Mystery emo skinks of Tonga!

There are about 3800 lizard species living on the planet today; accordingly, it can sometimes be a bit difficult to keep track of them all. Furthermore, new species are described on a very regular basis, and there's little doubt that many more species await discovery. Matty Smith (from New Zealand) recently encountered the lizard you see here while in Tonga, and he's been having trouble identifying it.

Ever one to take on a tetrapod-themed identification challenge, I had a go at identifying it, and I'd be interested in your opinion should you be able to help.

It's clearly a skink (as demonstrated by its streamlined look, shiny, imbricate scales and relatively large, regularly arranged head scales) - this narrows it down to about 1300 species (Scincidae is the largest lizard 'family'). But we can go further than this: Matty suspects that it might be a species of Emoia, and I'm inclined to agree. Emoia skinks, sometimes known as emoias or emo skinks (presumably on the basis of their taste in fashion and music), consist of about 75 species, split according to Brown (1991) into eight 'species groups'. Here's a close-up of the Tongan skink's head...

Oh, Emoia is yet another of those peculiarly odd yet likeable names invented by John E. Gray (other examples I've covered include the whale names Kogia and Caperea). As with most of his names, we have no idea what it might mean (so far as I know) (for some discussion of the Australasian reptile names Gray coined, see John Scanlon's comments here and here).

Emoias occur across south-east Asia and the islands of the Pacific and Australasian region. They are part of the large lygosomine skink group (e.g., Greer 1979, Honda et al. 2003, Smith et al. 2007). They tend to have a dorsally flattish, slightly pointed snout, their frontoparietal scales are united, they possess supranasal scales, and they have a transparent window in the lower eyelid (I'm sure I'm preaching to the choir, but to know lizards you have to know cephalic scale terminology). Some emoias are good climbers that spend a lot of time in trees; others are well known for being denizens of beaches and other littoral habitats. These littoral species - a good example is the widespread Littoral or Mangrove skink E. atrocostata - make a living by digging up small crustaceans from beach sediment (Auffenberg & Auffenberg 1988).

So, what species are we looking at? Matty reports that the striped Tongan emoia shown here frequents coastal environments, lives in trees and around tree roots, and is about 130 mm long in total length. Though the photo is a bit out of focus (sorry), we can clearly see a light brown, mid-line stripe that runs along the entire dorsal surface, from the snout-tip to the abdomen. Two narrower light brown stripes run in parallel from immediately above the eyes to near the hips, and a series of broken black stripes also run in parallel along the sides of the neck and body. Most of the dorsal colour is pale brown; the limbs and tail are turquoise (I think. My colour vision is not very good).

Taking the express train to Polynesia

Tonga is already well known as the home of several Emoia species, including E. tongana, the phenomenally widespread Copper-tailed skink E. cyanura (in the old, inclusive sense [read on] it occurs across much of the Pacific, from New Guinea in the west to the Pitcairn Islands in the east, inhabiting 24 different island archipelagos (Adler et al. 1995)), Lawes' skink E. lawesii, Gibbon's emo skink E. trossula and the Pacific black skink E. nigra. At least some of these Tongan species (like Lawes' skink and the Black skink) also occur on Samoa and also American Samoa; here, their presence might be explained by a habit of stowing away in Polynesian boats. Worth noting is that human-mediated dispersal seems to have been pretty important in shaping the ranges of many Pacific lizard species: it apparently explains how some taxa - including E. tongana (Austin & Zug 1999) - took the so-called 'express train to Polynesia' (e.g., Austin 1999), spreading around the Pacific within a few thousand years.

Anyway, the presence of prominent dorsal striping in the Tongan animals immediately rules out several contenders and leaves us with a few 'favourite' potential identifications. I immediately find E. cyanura (the Copper-tailed skink) to be the most likely contender [E. cyanura shown above, from the excellent Cook Islands Biodiversity site. Photo © Gerald McCormack 2007]. Like Matty's skink, it has a light-coloured stripe along the dorsal mid-line, two similar, more lateral stripes, and darker areas in between. And it's also notable for having a tail than can be bluish or greenish: in fact, common names other than Copper-tailed skink include Blue-tailed skink and Azure-tailed skink. Some individuals have brown tails, so the species (errr, read on) is variable in this character (Schwaner & Ineich 1998).

And then it got complicated...

A complication here is that E. cyanura has turned out to be more than one species: Ineich & Zug (1991) presented data which indicated that E. cyanura was actually a pair of sympatric, highly similar species (one of which is called E. impar: the name Azure-tailed skink is now used specifically for this form). While the two can be distinguished on the basis of a few tiny diagnostic characters, they otherwise overlap in all features and, to make matters worse, both exhibit the same sort of polymorphism in colour, with individuals in some populations being uniformly dark brown and lacking in stripes [the two images above show two E. impar morphs, also from the Cook Islands Biodiversity site. Photo © Gerald McCormack 2007]. However, the hard-to-spot distinction between the two has been supported by allozyme and mtDNA data, and they're also distinct in habitat preference.

Things then became more complicated when Bruna et al. (1996) showed that the various geographically widespread populations of both E. cyanura and E. impar encompass pretty impressive amounts of genetic variation: more than expected within a 'species'. This perhaps indicates that both clades actually represent complexes of cryptic species. And despite the morphological similarity of the entities traditionally termed E. cyanura and E. impar, they may not even be each other's closest relatives, as other emoias (like E. pseudocyanura and E. isolata) have been found to be closer to E. cyanura than E. impar is (Bruna et al. 1996) [the adjacent cladogram, from Bruna et al. (1996), depicts relationships between various Pacific emoia populations. You might be able to see some of the details just discussed in the text].

