The slow worm is a fascinating legless lizard often mistaken for a snake or a worm. Despite its serpentine appearance, this reptile possesses unique characteristics. Understanding its true nature reveals a complex and adaptable species with an important role in its ecosystem.
Identifying a Slow Worm: Not a Snake, Not a Worm
Several anatomical features distinguish slow worms from snakes. Slow worms possess movable eyelids, a trait absent in snakes, whose eyes are covered by fused, transparent scales. Slow worms also have external ear openings, which snakes lack.
A slow worm’s tongue is broad and slightly notched, unlike a snake’s deeply forked tongue. When shedding skin, slow worms do so in patches, while snakes typically shed their entire skin in one piece. Slow worms can also detach their tails as a defense mechanism, a process known as autotomy. This ability gives them their scientific name, Anguis fragilis, meaning “fragile snake.”
Slow worms also have rigid bodies due to bony plates called osteoderms beneath their smooth, glossy scales, which do not overlap. This structure results in a less fluid, more deliberate movement compared to the undulating locomotion of snakes. Their jaw structure is also different, with slow worms having fixed jaws unlike the flexible jaws of snakes.
Life and Habits of the Slow Worm
Slow worms typically measure between 30 and 50 centimeters in length, with some individuals reaching up to 57.5 centimeters. Their coloration varies: males are often greyish-brown and may develop blue spots, while females are usually coppery-brown with dark sides and sometimes a thin dark stripe along their back. Juveniles are often golden or silvery with dark bellies and a distinct dark stripe down their backs.
These reptiles are widespread across mainland Britain and parts of Europe and Asia, though generally absent from Ireland except for a few introduced populations. They prefer habitats with dense ground cover such as gardens, allotments, heathlands, and woodland edges, often seeking warmth and shelter in compost heaps or under rocks and logs. Slow worms are semi-fossorial, meaning they spend considerable time burrowing in loose soil and decaying vegetation.
Their diet consists primarily of slugs, snails, spiders, and earthworms, making them beneficial residents in gardens as natural pest controllers. They possess backward-curving teeth, effective for gripping slippery prey. Slow worms are mostly nocturnal, hunting at dusk. Unlike some other reptiles, they do not typically bask in direct sunlight; instead, they warm themselves by lying under objects that have absorbed solar heat. They hibernate during colder months, typically from October to March, often in communal underground burrows or sheltered areas.
Slow worms are ovoviviparous, meaning females retain eggs internally and birth live young, usually between mid-July and September. This reproductive strategy allows the mother to regulate the temperature of developing embryos, advantageous in cooler climates. Mating can be an extended process, sometimes lasting up to 10 hours, with males often biting the female’s head or neck during copulation. Females typically give birth to 8 to 12 young, born encased in a thin, transparent membrane they quickly break free from.
These lizards are known for their long lifespans; in the wild, they can live for 20 to 30 years, and one captive individual reportedly lived for at least 54 years at Copenhagen Zoo. The loss of legs in slow worms is an evolutionary adaptation, linked to a burrowing lifestyle where limbs would be a hindrance. This leglessness has evolved independently in multiple lizard lineages, distinct from the evolution of snakes.
Protecting Slow Worms
Slow worms are protected under the Wildlife and Countryside Act 1981 in the UK, making it illegal to intentionally kill, injure, or sell them. They are also listed as a Priority Species under the UK Post-2010 Biodiversity Framework. Despite their widespread distribution, slow worm populations have faced declines in some areas, primarily due to habitat loss from urbanization and agricultural expansion.
Predation by domestic cats, badgers, and certain birds poses a threat. Their role as controllers of garden pests underscores their importance in local ecosystems. Public efforts to protect slow worms can include creating wildlife-friendly gardens with log piles, rockeries, and compost heaps, which provide essential shelter and foraging areas. Reducing pesticide use and being mindful when gardening can further contribute to their conservation.