January 30, 2012 | 30
I really like finding out about, and writing about, obscure tetrapods. And that’s not a difficult thing to do, since there are some pretty big, pretty diverse tetrapod groups out there that contain huge numbers of poorly known, little-mentioned species. I’ve come back to obscure snakes on a few occasions, and here’s another article where I aim to bring your attention to but a small number of snakes that you might not have heard about before. Of course, whether a species is ‘obscure’ depends on your point of view, and some of the species I’m discussing here are actually familiar animals to those who know their snakes. All of the snakes we’re looking at here are members of the gigantic group traditionally known as Colubridae. See below for the small-print on the use of this name.
The photos were taken by Bangor University’s Wolfgang Wüster and Whitman College’s Kate Jackson (unless stated otherwise) and are used with their kind permission. I’d also like to note that – when I want to know basic stuff about a snake I don’t know much about – my first ports-of-all are still Chris Mattison’s The Encyclopedia of Snakes and Harry Greene’s wonderful Snakes: the Evolution of Mystery in Nature.
So, this is Ahaetulla prasina, photographed in Thailand. It’s one of eight or so Ahaetulla species, often known as Asian vine snakes, tree snakes or, unfortunately, as whip snakes*. They’re found throughout India, south-east Asia and on various Pacific islands. They’re colubrine colubrids, found in some phylogenies to be the sister-taxon to the rest of Colubrinae (Lawson et al. 2005). This is a surprisingly ‘basal’ position for such a strongly modified snake: the outgroups to Colubrinae are mostly terrestrial, short-skulled generalists. Pyron et al. (2011) also found this ‘basal’ position for Ahaetulla, but found evidence that it belongs in a little clade that also includes the Australo-Asian bronze-backed snakes or bronze-backs (Dendrelaphis) and the Asian flying snakes (Chrysopelea) [photo of C. ornata below by LA Dawson, from wikipedia].
* I say “unfortunately” because the name ‘whip snake’ is used for diverse, distantly related slender-bodied snakes from all around the world.
It seems, then, that this specialised clade of highly arboreal, very slender-bodied, mostly skink-eating colubrines diverged early on in colubrine history from a generalised, robust-bodied, predominantly terrestrial colubrine ancestor. To those primarily interested in ecology and behaviour and who don’t see the point of all this obsessing with phylogenetics, I care about this stuff because it allows us to hypothesise about ancestral states and about the changes lineages undergo across time. Basically, it provides the historical skeleton that we need if we want to make inferences about patterns in evolution. Asian vine snakes and flying snakes are specialised predators of arboreal lizards, and bronzebacks mostly eat lizards as well. If these three colubrine lineages do form a clade, was their emergence and subsequent diversification driven by the skink radiation? Questions like this can only be examined when you have phylogenies to work with (and time-calibrated phylogenies at that).
Lawson et al. (2005) found the four African water snakes (Grayia) to be the sister-taxon to the Ahaetulla + other Colubrinae clade. As suggested by the common name, African water snakes are semi-aquatic and eat fish, frogs and tadpoles. They’re robust and large, reaching 1.7 m and even 2.5 m in a few places in western Africa (Spawls et al. 2004). Grayia has always been difficult to place phylogenetically. Some authors have allied it with natricines or xenodontines, or have suggested that it might be outside a natricine + calamariine + colubrine clade (Kelly et al. 2003). Hemipenis morphology in snakes provides a great deal of phylogenetically important information and the hemipenis of Grayia differs quite strongly from that of colubrines – the organ is symmetrical and has a forked sulcus spermaticus in Grayia, whereas it’s asymmetrical in colubrines. Kelly et al. (2003) gave Grayia its own ‘subfamily’ (Grayiinae) in order to emphasise its distinction relative to Colubrinae (in phylogenetic terms, such acts are redundant, since Grayia already stands as a distinct lineage).
Anyway, I digress. I think that most people will find the name Ahaetulla unfamiliar, but not necessarily the snake itself – Asian vine snakes were often known by the junior synonym Dryophis until quite recently (the former name is Ahaetulla Link, 1807; the latter is Dryophis Dalman, 1823). These are very slender bodied snakes with long, pointed snouts and enlarged fangs at the front of the lower jaws (a possible adaptation for grabbing smooth-scaled lizard prey). Their most peculiar and remarkable feature is of course their horizontal pupils, shaped in some species like a horizontally rotated keyhole (that is, with a circular portion posteriorly, and a tapering, bar-shaped portion anteriorly). They’re rear-fanged, and are further unusual in being viviparous. Elsewhere in Colubrinae, viviparity is – I think – only present in smooth snakes (Coronella) and mock vipers (Psammodynastes). Most Asian vine snakes (including A. prasina) eat lizards, occasionally small mammals, but A. fronticincta reaches down from branches over water to grab fish.
