Slow Worms Anguis fragilis grow up to 40 cm long, the females being slightly larger than the males. The females are brown, reddish-brown or copper above, with contrasting dark brown or black flanks. Many have a thin dark central stripe down the middle of the back. The flanks have similar stripes or rows of spots. Males have a similar dorsal background colour, but with a broader range of hues, and without spots and stripes. Many animals in our region are a milk-chocolate brown. Both sexes are grey, bluish or black underneath, with paler markings. A small proportion of adult males have small blue spots near the head, but this is rarely recorded in our region. Juveniles have the adult female background colours, but have a distinctive strong black stripe from the head to the tail (Beebee and Griffiths, 2000).
Underneath their scales Slow Worms have a second layer of plates called “osteoderms”. These make Slow Worms feel less flexible than a snake, and rather like holding a large millipede.
They feed mainly on small slugs, snails and earthworms, caught on the surface or under cover. Larger snails are eaten from the shell, leaving the shell behind, sometimes creating a “thrush’s anvil” effect, but without the shells being shattered. The mostly intact shells can be taken as a good field sign for Slow Worms in suitable habitat.
Slow Worms warm up in a different way to our other reptile species. The others bask in sunlight, warming both directly from the sun and also from heat radiating back from the substrate on which they are basking. Slow Worms prefer to avoid the sun and warm indirectly by being in contact with rocks, wood or metal that is being warmed by the sun. Often they are underneath a piece of wood or a survey “tin”. They also spend more time underground than our other reptiles and can burrow in grass tussocks and soft soils. These habits may help to reduce predation. Consequently, in contrast to Common Lizard Zootoca vivipara and Adder Vipera berus, three quarters of our Slow Worm sightings are from deliberate surveys and only one quarter from casual encounters.
Hibernation is underground, in mammal burrows, natural crevices or stonework below the level to which frost penetrates. Emergence is in April, dependant upon the spring weather. The males emerge first. The rest of the year is spent in close proximity to the hibernation site. Mating takes place in the summer, with the average litter of eight young being born later in the autumn.
Like Common Lizards, the young are born in an egg sac and quickly free themselves. At birth they are seven to ten cm long, doubling this in their first year. They reach 23 cm in their second year. Breeding starts at age three for males, four years old for females. Slow Worms live longer than Adders and Common Lizards, perhaps to 10-15 years (Beebee and Griffiths, 2000).
Predators are the usual suspects: birds of prey, crows, carnivorous mammals and Hedgehogs Erinaceus europaeus. Newborn young are vulnerable to a broader range of birds and mammals.
Slow Worm range and habitats in our region are similar to Common Lizard, though slightly more restricted, as they are less likely to be found in the higher altitude moors and in “mid county” areas. In Northumberland, they are not recorded from large areas of the county north of the Tyne/ South Tyne Valley, and are sparse at Kielder and the Cheviots, especially when compared with Common Lizard and Adder. They are regularly recorded in the South Tyne and Allen dales, which account for most of the Northumberland records, and at Kyloe Hills.
In County Durham, the “Heart of Durham” area, as described under Common Lizard, is a very important centre for this species. The northern edges of the North York Moors also have important populations, extending beyond the moorland edge to woods and quarries at Guisborough and east of Guisborough.
Urban Slow Worms were unknown until recently, when a population was discovered near the Monkseaton/Whitley Bay area of Tyneside, using a variety of habitats including a disused railway line and adjacent scrub and gardens. The other occasional urban records are from known escapes.
Unlike Common Lizard, there is no “mid county” lowland distribution, though there may have been historically. Like Common Lizard, there is a coastal distribution, certainly south of Sunderland, particularly between Hawthorn and Castle Eden Denes. This situation also occurs on the Cleveland coast, along the cliffs and in the wooded coastal gills in the Saltburn/Guisborough/Loftus area. On the Northumberland coast, it is unclear whether there is a continuous or fragmented distribution, as there is too little data (Durkin, 2012B).
Many textbooks refer to the frequency of Slow Worms being found in gardens, churchyards, parks and allotments particularly around compost heaps, but this seems to happen very rarely in the North East.
Slow Worms are often found in the same upland habitats and locations as Adders and Common Lizards, and in the same coastal habitats as Common Lizards. Adders will occasionally predate Slow Worms, particularly juveniles.
by John Durkin