Slow Worms

Male Slow Worm (© John Dickson)

Despite its serpentine appearance, the slow worm Anguis fragilis is a legless lizard. At some point during the species’ evolutionary history its lizard ancestors took to life underground where legs became something of an impediment and were gradually lost.  However, they retain their internal pelvic structures together with many other lizard characteristics, clearly differentiating them from snakes. Most conspicuous among these are the presence of eyelids, which are absent in snakes, and a broad, fleshy, notched tongue as opposed to the fine forked tongue of a snake. Additionally, the presence of osteoderms, small mineralised plates embedded in the skin beneath the scales, give their movements a rather stiff, lizard-like appearance compared to the sinuous flexibility of a snake.

Male Slow Worm (© Kevin Palmer)

Slow worms are very secretive animals, spending most of their lives underground, yet they are surprisingly abundant. They are undoubtedly the most abundant reptile in Britain being highly adaptable and consequently found in a wide range of habitats, including grassland, hedgerows, open woodland, commons and heaths as well as both rural and urban gardens and allotments. Curiously, slow worms seem to positively thrive in disturbed habitats, frequently being found in greater numbers on brownfield sites than in pristine habitats.

Female Slow Worm (© Liam Russell)

Little is known about how slow worms use the space available to them, although they are generally thought to be rather sedentary, occupying small home ranges. Anton Stumpel, studying slow worms in the Netherlands, found that the majority of movements among his study group did not exceed 30m and the longest distance he recorded an individual moving was 130m. The species is not territorial and many individuals may share overlapping home ranges.

Juvenile Slow Worm (© Liam Russell)

Slow worms are sexually dimorphic, the sexes being readily distinguished once maturity is reached. However, as juveniles both sexes are alike. Young slow worms are striking animals appearing smooth and highly polished with a deep bronze or copper dorsal colouration and a thin black vertebral stripe. The flanks and ventral surface are black, with a well-defined edge between the bronze on the back and the black on the flanks.

As the animals grow so their colouration alters, although the changes are more marked in males than in females.

Females usually retain the pattern of the juveniles once maturity is reached. The intensity of both the bronze dorsal colouration and the black on the flanks may fade slightly but both are usually still apparent, while many females also retain the thin black vertebral stripe. Males on the other hand are usually a paler, less striking colour than the females. The black on the flanks and the black vertebral stripe are usually lost altogether, or at least greatly faded, and the animals tend to be uniformly grey or light brown. Males also tend to be of a heavier build than the females, particularly noticeable in their broad, thick set heads, and as they age a proportion of the males acquire small blue spots across the body.   

The length at birth is about 10cm and adults can reach up to 35 – 40cm, although exceptional individuals may exceed this. As with other British lizards, slow worms have the ability to shed their tails when grasped, an ability known as autotomy. Regeneration is slow and the original length is rarely ever attained again, most often appearing simply as a tapered stump.

Slow worms are live-bearers, with each female believed to breed biannually. Mating usually occurs in May, the male firmly holding the females head in his jaws during copulation. The embryos develop over the course of the summer and an average of 6-12 young, are born fully developed and wholly independent, during late August and September. Growth is rapid and sexual maturity it thought to be reached after about three years.

Slow moving, soft bodied invertebrates such as slugs and earth worms make up the bulk of the diet, with the small white slugs thought to be a particular favourite.

Slow worms are thigmothermic, preferring to raise their body temperature by conduction, when in contact with warm objects, rather than by radiation directly from the sun. This means that slow worms are rarely found basking in the open but are more often found beneath objects, such as rocks and logs that are themselves warming up in the sun. In fact, observing any slow worm activity above ground is infrequent but occurs most often on warm evenings or just after a rain shower, when presumably the slugs are abroad.

Corrugated iron or roofing felt can make excellent substitutes for rocks and other natural objects under which slow worms thermoregulate, and these artificial ‘refugia’ can attract large numbers of them. The animals can become uncharacteristically predictable where refugia are used; many individuals often sharing the same piece of tin and spending long periods of time there. Yet while this might provide good opportunities for surveying slow worm populations, it is unwise to leave refugia in one place for too long, as a ready supply of slow worms may attract predators such as weasels.