Convergent evolution is a fascinating phenomenon, and here [above] is the South American colubrid Oxybelis fulgidus (this one was photographed in Brazil), well known for being highly similar morphologically to Ahaetulla. The two are not close phylogenetically. While Ahaetulla is seemingly outside the enormous clade that includes the vast majority of colubrine taxa, molecular studies find Oxybelis to be deeply nested within a tropical American colubrine clade that includes cribos (Drymarchon), coachwhips (Masticophis) and tiger snakes (Spilotes), and to be the sister-taxon to the arthropod-eating green snakes (Opheodrys) (Lawson et al. 2005, Pyron et al. 2011). Most of these colubrines are generalised, predominantly terrestrial predators of small vertebrates, but some (like Spilotes) are also narrow-bodied, long-headed arboreal specialists.
The four Oxybelis species are often known as vine snakes – they occur from Arizona in the north to Peru in the south and, like Ahaetulla, possess the really slender bodies and long, pointed snouts typical of highly arboreal snakes. Again, they’re back-fanged diurnal predators, mostly of lizards, but (unlike Ahaetulla) their large eyes have round pupils.
Other colubrids are convergently similar to Ahaetulla and Oxybelis as well. Madagascar is home to the leaf-nosed snakes (Langaha), also sometimes called vine snakes (sigh, people are so unoriginal when it comes to common names) and best known for their crazy, sexually dimorphic nose-leaves (note that this structure isn’t unique to L. madagascariensis as you might think from most books). Langaha is not close to colubrines of any sort, instead being part of the pseudoxyrhophiine clade. Then there’s Xenoxybelis, a dipsadine [read on] probably close to (or part of) Philodryas. One of the Philodryas species is shown here.
Here’s a Black-striped snake Coniophanes imperialis, a rear-fanged, oviparous colubrid, one of 12 Coniophanes species. They occur from Texas in the north to Peru in the south. All Coniophanes snakes have longitudinal stripes on their bodies and smooth, shiny scales. Coniophanes is part of Dipsadinae, a major colubrid clade that doesn’t really include any well known species, though the North American hognose snakes (Heterodon) are part of this clade if it’s understood to include the xenodontines. Indeed, there’s a reasonable amount of confusion as to whether Dipsadinae is synonymous with Xenodontinae (Dipsadidae Bonaparte, 1838 has priority over Xenodontinae Bonaparte, 1845), whether the two are related – but distinct – clades, or whether Xenodontinae is a clade within a more inclusive group that can be termed either Dipsadidae or Dipsadinae. I think the last of those options is most likely.
American cat-eyed snakes (Leptodeira) and the blunt-headed tree-snakes or blunt-headed vine-snakes (Imantodes), famous for their bulging eyes and exceptionally slender neck regions, are almost certainly dipsadines too. Night snakes (Hypsiglena) and their relatives are also dipadines (see this Tet Zoo ver 2 article), as are the goo-eaters. Yup, I said goo-eaters. Really must write about them at some stage…
Pyron et al. (2011) found Coniophanes to be part of a dipsadine clade that also included the terrestrial, sometimes semi-fossorial graceful brown snakes (Rhadinaea) and the Central American rainforest snake Amastridium veliferum. The Coniophanes species shown above (the Black-striped snake) is most often found under piles of leaves and other debris and preys on lizards and nestling rodents. It seems to be highly adaptable and ecologically flexible, occurring in dry savannahs and farmland in the north to moist forest and even marshy areas further south. It seems to be tolerant of human disturbance and this, combined with its flexibility, means that it is presently of little conservation concern and seemingly remains in healthy numbers throughout its range.
The whole colubrid mess, again
I mentioned at the top that “small print” has to be used whenever the term ‘Colubridae’ is discussed. This is because there are differing views as how this name should be used. Colubridae as traditionally conceived is an enormous group containing nearly 2000 snake species, and such is its diversity that numerous separate ‘subfamilies’ have been named for clusters of species. The discovery that these ‘subfamilies’ are not really closely related (some seem to be outside the clade that includes viperids, elapids and other ‘colubrids’ while others are closer to elapids than to other ‘colubrids’) means that some authors now use a taxonomic system where just about all traditional ‘subfamilies’ are elevated to ‘family’ level (Vidal et al. 2007, Zaher et al. 2009). The taxonomy and phylogeny favoured by these authors is depicted in the adjacent diagram.
