Surrey Amphibian and Reptile Group (SARG) 

Slow Worm

  

Nomenclature

Common name:  slow worm 
Taxonomy:  Anguis fragilis (Linnaeus 1758)
Other Names:  Blind worm
Neidr ddefaid (Welsh)
Deaf-adder (Archaic)

Introduction

The Slow worm is often mistaken for a snake.

Despite its name, the Slow worm is neither slow nor a worm. It is a legless lizard, of between 30-50cm (about 18 inches) in length. Whilst superficially looking like a snake, 3 main differences are that the slow worm does not have a distinctive head, it has visible eyelids, and it will readily shed its tail if threatened.

Slow worms spend the majority of time in deep vegetation or underground in humid, overgrown areas of rough grassland, woodland edges, heathland, scrub, and urban areas such as gardens & allotments. There is concern that Slow worm numbers may be in decline, due to destruction of their habitat.

Although widespread across Britain, the slow worm is most commonly reported in the Southern counties, Wales and South-West Scotland.

They have been recorded to live for up to 30 years in wild, and the record age for a Slow worm in captivity is 54 years! (Copenhagen Zoo).

The part of their scientific name 'fragilis' (fragile) comes from the tendency of this species to shed its own tail, when threatened by predators, or if handled too roughly (caudal autotomy).


Identification

The Slow worm has eyelids and tight, interlocking scales.

The Slow worm reaches a length about 45cm (18 inches) from nose to tail, with a slim build. Although they are normally sluggish creatures, they can be fast moving when startled. The obvious feature is the very smooth, glass-like coat, which usually has a glossy, metallic finish.

Unlike snakes, the Slow worm has both eyelids and visible ears. The Slow worm has a rounded, notched tongue, unlike the forked tongue of a snake.

When Slow worms shed their skin, the sloughs fall away from the body in sections, as with other lizards, rather than shedding one complete slough, as is usual for snakes.


Morphology

An extreme blue-spotted morph, adult male Slow worm. © Dave Bird

Slow worms exhibit a wide variation in colour, from copper to a silvery grey. Adults have a smooth, shiny appearance, and a grey or bluish belly. Occasionally, individuals may have blue spots, a feature that is more common amongst males than females. This blue colouration is more common in coastal or island populations, and may vary over the year. It does not usually occur until an animal is at least three years old.

Melanistic (black) Slow worms occur, but are very uncommon. Albino specimens are very rare indeed.

Various Slow worm morphs including melanistic (black)
Various Slow worm morphs including melanistic (black)
 Large blue-spotted male
Large blue-spotted male
 Ultra-rare albino Slow worm © Wolfgang Wüster
Ultra-rare albino Slow worm © Wolfgang Wüster

Sexual Dimorphism

The male is uniformally coloured and is generaly a lighter shade than the female which usually has a thin dorsal stripe.

Males and females are different in appearance (sexually dimorphic);

Females are brown, copper coloured or red on the back, with brown or black sides, often with lighter iridescent flecks. In many individuals there is a dark stripe passing along the middle of the back and stripes running along the sides of the body.

Males vary in colour, they may be greyish, brown, coppery or reddish-brown, and typically do not have stripes; furthermore, they have broader and longer heads. The flanks of males are usually the same colour as their back.

Both males and females often have body scars; the males from fights, and the females from mating. Scars on females are usually to the head, neck and upper body.


Could be mistaken for

Young slow worms are occasionally mistaken for earthworms

Slow worms are most commonly mistaken for any species of snake, and most reports of the rare and elusive Smooth snake, turn out to be the widespread Slow worm.

Even exceptionally large earthworms are sometimes mistakenly reported to be slow worms.

Adder
Adder
Smooth snake
Smooth snake
Grass snake
Grass snake

Distribution

Global distribution for slow worm © http://seh-herpetology.org

This species has a broad distribution in continental Europe, where it is found from Scandinavia south to northern Spain and Portugal, and eastwards to southwest Asia and western Siberia.

Although widespread throughout Britain, it is naturally absent from Ireland (except for the area of the Burren, which has an introduced colony).

UK distribution for slow worm © NBN
UK distribution for slow worm © NBN
Surrey distribution for slow worm © SARG
Surrey distribution for slow worm © SARG

Habitat

Surrey dry heathland is not ideal habitat, but supports widespread slow worm populations.

The Slow worm can be found in most habitats where there is sunlight and dense vegetation. Favourite haunts include woodland margins, railway embankments, unmown grassland, heathland, gardens and allotments. Damper locations are favoured, due to the abundance of soft-bodied invertebrates which make up the major food source.

Residential gardens and allotments are key habitats for this animal, and they are strongly associated with compost heaps, where they can find both warmth from decaying vegetation, and food in the form of slugs and snails.

Their adaptivity, which allows them to share the urban environment with humans, has allowed this secretive reptile to survive across a very wide area, however it has also brought them into contact with domestic cats, which probably account for more Slow worm deaths than any natural predator.


Activity

The Slow worm is mostly diurnal, but due to its semifossorial (living in leaf-litter, deep undergrowth and in soil) nature, it is rarely seen in the open. On warm nights, there is good observational evidence to suggest that nocturnal activity also occurs. Like all British reptiles, it requires the warmth of the sun (although often indirectly for the Slow worm) to gain the energy it needs to hunt, but on warm nights it can gain some energy through conduction from the warm ground.

In Spring, males can be seen basking, but they are usually partly obscured by vegetation. Females are rarely observed in the open, unless crossing between areas of cover. Most sightings come from lifting refugia, including corrugated iron sheets, road signs or stones.