Slow worms are the reptile most regularly encountered in gardens and allotments. This is probably because these places tend to provide a wide range of features and habitats in a small area. Features of gardens that may be attractive to slow worms include compost heaps, stone walls or stone piles, hedges, patches of rough grass, areas of disturbed soil where it is easy to burrow, and piles of garden rubbish. Additionally, during dry conditions, gardens and allotments tend to get watered, promoting continuous slug activity. Their preference for sites close to human habitation makes them frequent victims of predation by domestic cats.

As with the other native reptiles, slow worms hibernate underground during the winter months, often in sizable aggregations. Emergence usually occurs during March with the males appearing first and, depending on the weather, they usually return into hibernation during October. It is not known whether they return to the same hibernaculum each year.

The most unusual slow worms in the county are to be found on the island of Steep Holm. Steep Holm is a small limestone outcrop, a westward extension of the Mendip Hills, rising out of the Bristol Channel approximately 9km off the coast at Weston-Super-Mare. The island is managed by the Kenneth Allsop Memorial Trust as a nature reserve, predominantly for the large colony of nesting seabirds, but is also home to an extraordinary population of slow worms, the only species of reptile found there.

One curious feature of the Steep Holm slow worms is the colouration of the males. On the mainland  a proportion of aging male slow worms may develop a pattern of small blue spots across the body, yet some of the males on Steep Holm develop these spots so excessively that they merge into a dense continuous sky blue band running down the back and sides. The animals are literally blue! As if that was not odd enough, the slow worms on Steep Holm also exhibit gigantism, regularly surpassing the maximum sizes of animals found on the mainland.

Some of the more improbable claims about Steep Holm slow worms seem to owe much to the tradition of fisherman’s tales, such as the claim from an International Voluntary Service team in 1975 that they found an animal a metre in length, but others are more credible.

In their book ‘Steep Holm Wildlife’, Rodney Legg and Tony Parsons describe a number of specimens collected on the island that exceed 16 inches (406mm) in length. Tony Phelps, a herpetologist working for the Nature Conservancy in Furzebrook Dorset, claimed in 1975 to have found a specimen eighteen inches (457mm) long. Then in 1984 Nick Smith from Southampton University captured a specimen that was lacking most of its tail but still had a snout to vent length of 250mm, a total length of 391mm and weighed in at 60 grams. The longest slow worm yet found in Britain was captured on Steep Holm in 2001 by a group of Blenheim Venture Scouts and measured 535mm. A population incorporating a number of exceptionally large, bright blue animals makes the Steep Holm slow worms unique in the UK!


Slow worms are widespread across Britain and mainland Europe, although they are absent from Ireland. They can be found on the Atlantic coast of Portugal from where they range across the northern half of the Iberian Peninsula, extending north as far as Scandinavia. In the south they extend through Italy and the Balkans to the Mediterranean, while reaching east as far as western Siberia and the Caspian Sea.

Locally, they are common and widespread throughout Somerset and the unitary authorities at all altitudes and in most habitats, although they may be scarce in the most intensively farmed landscapes and absent from arable lands.

John Burton writing in 1974 suggests that the status of slow worms in Somerset has changed little since 1851 when William Baker described them as ‘common throughout the county’, and the same is probably true today. Baker however, also suggests that they are not common in marshy places. While they certainly tend to have a preference for light, well drained soils I have not found Baker’s assertion to be entirely correct and they can be found on heavy soils, including clays, and in remarkably damp habitats, for example on the levels and moors or in the mires of the Blackdown Hills.

Other authors similarly refer to their frequency, Charbonnier having found them to be ‘common in fields and hedgerows’ around the Bristol area in 1887 and more recently Avery having described them as ‘found in a wide variety of habitats’ on the Mendip Hills.

Their apparent affinity with disturbed, or brownfield, sites means they are often most abundant close to human habitation, reaching their greatest densities in locations such as gardens, allotments or churchyards. Surveys of several closed landfill sites across the county have also revealed significant populations of slow worms. However, their secretive nature means they are infrequently encountered except when activities involve lifting stones, logs or other objects from the ground, or moving compost heaps, when they can sometimes be found in large numbers.