I’ve returned several times on this blog to the Slow-worm Anguis fragilis, a legless anguid lizard that occurs across Europe and Asia as far east as western Siberia. I find slow-worms very charismatic animals. Part of the appeal might be that they are easy to find in the places where I’ve lived, part of it might be that we Brits have such a poor reptile fauna that we hold those few species we do have in special regard, and part of it might be that they’re really cute and cool to look at. Slow-worms (there are actually two species: A. fragilis, and A. cephallonica of the Peloponnese and Ionian islands), and their close relatives within Anguidae, are relatively intelligent by lizard standards, and captive individuals exhibit behaviours suggesting that they learn to recognise their keepers. They’re long-lived; a captive slow-worm at Copenhagen lived for 54 years.
However, despite my affection for them, there are some members of the species that I, personally, find shockingly alien and difficult to come to terms with…
One thing that I find particularly interesting about slow-worms is the question of how big they get. I find this subject so interesting because, of all the slow-worms I’ve seen, none have ever been any longer than 25 cm. Other British people with experience of this species talk of individuals of between 25-30 cm as being exceptionally big, so I know I’m not alone [the individual shown at top, photographed in Cardiff, was reported as exceptionally large yet is ‘only’ about 30 cm long]. Despite this, some slow-worms well exceed these lengths, with the world record being 48.9 cm for a male discovered in Portsmouth (Fairfax 1965). That’s close to home: I don’t live in Portsmouth, but it’s the location of my academic base (the University of Portsmouth).
Particularly big slow-worms are also reported to occur on Flat Holm and Steep Holm, two small islands in the Bristol Channel [in the adjacent photo, Steep Holm is the dome-shaped island on the right]. In 1975 British herpetologist Tony Phelps discovered a Steep Holm slow-worm that was 45 cm long, and estimated by him to be between 60 and 70 years old (presumably on the basis of its size). In 1984 an individual 39 cm long was photographed on Steep Holm by slow-worm expert Nick Smith but, because it had a regenerated tail, its original length (i.e., with its intact original tail) would have been about 530 mm, thus making it a new world record.
I’ve never visited Flat Holm or Steep Holm, but I have seen them, if that helps. It makes sense that exceptionally big slow-worms occur on small islands like this, but the fact that the global record-holder comes from Portsmouth is surprising, especially given that Portsmouth is highly urbanised and with very little good space for wildlife: for a flavour of the city, have a look at Graeme’s photos.
Having mentioned Steep Holm, I can’t resist saying a bit more about its wildlife. In 1977, Chinese muntjac Muntiacus reevesi were introduced to the island. Muntjac are all over Britain today following repeated waves of introduction that occurred from the 1870s onward, though quite why anyone would want them on a steep-sided island that has a total area of less than 0.25 sq km (63 acres) is a bit of mystery. Unsurprisingly, Steep Holm’s muntjacs have repeatedly tumbled to their deaths, with at least seven of the animals dying this way between 1984 and 1986. Interestingly, stomach contents showed that these unlucky muntjacs were in the habit of clambering down the steep cliffs to eat seaweed on the beaches (Legg & Parsons 1990). Elsewhere in the world, various populations of sheep, cattle and red deer visit beaches to eat seaweeds, so this behaviour is not without precedent.
Even stranger than the introduction of muntjac is the 1907 introduction of the Orkney vole Microtus arvalis orcadensis to Steep Holm. This was the brainchild of Cardiff chemist Robert Drane, who ‘looked forward to the surprise of someone happening to find [Orkney voles] on Steep Holm’ (Legg & Parsons 1990, p. 46). The attempt failed, however. The Orkney vole was first described as a new species in 1904 and initially thought to be a relict endemic that found refuge on the Orkneys after the spread across Pleistocene Britain of the Field vole M. agrestis (Hinton 1910). We now think that the Orkney vole was introduced by people to the Orkneys, with chromosomal studies indicating that the original population came from France or Spain. Anyway, I digress.