Food

This Slow worm skull demonstrates the backward curved teeth that it uses to grasp slippery invertebrates such as slugs.

The Slow worm's diet comprises a wide range of invertebrates, including: slugs, snails, mealworms and crickets.

They are often referred to as 'The gardener's friend', as they prey upon many pest species that attack flowers and vegetables.

The use of slug-pellets in gardens and allotments should be avioded, as this not only poisons the slugs, but also poisons the helpful predators that eat slugs, such as the Slow worm, song thrush and hedgehog.

Slug
Slug
Earthworm
Earthworm
Cricket
Cricket

Predators

Although snakes are known predators; it is unsual for a slow worm to be on the menu of a grass snake. © Jukka-Pekka Tikkanen

The slow worm suffers predation from birds, snakes and mammals.

Of our native snakes, both the smooth snake and adder are saurophagous (lizard eaters). The smooth snake is particularly noted for its reptillian diet, and slow worms are a major prey item. Recent research has shown that slow worms are able to detect the scent of snakes, (particularly smooth snakes), and will quickly move away once such a scent is detected.

Many species of bird will readily take slow worms, including pheasants, corvids, birds of prey and even woodpeckers. Pheasants pose the greatest threat, as they are ground birds, and hunt in the deep vegetation that is the preferred habitat of the slow worm.

Mammalian threats are mainly posed by hedgehogs and badgers. The hedgehog in particular, hunts the hedgerows than can be ideal habitat for slow worms.

The slow worm is not a swift reptile, but will try to avoid predators and can muster a surprising turn of speed when required.

If the slow worm cannot avoid contact with a predator, it has two major defence mechanisms: When caught, the slow worm will readily defecate, and although the faeces is not as foul smelling as that of the grass snake, it may be sufficiently unpleasant to deter some predators.

The main defence machanism of the slow worm is caudal autotomy. It can voloutarily shed its tail, which will wriggle violently for several minutes, catching the predators attention, whilst the slow worm escapes. The tail will regrow, but never reaches its original length, and may appear discoloured compared to the rest of the body.

Slow worms will bite in defence, but their teeth do not protrude far from the gums, and the bite is relatively ineffective. Bites to humans are rare, and virtually painless. Such bites do not generally break the skin, and the reaction is more usually shock, than pain.

If a slow worm is handled too roughly by humans, it will readily lose its tail.

Nationally; the domestic cat probably kills more slow worms than any other predator.© Chris Chesher
Nationally; the domestic cat probably kills more slow worms than any other predator.© Chris Chesher
Corvids; such as crows will readily predate a slow worm. © Nick Green
Corvids; such as crows will readily predate a slow worm. © Nick Green
Hedgehog
Hedgehog

Shelter

In common with most reptiles, Slow worms require dry, frost free hibernation sites (hibernacula) that are safe from predators. For the Slow worm, this is commonly at the base of a compost heap, or under rubble in urban areas. In wild areas, ant nests are sometimes used, along with grass tussocks, root systems and other deep vegetation structures.


Reproduction

Although fights between rival males occur, it is a rare event.

During courtship, the male takes hold of the female by biting her head or neck, and they intertwine their bodies. Courtship may last for as long as 10 hours before copulation occurs.

Most females tend to mate once every two years in Britain. Slow worms start to mate around April and May, but cannot actually conceive until June when the females’ eggs pass into their oviducts.

Although territorial disputes between males are usually settled by scent, occasionally they will fight with each other for possession of females. A female may pair with several males throughout the breeding season.

The young slow worms are initially encased in the egg membrane at birth, but almost immediately break free. They measure from 70-100mm (3-4 inches) in length, and will be fully grown after 6-8 years, becoming sexually mature by 3 or 4 years in males and 4 or 5 years in females.

An average of 6-12 live young are born from mid-August to mid-September, however, as many as 26 young are possible!


Juveniles

Both sexes of juveniles are gold, copper, brown or reddish in colour with a dark brown/black belly & sides. They have a thin dark brown stripe along the spine. The dark dorsal stripe of the male juveniles disappears after a year or two.

Juveniles have dark sides with a black dorsal stripe.
Juveniles have dark sides with a black dorsal stripe.
 Red or orange juveniles are not uncommon and resemble earthworms. © Liam Russell
Red or orange juveniles are not uncommon and resemble earthworms. © Liam Russell

Conservation

The Slow worm is legally protected (Wildlife and Countryside Act 1981) from intentional or unintentional killing and injury, and from sale, although its habitat is not. However, Slow worm numbers have dropped and populations are at high risk from development, especially on so-called 'brown field' sites such as railway sidings, under-used allotments, etc.

It may occur in localised high densities, but it is extremely poor at dispersal and entire isolated populations can be wiped out during unsympathetic habitat management or development operations. Ironically this risk is increased due to its cryptic nature as it often overlooked during surveys, a trait which enables it to survive threats from predators such as cats.

Current factors affecting the species include:

  • Loss of suitable habitat due to development, natural succession, etc.
  • Loss of suitable hibernation sites due to decline in traditional farm practices (less long-term storage of manure, loss of dry-stone walls).
  • Human persecution (due to mistaken identity as a snake).
  • Probable locally high rates of predation by domestic cats.
  • Unecessary over-tidying of allotment plots.


Links


Information

Principal Author(s):Main Illustrator:Edited by:Updated:
Sarah DroverSteve LanghamSteve Langham05 Sep 2018