That cute little Mexican snake was, obviously, a ‘colubrid’. That means, essentially, that it’s a colubroid snake that isn’t a viperid, elapid, or a member of any of the other obviously distinct colubroid clades (more on this matter below). Its small size, short-snouted, wide head and proportionally enormous eyes at least suggest that it’s a juvenile. The fact that it was photographed in Mexico, and in a region of scrubland and desert, helps narrow down the possible identities. I initially thought it was a lyre snake (Trimorphodon): they tend to have a light grey ground colour, dark grey/brownish dorsal blotches (one of which – the lyre-shaped one – extends across the back of the head, just reaching level with the eyes), and very large eyes with vertical pupils. The head is dorsoventrally flattened in lyre snakes, perhaps so that they can better reach into crevices when searching for lizards and other prey. But some things weren’t right: in particular, the dark marking that runs along the side of the Mexican snake’s face, ‘through’ the eye. That’s not seen in lyre snakes [photo by Dave Hone].
In fact, the combination of (1) dark blotches that reach around the back of the head and then travel forwards towards the nostrils, (2) the dark, finger-like marking on the back of the head, (3) the large eyes with vertical pupils, and (4) the brownish/greyish ground colour show that this is almost certainly a Night snake Hypsiglena torquata: a small colubrid that occurs in woodlands, deserts, chaparrals and other habitats across the western and south-western United States and then south to Costa Rica. Night snakes are typically 36-41 cm long as adults, with 50 cm being the world record. They’re nocturnal (as you might guess) and feed on frogs and lizards, using enlarged grooved teeth at the back of the jaws. Several subspecies have been named; they vary in the amount of spotting, in scale counts, and in the pattern of blotches on the back of the head and neck. This is in fact such a widespread snake across the USA in particular that I imagine it to be pretty familiar: how come all you Americans didn’t recognise it straight away? Congrats at least to Sven, chris (with small ‘c’), and everyone else who went with this identification.
Hypsiglena has conventionally been classified within Xenodontinae, a large group of American ‘colubrids’ that appear pretty heterogenous and diverse but, like so many snake groups, have been united on the basis of hemipenis characters (see Zaher 1999). The mussuranas (Clelia), ringnecked snakes (Diadophis), hognose snakes (Heterodon), snail-eating and slug-eating snakes (Sibon) and many others (many mightily obscure) are all in here. So, if you identified the snake as a cat-eyed snake (Leptodeira) or a slug-eater, you were at least in the right ball park [adjacent Heterodon from wikipedia].
Some authors think that the xenodontines, while monophyletic, should be split into two groups, with Xenodontinae proper being used for the South American clade and Dipsadinae for the Central American clade (e.g., Cadle 1984). This has been generally supported by recent studies. Lawson et al. (2005) found night snakes to form a clade with the graceful brown snakes (Rhadinaea) and also suggested that at least some of the North American xenodontines (including Farancia (Rainbow snake F. erytrogramma and Mud snake F. abacura), night snakes and hognose snakes) might form a clade [a Northern cat-eyed snake Leptodeira septentrionalis shown below, from wikipedia].
Like so many traditionally recognised ‘families’ (I’m sure we’ve covered this sort of thing before), Colubridae has, some might argue, been maintained by ‘social inertia': defining characters have never been identified, and the numerous constituent groups have been allied through convenience, and by absence of characters rather than possession of them. I will avoid discussing this issue further, as it involves so many snake groups and so many contributions to the literature that I’ll be here all night, but for the purposes of the discussion here all you need to know is that Zaher et al. (2009) raised the xenodontine clade to ‘family’ status as Dipsadidae (Dipsadidae Bonaparte, 1838 has priority over Xenodontinae Bonaparte, 1845). It includes Dipsadinae, Carphophiinae and Xenodontinae, as well as a huge raft of genera that have yet to be classified satisfactorily. Zaher et al. (2009), in fact, decided to explode ‘Colubridae’ of tradition and to raise many of its constituent clades to ‘family’ level. They also coined the new names Colubroides (for xenodermatids and colubriforms) and Endoglyptodonta for the colubriform clade that includes viperids, homalopsids, elapoids and colubroids. Oh, and there’s a mild controversy over exactly what ‘Colubroidea’ means, as molecular scientists have developed their own ‘special’ meaning of the term that’s contrary to herpetological tradition. Yikes – – what did John Scanlon say about avoiding this mess?
Finally… while we’re on the subject of herpetology… here’s me with something that arrived in the post this morning. Yes, if you read this article you’ll know exactly what it is, and what a big deal is it for me to hold my own cherished, much loved copy. To the person involved, thank you once again.
Finally finally… tonight saw the screening (on the UK’s Channel 4) of the last episode of Inside Nature’s Giants, a four-part TV series that, via dissection, discussions of phylogeny, functional morphology, ecology and behaviour, presented a huge amount of anatomical information on elephants, rorquals, crocodiles and giraffes to an enthralled public. A bold and fantastic series. I aim to discuss it at length some time. Stay tuned [Will took the adjacent photo. Not bad for a 7-year-old].
Refs – –
Cadle, J. E. 1984. Molecular systematics of neotropical xenodontine snakes: III. Overview of xenodontine phylogeny and the history of New World snakes. Copeia 1984, 641-652.
Lawson, R., Slowinski, J. B., Crother, B. I. & Burbrink, F. T. 2005. Phylogeny of the Colubroidea (Serpentes): new evidence from mitochondrial and nuclear genes. Molecular Phylogenetics and Evolution 37, 581-601.
Zaher, H. 1999. Hemipenial morphology of the South American Xenodontinae snakes, with a proposal for a monophyletic Xenodontinae and a reappraisal of colubroid hemipenes. Bulletin of the American Museum of Natural History 240, 1-168.
– ., 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.