I personally prefer this system. I think it better reflects diversity and disparity to have the clades identified as ‘families’, and I think that biologists and other people tend to understate and underestimate extant lizard and snake diversity and disparity because, for reasons of social inertia, these animals have (unlike birds and mammals) historically been grouped into a low number of ‘families’. Amphibians are affected by the same problem.
However, not everybody agrees with this approach. Pyron et al. (2011) simply stated that changing the status of these clades is unnecessary. But the fact that a large number of ‘colubrid’ ‘subfamilies’ grouped closer to elapids than to colubrines meant that they opted to united the respective groups within an inclusive, ‘family’-level clade termed Lamprophiidae. Colubridae, in their system, was therefore limited to the clade that included natricines, pseudoxenodontines, dipsadines, scaphiodontiphiines, calamariines, grayiines and colubrines (see adjacent phylogeny, from Pyron et al. (2011)). We’re looked at this area (that is, the content and membership of Colubridae sensu lato) a few times before on Tet Zoo (see links below): sloooowly but surely, I aim to say at least something about all of these complex, diverse snake clades.
Lest we forget…
Lest we forget, colubrines and all other kinds of snakes are threatened worldwide by habitat degradation and loss. Species in Madagascar, the American tropics and south-east Asia in particular are potentially in great danger as forests, scrublands and deserts are reduced in size or damaged ecologically as a result of climate change, drought, or the loss of neighbour species in their ecosystems.
In tropical Asia, major and mostly unregulated harvesting of wild snakes for their meat, gall bladders and skin is definitely having an impact on the populations of some species. In Vietnam, local people earn three to five times as much income from trading in snakes than they do from cultivating crops and selling vegetables (more on that here; free pdf here), so it’s not surprising that a massive and unsustainable trade in snakes is underway. Indeed, CITES recently described the Asian snake trade as “one of the largest under-regulated trades in terrestrial wildlife globally”. It’s difficult to know what can be done about the Asian market in snakes – I doubt that many readers of this blog are in the regular habit of consuming snake meat (especially that from unsustainably harvested, wild-caught snakes), and it seems that the vast percentage of the consumer base have no interest in, or concern about, animal welfare, conservation or sustainability.
Ok, my initial plan was to get through a much higher number of taxa. Alas.
For previous Tet Zoo articles on snakes (some of which are very obscure, some of which are not), see…
Refs – -
Lawson, R., Slowinski, J., Crother, B., & Burbrink, F. (2005). Phylogeny of the Colubroidea (Serpentes): New evidence from mitochondrial and nuclear genes Molecular Phylogenetics and Evolution, 37 (2), 581-601 DOI: 10.1016/j.ympev.2005.07.016
Kelly, C. M. R., Barker, N, P. & Villet, M. H. 2003. Phylogenetics of advanced snakes (Caenophidia) based on four mitochondrial genes. Systematic Biology 52, 439-459.
Pyron, R. A., Burbrink, F. T., Colli, G. R., Montes de Oca, A. N., Vitt, L. J., Kuczynski, C. A. & Wiens, J. J. 2011. The phylogeny of advanced snakes (Colubroidea), with discovery of a new subfamily and comparison of support methods for likelihood trees. Molecular Phylogenetics and Evolution 58, 329-342.
Spawls, S., Howell, K., Drewes, R. & Ashe, J. 2004. A Field Guide to the Reptiles of East Africa. A & C Black (London).
Vidal, N., Delmas, A.-S., David, P. Cruaud, C., Couloux, A. & Hedges, S. B. 2007. The phylogeny and classiﬁcation of caenophidian snakes inferred from seven nuclear protein-coding genes. C. R. Biologies 330, 182-187.
Zaher, H., Grazziotin, F. G., Cadle, J. E., Murphy, R. W., Cesar de Moura-Leite, J. & Bonatto, S. L. 2009. Molecular phylogeny of advanced snakes (Serpentes, Caenophidia) with an emphasis on South American xenodontines: a revised classification and descriptions of new taxa. Papéis Avulsos de Zoologia, Museu de Zoologia da Universidade de São Paulo 49, 115-153.
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