slow Worm Anguis fragilis they are scarce or absent on the higher moors. They are less adaptable to the small areas of habitat that might support Common Lizard.
In Northumberland, where they are much better recorded than the other reptiles, there are good populations at Kielder, Redesdale, Kyloe Hills, the College Valley/Cheviots, Redpath/Fontburn and Quarryhouse Moor.
In County Durham they are largely absent from the area east of the A68, except for a small population on Hedleyhope Fell and a possible, elusive population in the lower Derwent Valley. As with Common Lizard and Slow Worm, the great majority of records are in the “NY9-NZ0” zone, with the best sites at the “Heart of Durham” area, as described under Common Lizard, in and around Hamsterley Forest, and in the Stang/River Greta area.
The northern edges of the North York Moors, especially around Scaling Dam, have a strong population, though they may be declining there (Durkin, 2012B).
In ideal habitat, population densities of one Adder per 10 metres of linear habitat surveyed have been found at Kyloe Crags, Hamsterley Forest and at Pow Hill. These ideal habitats are usually quite small in extent.
There is no mid-county lowland or coastal population; there are occasional records but these are probably escapes from captivity. Occasional records at Chopwell Wood and Gibside could be a small population, or may possibly be escapes. There is a curious record of Adders in Gosforth Park during the Second World War. Large stocks of gravel had been brought in from Biddlestone in north Northumberland, ready to use for infilling bomb craters. One or several Adders were seen basking on the stockpiles for several years afterwards. It seems likely that Adders, perhaps young animals, were accidently transported along with the gravel (Bob Wilkin, pers. comm., 2012).
Two thirds of our Adder sightings are from casual encounters, often by botanists and foresters. Gordon Simpson is the champion recorder. Surveyors use a combination of keen observation and quiet approach, trying to spot Adders basking on suitable rocks without disturbing them. Binoculars help the observer to see them from a distance. Sheets of corrugated iron or felt roofing material can be placed at strategic, sunny spots, to encourage animals to come out into the open to bask at points where they can be seen from a safe distance. The surveys are usually done at a time of day when the Adders are just warming up and before other people can cause disturbance.
Adders have given us a number of place names, all within their current range, such as Adder Wood, Adder Crags, Adderstone and the White Adder Water.
Adders are often found in the same upland habitats and locations as Slow Worms and Common Lizards, but are absent from mid county lowland areas and from the coast. Adult Adders will predate juvenile Common Lizards and Slow Worms.
by John Durkin