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Slow-worm

Anguis fragilis

General description

A harmless leg-less lizard, the slow-worm is still all too often mis-identified as a snake.  To the trained eye, Anguis fragilis is instantly recognizable as a lizard and is very different to snakes of any species found in Britain.  Overall, a slow-worm’s locomotion is far less fluid than is the case with a snake.  In the hand, a variety of features that identify the slow-worm as a lizard and not a snake become plainly apparent. 

A slow-worm has eyelids, a feature present in all lizard species but absent in snakes.  The unblinking stare of a snake is very different to the comparatively small blinking eyes of a slow-worm.  There are multiple rows of scales on the belly in contrast to the single row of scales present in the case of snakes, and the typical broad and flat lizard tongue of a slow-worm also requires the animal to open it’s mouth slightly in order to flicker the tongue as the indentation in the upper mandible which allows a snake to tongue flicker without the need to open its mouth is of course absent.

The slow-worm’s skin is extremely smooth and polished in appearance.  The ground colour varies from grey through various shades of brown to copper or reddish brown, with melanistic animals occurring with less regularity than is the case with Zootoca vivipara.  The belly is a dark charcoal to bluish black, often mottled to varying degrees with paler colouration.  Slow-worms can reach 45cm in length.

Sexing slow-worms

The sex of adult slow-worms is on the whole, fairly simple to determine.  Males tend to be uniform grey/brown to coppery reddish brown.   Occasionally individuals exhibit darker sides, and darker flecks of colour can occur throughout the dorsal and head area.  Some male slow-worms also display noticeable blue flecks or spots on the sides as well as on occasion on the dorsal area.  The head of an adult male is noticeably larger in proportion to the overall body size than is the case with a female slow-worm.

Female slow-worms generally feature more patterning than males and commonly display dorsal colouration of brown, reddish or copper with noticeably contrasting darker sides.  The majority of adult female slow-worms encountered by the author have displayed one or more vertebral lines running the length of the body, as well as longitudinal lateral stripes each side of the body.

Habitat

Slow-worms thrive in thickly vegetated areas with exposure to sunlight.  The author has encountered slow-worms at roadside hedgerows, dry stone walls and railway embankments.  The edges of woodland, heathland and rank grass lands and bracken banks are also popular haunts.  Allotments are also favoured, and instances where some plots are untended and therefore overgrown with rank grass and bramble patches provide ideal conditions for slow-worms.  An allotment site known to the author in the Llanelli area regularly turns up large numbers of slow-worms, which thrive on the abundant invertebrate population and make good use of the compost heaps & piles of bark chippings for feeding and thermo regulation. 

Ex industrial land or Brownfield sites often provide ideal habitat for slow-worms.  One such site known to the author in the Port Talbot area supports large numbers of slow-worms as well as viviparous lizards and grass snakes.  The combination of areas of dense overgrown bramble and rank grass growing through well drained banks of clinker, piles of stones and rubble provide multiple refuge choices.  All manner of discarded refuse at the site provide the resident slow-worms with prime refuges with increased thermoregulation opportunities.   Sheets of tin, plastic and discarded tyres overgrown with bramble and exposed to sunlight litter the site.  During the author’s first visit to the site, a roofing tile lying amongst rank grass in the sun and close to a bramble patch, upon investigation revealed an aggregation of seven adult slow-worms.

Behaviour

Very little is known about the slow-worm’s life cycle.  Although relatively common throughout south Wales, the animal`s reclusive lifestyle does not make it an easy study subject.  Little is known of the slow-worm’s year, but it is likely that the slow-worm is chiefly fossorial, spending a good proportion of its life under ground in loose soil or in thick surface herbage, both situations providing high levels of invertebrates.

Slow-worms thermoregulate rather than bask in the conventional lizard manner, and maintain body temperature by physical contact with heat retaining surfaces such as discarded tyres out in the sun, or flat stones, sheets of ply wood or tin.  Heat generating piles of bark chip, rotting vegetation and compost heaps are also commonly utilized.  Of the many thousand individuals that the author has encountered over the years, the amount that have been found on the surface “mosaic” basking can literally be counted on both hands. 

Slow-worms emerge from their hibernacula below the frost line sometime in March, the males being the first to emerge.  Hot days are not the best conditions to find them, as the heat causes the slow-worms to seek refuge deeper in the cool soil.  Contrastingly, relatively cool days i.e. overcast with the occasional sunny spell, regularly provide the best viewing opportunities.  Such conditions dictate that the slow-worms must thermoregulate for longer periods beneath surface refugia such as flat stones.  Slow-worms can be found in considerable numbers in suitable habitat and do not appear to be territorial.

The chief defence strategy for slow-worms is autotomy, the ability to shed the tail.  When first captured a slow-worm will often thrash wildly in the hand and more often than not will also defecate.   The animal very quickly settles down however and will rest seemingly contentedly in the hand.  Tail’s can be shed voluntarily but it would appear to be the case that some populations are more inclined to do so than others.

Diet

Very little is known about slow-worm diet.  The author’s scant knowledge of slow-worm feeding behaviour is based purely upon observations of captive animals.  Earthworms of a suitable size, small white slugs and on occasion snails were all seen to be on the slow-worm menu.  Legged invertebrates such as spiders and crickets were also taken, but such instances were very much a hit and miss affair and attempts at such prey items were rarely successful.  In captive situations, slow-worms were seen to patrol the enclosure, passing through soil and leaf litter with the tongue flickering regularly.  When a small white slug or earthworm was encountered, there was always a great deal of tongue flickering and close inspection of the prey.  The whole process is generally painfully slow to the point of being frustrating for the observer.  Just when it seems that the animal will perhaps never commit to feeding, and when the slow-worm’s snout has been literally touching the potential prey for some time, a movement from the prey stimulates a lunge from the slow-worm and the prey is seized in the jaws and gradually swallowed, the slow-worm mouthing and manoeuvring the prey item the entire time whilst moving it’s head from side to side until the prey is swallowed completely.

Breeding

The author has witnessed slow-worms copulating on two occasions.  Both pairs of animals were in a captive situation and both observations were made during late May.  In the first instance, the animals were discovered already copulating, the male grasping the female’s neck in his jaws and the two animal’s bodies entwined.  It was not known at what time copulation began but the animals were observed in the copulatory position for over five hours.   In the second instance, the male was seen to pursue the female clumsily, tongue flickering the whole time as well as displaying slight head nodding.  The male then grabbed the female by the side of the head and the bodies intertwined, both animals waving their tail ends rhythmically.   The male eventually entered one of his paired hemipenis into the female’s cloaca and the animals were seen to copulate for four hours and forty minutes.

The majority of females in Great Britain do not breed annually but every other year.  The Slow-worm is ovi-viviparous; between 3 and 26 young are born in an egg membrane that breaks soon after birth.  New born slow-worms are 7 to 10cm in length and are quite stunning to look at.  Varying from silver to bright gold or coppery red baby slow-worms are living jewels.  There is a noticeable black vertebral stripe and the flanks and belly are black.

Predators

Slow-worms are on the menu of a variety of predators.  No doubt the usual suspects
such as foxes take advantage of slow-worms if discovered, and the author has witnessed magpies attacking and killing an adult male slow-worm on a tarmac road.  In southern England, the smooth snake (Coronella austriaca) also regularly feeds upon slow-worms.  Domestic cats are avid predators of slow-worms, the author having lost count of the occasions when he has witnessed cats preying upon slow-worms in the field.

Longevity