Despite my failure to ever see a remarkably big slow-worm, a few of my friends and colleagues have been more lucky. One of my closest friends and allies is Stig Walsh: taphonomist, palaeontologist, artist, woodsman and all-round outstanding human being [adjacent photo features (left to right) me, Stig and Monja, Stig’s girlfriend. Note the Steve Irwin t-shirt]. Curiously enough, Stig’s Portsmouth garden was once home to what appears to have been a particularly big adult male slow-worm, though unfortunately I never saw it and hence was never able to measure it. Stig got to photograph it however, and below is one of his images. As you can see, this animal was co-operative enough to align itself against some bricks that would allow its length to be estimated. I don’t yet know exactly how big those bricks are, so haven’t done this, but my estimate is that the animal exceeded 30 cm.
More recently, my good friend Markus Bühler (whom regular blog readers might know better as Sordes) was lucky enough to encounter a particularly big male slow-worm in Germany. It was dead, perhaps as a result of ingesting poisoned prey, and measured about 45 cm. Markus has preserved the specimen and has kindly supplied photos [one is shown below]. Again, from my perspective the animal is a monster, and well outside my range of experience.
What’s weird is that some of these especially big slow-worms look quite different from the smaller animals I’m used to, and in fact this goes for many of the slow-worms that come from continental Europe. Superficially, what with their bulkier proportions and more clearly demarcated scales*, they look rather more like glass lizards Ophisaurus than do normal British slow-worms. Glass lizards are legless anguids that are sometimes described as particularly big slow-worms. Named for their particularly fragile autotomising tails, there are about 15 species that inhabit woodland and steppe habitats across North America, Morocco and Eurasia. The best known of them is O. apodus, the Sheltopusik. This is a huge Eurasian species that can reach 1.4 m.
* Slow-worms and glass lizards don’t just possess scales: they’re actually encased in subrectangular osteoderms. These are highly distinctive as fossils and help explain why the group has such a good fossil record.
What makes the similarity between some slow-worms and glass lizards even more interesting is that a recent phylogenetic analysis found that slow-worms are not just closely related to glass lizards, they apparently are glass lizards; in other words, they are deeply nested within Ophisaurus, being most closely related to O. apodus (Macey et al. 1999). My speculation is that the Ophisaurus-like slow-worms of continental Europe are basal members of the slow-worm lineage, and that British populations form an unusual, presumably young, clade. This could all be nonsense, but it’s a working hypothesis. Population-level work of the sort needed to test it has been published on some Asian glass lizards (Lin et al. 2003), but not on slow-worms so far as I know.
Are slow-worms really glass lizards? If they are, this raises a few problems for the classification and taxonomy of these lizards. There is also the alternative view, promoted by Sullivan (1987), that slow-worms, Old World glass lizards, and American glass lizards represent highly disparate lineages that have been separated since the Eocene at least and are only alike due to convergence. While I wanted to cover all of this here, it’ll have to wait, like so much else… to another time.
More on anguids soon, but this time on rather more exotic forms.
For previous articles on the British herpetofauna see Beasts of Portland, Spiky-frilled, lek-breeding amphibious salamanders, Britain’s lost tree frogs and Hunting green lizards in Dorset.
Refs – –
Fairfax, R. A. 1965. Very large English slow-worm. British Journal of Herpetology 2, 229.
Hinton, M. A. C. 1910. A preliminary account of the British fossil voles and lemmings; with some remarks on the Pleistocene climate and geography. Proceedings of the Geologists Association, London 21, 489-507.
Legg, R. & Parsons, T. 1990. Steep Holm Wildlife. Wincanton Press, Wincanton.
Lin, S.-M., Chang, W.-S., Chen, S.-L., Shang, G. & Lue, K.-Y. 2003. Taxonomic status of the legless lizard Ophisaurus (Squamata: Anguidae) in Taiwan: molecular data, morphology, and literature review. Zoological Studies 42, 411-419.
Macey, J. R., Schulte, J. A., Larson, A., Tuniyev, B. S., Orlov, N. & Papenfuss, T. J. 1999. Molecular phylogenetics, tRNA evolution and historical biogeography in anguid lizards and related taxonomic families. Molecular Phylogenetics and Evolution 12, 250-272.
Sullivan, R. M. 1987. Parophisaurus pawneensis (Gilmore, 1928), new genus of anguid lizard from the middle Oligocene of North America. Journal of Herpetology 21, 115-133.