Is a Slow Worm a Snake? How to Tell a Legless Lizard

Why the slow worm is a lizard

The slow worm (Anguis fragilis) belongs to the family Anguidae, a group of lizards that also includes the legless glass lizards of North America and the European glass lizard of the Balkans, as well as legged species such as the alligator lizards of the western United States. Its body plan is that of a lizard that has simply lost its legs.

Losing legs is surprisingly common in lizards. It has happened independently in many families, including skinks, Australian flap-footed lizards and anguids. A long, smooth, legless body is ideal for sliding through thick grass, leaf litter and loose soil, which is exactly where slow worms spend most of their lives.

Snakes evolved from lizard ancestors too, but much longer ago and along a separate line. So the slow worm and snakes look alike because they adapted to similar lifestyles, not because they are close relatives. Biologists call this convergent evolution.

How to tell a slow worm from a snake

The quickest checks are the eyes, the size and the shine:

If you see a small, very glossy, brown, bronze or gray “snake” in a garden or compost heap that closes its eyes, it is a slow worm.

Eyelids, ears and tongue

The eyelids are the clearest clue. A slow worm’s eyes are small, with round pupils, and it can close them with true lids. A snake’s eyelids are fused into a transparent scale called a spectacle, so a snake always stares with open eyes.

The slow worm also has ear openings, a lizard feature. They are tiny and usually covered by scales, so they are hard to see, but snakes lack external ears completely.

The tongue is different too. Slow worms flick their tongues to pick up scents, just as snakes do, but the tongue is short, broad and only notched at the tip. Snakes have a long, deeply forked tongue and a small gap in the upper lip through which it can slip in and out while the mouth is closed. A slow worm has no such gap and must open its mouth slightly to flick its tongue. Read more in why do snakes flick their tongues?.

Tail, scales and skeleton

Like many lizards, the slow worm can shed its tail when grabbed. The broken tail twitches for minutes and distracts the predator while the lizard escapes. The slow worm regrows only a short stump, so many adults have blunt tails. Snakes cannot regrow a lost tail at all.

The scales also differ. A slow worm’s skin looks almost polished, with small, smooth scales that are much the same on the back and the belly. Under them lie tiny bony plates called osteoderms, which make the body stiff, so a slow worm wriggles rather clumsily when picked up. Most snakes have one row of broad belly scales that grip the ground and help them move.

Inside, the skeleton still shows the lizard ancestry. The slow worm keeps small remnants of a shoulder girdle and hip bones, and its lower jaw is firmly joined at the chin. In snakes the two halves of the lower jaw are linked by stretchy ligaments, which lets them swallow very large prey.

Slow worm, adder or grass snake?

In Britain, the slow worm is most often confused with the country’s three native snakes:

• Common European adder: a thick-bodied, venomous snake, usually 55–65 cm (22–26 in) long, with a dark zigzag down the back, red to coppery eyes with vertical pupils and rough, keeled scales. It lives mainly on heaths, moors and woodland edges rather than in gardens.
• Grass snake: Britain’s largest snake, reaching about 1 m (3 ft 3 in) or more, olive green to gray with a yellow or cream and black collar behind the head and black bars along the sides. It is harmless and often swims in ponds. The grass snakes of Britain and western Europe were split off as a separate species, the barred grass snake (Natrix helvetica), in 2017.
• Smooth snake: a rare, slim, gray or brown snake about 60–75 cm (24–30 in) long, found in Britain only on a few southern heathlands.

The slow worm is smaller than all of them, has no zigzag and no collar, and looks much shinier. Where gardens are concerned, a “snake” in the compost heap is most likely a slow worm or, in warmer lowland areas, a grass snake, which lays its eggs in rotting vegetation. Our comparison of adder vs grass snake covers the two snakes in more detail.

Why the confusion matters

Mistaking slow worms for snakes has cost many of them their lives. Across Europe, people have killed slow worms on sight in the false belief that they are venomous. Old names such as “deaf adder” did not help.

In fact slow worms are harmless and secretive, and their numbers are falling in many areas because of habitat loss, road traffic and garden cats. In Great Britain they are protected by law under the Wildlife and Countryside Act 1981, so it is illegal to intentionally kill or injure them. If you find one, the best thing is to leave it where it is and let it slip back into cover; picking one up can make it shed its tail.