Oh, and - - Emoia as a whole may well not be monophyletic (surprise surprise) (Smith et al. 2007), but I'll leave that can of worms alone for the time being. Also worth noting is that another striped Pacific emoia has an element of mystery about it. During the 1970s, a population of E. caeruleocauda was discovered on Fiji: this is well east of Vanuata and the Carolina Islands, the accepted eastern-most part of this species' range. Brown (1991) suggested that this Fijian population represented an anthropogenic introduction. However, later study by Zug & Ineich (1997) - which found the Fijian emoias to be homogenous and morphologically distinct compared to other populations - indicated that these lizards are most likely long-isolated, previously overlooked natives [the adjacent E. caeruleocauda photo, by Eugene van der Pijll, is from wikipedia].

Getting back to the Tongan emoias, a good question needs asking at this point. My identification of Matty's skink rests entirely on gross external morphology. On this basis, the skink is (most likely) E. cyanura. But, now that 'E. cyanura' of traditional has been split into two named species, and probably consists of numerous, substantially different lineages (several of which differ more from each other than they do from other long-recognised emo skink species), I actually can't confirm this, and nor can anyone without a genetic lab and the skink's tissue to hand. Hey, I'm not complaining, I'm just sayin' it like it is.

Does anyone know if a revised taxonomy for either the E. cyanura or E. impar complexes have been published? If so, any news on what happened to the Tongan 'E. cyanura' population? Finally... you do realise that the long, rambling bit of text you've just read started out as 'Can you identify this lizard'?

For previous Tet Zoo articles on neat squamates see...

• Mosasaurs might have used the same microscopic streamlining tricks as sharks and dolphins
• Tongues, venom glands, and the changing face of Goronyosaurus
• Dinosaurs come out to play (so do turtles, and crocodilians, and Komodo dragons)
• Tell me something new about basilisks, puh-lease
• 'Cryptic intermediates' and the evolution of chameleons
• The Great Goswell Copse Zootoca
• Of giant plated lizards and rough-necked monitors
• Ermentrude the liolaemine
• Evolutionary intermediates among the girdled lizards
• Hell yes: Komodo dragons!!!
• Amazing social life of the Green iguana
• Arboreal alligator lizards - yes, really
• Pompey and Steepo, the world-record-holding champion slow-worms

And don't forget the (still unfinished) series on gekkotans...

• The Tet Zoo guide to Gekkota, part I
• Gekkota part II: loud voices, hard eggshells and giant calcium-filled neck pouches
• Squirting sticky fluid, having a sensitive knob, etc. (gekkotans part III)
• Lamellae, scansor pads, setae and adhesion... and the secondary loss of all of these things (gekkotans part IV)
• The incredible leaf-tailed geckos (gekkotans part V)
• 300 years of gecko literature, and the 'Salamandre aquatique' (gekkotans part VI)
• Whence Uroplatus and... there are how many leaf-tailed gecko species now?? (gekkotans part VII)
• Ptychozoon: the geckos that glide with flaps and fringes (gekkotans part VIII)

Refs - -

Adler, G. H., Austin,. C. C. & Dudley, R. 1995. Dispersal and speciation of skinks in the tropical Pacific Ocean among archipelagos. Evolutionary Ecology 9, 529-541.

Auffenberg, W. & Auffenberg, T. 1988. Resource partitioning in a community of Philippine skinks (Sauria : Scindidae). Bulletin of the Florida State Museum, Biological Sciences 32, 151-219.

Austin, C. C. 1999. Lizards took express train to Polynesia. Nature 397, 113-114.

- . & Zug, G. 1999. Molecular and morphological evolution in the south-central Pacific skink Emoia tongana (Reptilia: Squamata): uniformity and human-mediated dispersal. Australian Journal of Zoology 47, 425-437.

Brown, W. C. 1991. Lizards of the genus Emoia (Scincidae) with observations on their evolution and biogeography. Memoirs of the Californian Academy of Sciences15, 1-94.

Bruna, E., Fisher, R., & Case, T. (1996). Morphological and Genetic Evolution Appear Decoupled in Pacific Skinks (Squamata: Scincidae: Emoia) Proceedings of the Royal Society B: Biological Sciences, 263 (1371), 681-688 DOI: 10.1098/rspb.1996.0102.

Greer, A. E., 1979. A phylogenetic subdivision of Australian skinks. Records
of the Australian Museum 32, 339-371.

Honda, M., Ota, H., Köhler, G., Ineich, I., Chirio, L., Chen, S.-L. & Hikida, T. 2003. Phylogeny of the lizard subfamily Lygosominae (Reptilia: Scincidae), with special reference to the origin of the New World taxa. Genes & Genetic Systems 78, 71-80.

Ineich, I. & Zug, G. R. 1991. Nomenclatural status of Emoia cyanura (Lacertilia, Scincidae) populations in the Central Pacific. Copeia 1991, 1132-1136.

Schwaner, T. D. & Ineich, I. 1998. Emoia cyanura and E. impar (Lacertilia, Scincidae) are partially syntopic in American Samoa. Copeia 1998, 247-249.

Smith, S. A., Sadlier, R. A., Bauer, A. M., Austin, C. C. & Jackman, T. 2007. Molecular phylogeny of the scincid lizards of New Caledonia and adjacent areas: evidence for a single origin of the endemic skinks of Tasmantis. Molecular Phylogenetics and Evolution 43, 1151-1166.

Zug, G. R. & Ineich, I. 1997. Striped skinks in Oceania: the status of Emoia caeruleocauda in Fiji. Pacific Science 51, 183-188.