Species of the day: Slow-worm

03 July 2020 | Posted in James Duncan , reptiles
Species of the day: Slow-worm
Slow-worm © Derek Middleton

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

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 (viviparousreproduction, 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. 


  • sophie Carnell:

    09 Jul 2020 10:26:00

    Thank you for this interesting article.

  • Ian Palmer:

    09 Jul 2020 10:50:00

    I found a baby slow worm, under a stone on my rockery, but have never seen any adults, only on the railway bank, which is over 50yds away!, would they travel that far? or have I got them in my garden?

    Hi Ian. Slow-worms are not particularly social - owing to spending much of their active lives beneath the surface, it can make them difficult to observe (outside of refugia and compost heaps!) They tend to disappear for lengthy periods of time, making their habits hard to study. But, as a whole they're not wide-ranging and there's usually significant overlap between individual home ranges. Studies seems to indicate that 50m is probably significantly more than the average distance travelled but your discovery of a juvenile obviously points to the fact you do have adults, they're just keeping hidden. James

  • Pat Winter:

    09 Jul 2020 11:32:00

    I’m glad to know so much more about my glossy neighbours living in the compost bin, especially about living deeper, and mating habits (I’ve seen neck biting while a pair was coiled together. Their colour and stripes vary widely, so I can distinguish individuals. I think their arrival coincided in a marked drop of brandling worms, though kitchen veg waste plenty of fruit flies which they may like. Good news about the molluscs!

  • 09 Jul 2020 11:52:00

    Fascinating details. We normally see a few in the orchard but this year none. However a few common lizard and adder and black adder were spotted

  • Laurain Carr:

    09 Jul 2020 12:29:00

    Thank you for such an interesting article. Who would have thought. I hope to see some now and enlighten others about these interesting lizards!

  • Irena Greaves:

    09 Jul 2020 13:14:00

    Thanks so much for this truly interesting article. We have seen a few Slow-worms basking on paths around Woolbeding Common. You have to look carefully where you tread because they like to hang around right where one might walk! We were really taken by the beautiful markings of these delightful creatures.
    Thanks for all the fascinating information!

  • Denise Fossey:

    09 Jul 2020 14:51:00

    Very interesting, thank you.

  • Cherry Baden-Powell:

    09 Jul 2020 14:59:00

    My compost heaps are a mecca for slowworms and I get a good chance to study them and to show local quiet children . I strongly recommend covering the heap with stout old carpet , which allows slowworms to bask under it and cats to bask on top , ignorant of one another . Cats like to kill them and then are sick , but it doesn’t put off the cats , which are the slowworms’ greatest threat in gardens

  • Robin Young:

    09 Jul 2020 15:34:00

    We have many slow worms living in a compost bin which I now call a “slow wormery”. We often also see them at various places in the garden. Sadly yesterday while mowing the lawn I inadvertently lopped part of the tale of one of the slow worms. Will it grow back ?

  • Joann Wyatt:

    09 Jul 2020 17:11:00

    I happened across an adult slow worm, last week, basking in the sun on an overgrown verge on the lane near my house. I was thrilled, as I haven’t seen a slow worm, in years. This is a rural area and the council almost never mow the verges now. The next day, the verge had been cut, almost back to soil level. I couldn’t believe it. Possibly, the local farming business did it as the verges were on a dangerous bend. The rest of the area was left untouched.

  • Chris Meachen:

    10 Jul 2020 00:56:00

    Our garden is teeming with slow worms, helped by the fact we try & manage it to encourage all wildlife. The mature females stay copper-coloured, while the males turn a steely grey, with small bright blue spots when in breeding condition. We’ve arranged several dry brick retaining walls on our sloping plot, with voids incorporated so they can hibernate in safety, & plenty of long, dense vegetation so they can move around in relative safety. We’ve also been rewarded with Smmoth & palmate newts, frogs, toads, grass snakes & adders.. We find it best not to keep the garden too tidy if you want to encourage wildlife!

  • jane holbrook:

    10 Jul 2020 05:44:00

    Enjoyed the article, have learnt a lot about the slow worm habits.
    Have seen them occasionally in the garden, but will look more carefully in future. Also liked reading other members comments.

  • Julie Keane:

    10 Jul 2020 11:36:00

    Slow worms congregated under some astroturf used as weed suppressant on my allotment. I used to collect them in a bucket to transfer to the compost bin when I needed to. Babies are particularly lovely

  • Neill:

    10 Jul 2020 11:36:00

    Ive come across them a number of times under weed prevention membrane where I guess they can warm up and be safe from predation. Regulars in our copost heaps too

  • Christine Dafter:

    10 Jul 2020 13:11:00

    What a fascinating piece James.
    I seldom see Slow Worms, usually out on W. Mill reserve under the corruagated sheets, but recently I saw one just outside, in the lane by my house, sunbathing on the tarmac. It was Pink, and I was told it might have just emerged leaving its old skin behind. Unforunately I have no idea whether it was male or female. However I sent a photogaph to a colleague who said it would be forwarded to SxBRC, so I do wonder if there is any informaton as to why it was pink!

  • Leonie Mercer:

    12 Jul 2020 09:13:00

    Our slow worms bask under a clay bird bath and this spring have been joined by a grass snake which has been exciting BUT do grass snakes eat slow worms??

    Hi Leonie. Slow-worms and Grass Snakes are often found in close proximity, though surprisingly slow-worms rarely seem to feature in the snake's diet, which is much more heavily tilted towards frogs, toads and fish. James

  • Richard Bushell:

    13 Jul 2020 20:03:00

    Before retiring I worked at a lot of churches and a few times I came across a slow worm in the ground gutter surrounding the church. Churchyards are really good for all kinds of wildlife.

  • Tony Wetjen:

    03 Sep 2020 11:53:30

    An excellent article. Although I’ve seen Slow-worms on more than one occasion in the wild, I’ve yet to see one in the garden.
    Clearly I need to get a compost heap!

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