The slow worm is Britains' only species of limbless lizard. Whilst it may appear snake-like superficially it is differentiated from snakes by its moveable eyelids, different head shape, and different scaleation. The body scales of both adders and grass snakes have a keel running down the centre, whereas the slow worm's scales are extermely smooth and shiny. In addition this lizard is not as flexible as the native snakes.
Female slow worms usually have a dark stripe down the back, in addition to dark flanks. In contrast the male has little to no stripe down the back, and the flanks are not much darker then the rest of the body. The underside of both sexes is mottled black and grey. Adult slow worms can grow up to 50cm in total length, with the tail often constituting half of that length. Neonates and juveniles have a dark stripe down the back and are golden to silver on top, with black flanks and underside. This colour fades with time so that by the time thay are around two years of age they resemble smaller versions of the adults.
The slow worm is able to shed its tail if it needs to in a process known as cadual autotomy. Here, muscles contract tightly and break vertebrae across 'fracture planes' to allow the tail to seperate from the body. In time the tail re-grows, but never to its original length, and the colour is usually somewhat darker than the original tail. The regrown section of tail has no bones in it and thus cannot be shed lower down than previously.
Reproduction & hibernation
Breeding takes place in April or May; roughly the same time as the viviparous lizard. Male:male competition is displayed in the slow worm. Similar to the adder, male slow worms entwine their bodies and appear to wrestle. The stronger male may bite the neck of his opponent in an attempt to discourage the weaker individual, which is a behaviour not known to be exhibited by adders.
Once the female becomes gravid she will bask for long periods of time to speed up the development of her young. The internal development of the young of this species is an adaptation to a cooler environment and allows this species to live at high latitudes and altitudes. Adult female slow worms give birth to between six and twelve neonates in late summer, a few months before hibernation.
Slow worms appear to congragate around hibernation areas from early September. With colder nights and cooler days these lizards gradually reduce their activity until it reaches a point where they do not venture from their hibernacula. Normally around early to mid-March these lizards will re-appear to begin their active season.
Slow worms feed mostly on invertabrates such as beetles, spiders, snails, and slugs; their large and pointed teeth help to keep hold of slippery prey. Food items are generally held in the mouth, crushed, and swallowed whole.
Slow worms are semi-fossorial reptiles, so spend much of their time beneath cover. However it is not uncommon to see them hunting or basking in long grass or bracken on warm, damp, summer evenings. The type of habitats slow worms are likely to be found in include heathland, costal dunes, moors, scrubland, and long grassland. In addition, gardens in the right location appear to be easily colonised by this species, and compost piles can provide a great habitat for many slow worms. Common factors in all habitats occupied by this species are some higher, drier places which are more exposed to the sun which are used for hibernation during the colder months of the year. The slow worm has been recorded up to an altitude of around 300m in Cumbria.