By James Duncan
Learning and Engagement Officer
Both the naming and look of the Slow-worm (Anguis fragilis) may be confusing, for it's neither a worm nor particularly slow in its movements; nor is it a snake - as per its superficial look. It is of course Britain's one and only legless lizard. A regular denizen of your compost heap, the sleek and beautifully smooth slow-worm is undoubtedly a keen gardener's friend for its favoured meals tend to include both slugs and snails. Once classified as a serpent, this commonly misidentified lizard actually displays various key anatomical differences. Like us, it possesses a closable eyelid, covering a circular pupil that is wholly different from the unblinking stare of a snake. Its tongue is wider and fatter than that of a snake and displays a characteristic notch, rather than an obvious deep fork. Whereas a snake flicks its forked tongue through an indentation in its upper jaw, the slow-worm has no such feature and needs to open its mouth to achieve the same. Unlike the supple serpents, the body of the Slow-worm is encased by a series of bony plates known as osteoderms, which lie beneath the glassy outer scales. Translating directly to 'bone & skin' these give the lizard a rigid feel, something that'll be familiar to anyone who's ever handled one.
Though the Slow-worm may form the first reptilian experience for many a child, as a whole it's a surprisingly elusive species. Its widespread distribution, fondness for a range of habitats and propensity for living close to human habitation would lead you to imagine it's well studied. However, it's forever been one of our least understood reptiles, owing to difficulties in tracking its long-term habits. Though a diurnal (day-active) species, they're very much a creature of the subterranean, spending much of their lives in an underground network of tunnels or within thick vegetative matter at the surface. Like all reptiles they're ectothermic, relying on both the warmth of the sun and of surrounding surfaces (hence decomposing compost) to regulate their body temperature (thermoregulation). With a tendency to bask somewhat more secretively than snakes, early spring typically offers the best opportunity to spot them as they sunbathe more regularly post-hibernation in an effort to get into breeding condition. Often found beneath suitable surface refugia (such as corrugated metal sheets) they display a similar tolerance for extreme heat to us - once the surface temperature climbs above 35°c, they'll rapidly disappear to seek shelter.
Slow-worm & young © Derek Middleton
Once in fine fettle, breeding commences from late May and the usually docile slow-worm may enter a rather hostile phase. Their small, backward-facing teeth, typically utilised for gripping slippery prey, become weapons for the males to wield upon each other. Males become supremely territorial, wrestling furiously and biting savagely, something that may result in significant visible scarring. The biting isn't reserved solely for male to male combat and during copulation the male will hold and bite the female's neck or head during a period of intertwinement that may last for up to ten hours. Typically the slow-worm employs a two-year (biennial) breeding cycle in Britain, dependant on environmental factors. Bridging the gap between egg-laying (oviparous) and live-young (viviparous) reproduction, the Slow-worm is ovoviviparous, whereby the young hatch inside the body, feed upon the yolk, but are born live at around 7-10cm, enclosed within a thin egg membrane which soon ruptures.
The Slow-worm has also, like many lizards, evolved a remarkable ability against predation - that of autotomy - a capacity to shed its tail via muscular contraction. Quite incredibly, several vertebrae in the tail have in-built fracture planes, which separate if the slow-worm needs to assist a rapid escape. Should a slow-worm evade capture, a new tail will eventually (and slowly) develop, though it'll be somewhat stumpier than the original. Autotomy is, however, a restricted process and cannot be repeated in an identical manner - a desperate slow-worm may have the option to split another vertebra higher than the original severance, but then of course, it may not. The very naming of the slow-worm, 'fragilis' refers to the sacrifice of the 'fragile' tail. Unlike other lizards, the tail doesn't play a role in storing fat that can be diverted into egg production - the loss of the tail won't therefore affect a female's reproductive success. Considering their supreme vulnerability to a range of avian, mammalian and reptilian predators, the defenceless slow-worm may be exceptionally long-lived, perhaps thirty years in the wild. A record captive individual even lived for over twice this long. While the slow-worm is generally most common in the south and west of Britain, it's now a BAP (Biodiversity Action Plan) priority species owing to declines in population due to both degradation and loss of suitable habitat.